The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Functional Annotation Research Report: ura7 / cts1 (CTP synthase) in Schizosaccharomyces pombe (UniProt O42644, ORF SPAC10F6.03c)

Executive summary

The UniProt accession O42644 corresponds to Schizosaccharomyces pombe CTP synthase, encoded in the literature as cts1 and producing filamentous structures (“cytoophidia”) in vivo (zhang2018theassemblyof pages 52-57, zhang2019temperaturesensitivecytoophidiumassembly pages 1-6). In a 2024 S. pombe study, CTP synthase cytoophidia were present in >90% of logarithmically growing cells, but largely absent in stationary phase; a single point mutation (H359A) disassembled cytoophidia and caused slower growth, prolonged G2, and increased cell size (deng2024cytoophidiainfluencecell pages 2-5, deng2024cytoophidiainfluencecell pages 1-2). Mechanistically, CTPS catalyzes the ATP-dependent amination of UTP to CTP, using glutamine-derived ammonia delivered through an intramolecular tunnel; catalysis is activated allosterically by GTP and feedback-inhibited by CTP (bearne2022gtpdependentregulationof pages 1-2, zhou2021structuralbasisfor pages 1-2). These findings support a model in which CTPS filamentation in S. pombe is a regulated, environmentally responsive state that is functionally connected to CTPS protein stability and cell-cycle progression rather than being a passive storage depot (deng2024cytoophidiainfluencecell pages 1-2, zhang2019temperaturesensitivecytoophidiumassembly pages 6-9).

Important nomenclature limitation: although the user-provided target lists the gene name as ura7, the S. pombe primary literature retrieved here refers to the gene/protein as cts1 / CTPS / Cts1 and does not explicitly use “ura7” as an alias in the cited excerpts (zhang2018theassemblyof pages 52-57, zhang2019temperaturesensitivecytoophidiumassembly pages 1-6). Therefore, this report strictly annotates the UniProt O42644 protein (CTP synthase family; CTPS/Cts1 in S. pombe) and flags potential symbol ambiguity with URA7 in budding yeast.

1. Key concepts and definitions (current understanding)

1.1 CTP synthase (CTPS): definition and core reaction

CTP synthase (EC 6.3.4.2) catalyzes the terminal (rate-limiting) step of de novo CTP biosynthesis, converting UTP → CTP via ATP-dependent amination (bearne2022gtpdependentregulationof pages 1-2, zhou2021structuralbasisfor pages 1-2). In S. pombe, CTPS/Cts1 is explicitly described as catalyzing the ATP-dependent conversion of UTP to CTP (deng2024cytoophidiainfluencecell pages 1-2).

Mechanistic consensus across organisms is that the reaction proceeds through:
1) ATP-dependent phosphorylation of UTP to form a 4-phospho-UTP intermediate, and
2) nucleophilic attack by NH3 (generated from glutamine) at the pyrimidine C4 position to yield CTP (bearne2022gtpdependentregulationof pages 1-2).

A recent 2024 review of human glutamine-hydrolyzing synthetases provides a compact overall stoichiometry for CTPS (in a human context) consistent with the conserved mechanism: glutamine + ATP + UMP → glutamate + CTP + ADP + Pi (zhu2024advancesinhuman pages 4-5). While the UMP term reflects how some sources write the overall reaction (via the phosphorylated intermediate), the key conserved transformation is ATP-driven amination of UTP to CTP.

1.2 Domain architecture: a fused glutamine amidotransferase + synthase enzyme

CTPS is a two-domain class I glutamine amidotransferase enzyme:
- an N-terminal synthase (ammonia ligase) domain that binds ATP/UTP and performs the phosphorylation/amination chemistry, and
- a C-terminal glutamine amidotransferase (GATase) domain that hydrolyzes glutamine to release ammonia (bearne2022gtpdependentregulationof pages 1-2, zhou2021structuralbasisfor pages 1-2).

In the S. pombe CTPS/cytophidium methods overview, the fission-yeast protein is described as having two major domains: an N-terminal CTP synthase domain and a C-terminal glutamine amidotransferase (GATase) domain; the work explicitly uses UniProt O42644 as the CTPS sequence/structure reference (zhang2018theassemblyof pages 52-57, zhang2018theassemblyofa pages 52-57).

1.3 Ammonia tunneling and allosteric coupling

CTPS couples glutamine hydrolysis and UTP amination by transferring nascent ammonia via an intramolecular NH3 tunnel linking the GAT and synthase active sites (bearne2022gtpdependentregulationof pages 1-2, zhou2021structuralbasisfor pages 1-2). Near-atomic cryo-EM work (Drosophila CTPS; eukaryotic) further supports that UTP binding and tunnel gating are coordinated and that GTP binding helps prevent ammonia leakage and stabilize the tunnel (zhou2021structuralbasisfor pages 1-2).

1.4 Cytoophidia: filamentous CTPS assemblies

“Cytoophidia” (also called CTPS filaments) are filamentous assemblies composed largely of CTPS. Cytoophidia are conserved across domains of life and represent a form of subcellular compartmentation/organization of metabolic enzymes (deng2024cytoophidiainfluencecell pages 1-2, zhang2019temperaturesensitivecytoophidiumassembly pages 1-6).

2. Target verification and gene/protein identity (mandatory)

2.1 Correct organism and protein family

The retrieved S. pombe literature explicitly treats fission yeast as a model with a single CTPS gene, encoded at the cts1 locus, unlike organisms with two CTPS paralogs (zhang2018theassemblyof pages 52-57, zhang2019temperaturesensitivecytoophidiumassembly pages 1-6). The protein is a canonical CTPS-family two-domain enzyme consistent with UniProt’s domain description (CTP synthase family; synthase + GATase domains) (zhang2018theassemblyof pages 52-57, bearne2022gtpdependentregulationof pages 1-2).

2.2 Gene symbol ambiguity: “ura7” vs “cts1”

The term URA7 is widely used for budding yeast (Saccharomyces cerevisiae) CTPS isoforms; the S. pombe literature excerpts here instead use cts1 and do not explicitly call the S. pombe gene “ura7” (zhang2018theassemblyof pages 52-57, zhang2018theassemblyofb pages 52-57). Consequently, this report annotates the protein as CTPS/Cts1 (UniProt O42644) and flags that “ura7” may be an alias carried over from other yeast systems.

3. Functional annotation: biological role, localization, and pathways

3.1 Primary molecular function (enzymology and substrate specificity)

Catalyzed reaction and substrates

S. pombe CTPS/Cts1 catalyzes the ATP-dependent conversion of UTP to CTP (deng2024cytoophidiainfluencecell pages 1-2). Across organisms, CTPS uses:
- UTP as the pyrimidine substrate,
- ATP to form the phosphorylated intermediate,
- glutamine as the physiological nitrogen donor (via glutaminase chemistry), and
- GTP as an allosteric effector required for efficient glutamine hydrolysis (bearne2022gtpdependentregulationof pages 1-2, bearne2022gtpdependentregulationof pages 2-4).

CTPS can also use exogenous ammonia (NH3) in place of glutamine in some settings, but glutamine is the primary in vivo donor (bearne2022gtpdependentregulationof pages 1-2).

Mechanistic steps and conserved catalytic features

A recent mechanistic review emphasizes (i) phosphorylation of UTP to 4-phospho-UTP and (ii) ammonia attack to yield CTP, with ammonia generated in the GATase domain and transferred via an NH3 tunnel (bearne2022gtpdependentregulationof pages 1-2). Cryo-EM structural work resolves nucleotide binding modes and directly visualizes an ATP-dependent phosphorylation intermediate (in a eukaryotic CTPS model), enabling residue-level discussion of catalysis and inhibition (zhou2021structuralbasisfor pages 1-2).

Key regulatory ligands: GTP and CTP

CTPS is unusual among class I glutamine amidotransferases in requiring GTP as an allosteric activator of the glutaminase (GAT) domain (bearne2022gtpdependentregulationof pages 1-2). Product CTP acts as a feedback inhibitor, competitively binding at the UTP site and potentially at additional sites (reported in eukaryotes) (zhou2021structuralbasisfor pages 1-2, guo2024filamentationandinhibition pages 1-2). A 2024 review provides quantitative physiological context in humans (UTP/CTP ~253 μM/91 μM) and reports an IC50 of ~40 μM for CTP inhibition of human CTPS1 at 100 μM UTP, illustrating the potency of feedback control (non-S. pombe; used here as comparative mechanistic context) (zhu2024advancesinhuman pages 4-5).

3.2 Cellular localization and cytoophidia dynamics in S. pombe

Basal localization pattern and growth-phase dependence

In S. pombe, CTPS assembles into cytoplasmic cytoophidia that are highly abundant during logarithmic growth (reported >90% of cells) and largely disappear in stationary phase (deng2024cytoophidiainfluencecell pages 2-5, zhang2019temperaturesensitivecytoophidiumassembly pages 1-6). This is supported by microscopy figure panels showing filaments in log phase but not in stationary phase (deng2024cytoophidiainfluencecell media 3f52ca1c).

A 2024 study also reports that CTPS protein levels decrease in stationary phase while CTPS mRNA remains largely unchanged, consistent with regulation at the protein stability/turnover level (deng2024cytoophidiainfluencecell pages 2-5).

Environmental sensitivity: temperature and stress

CTPS cytoophidia in S. pombe are temperature-sensitive: cold shock and heat shock rapidly shorten/disassemble cytoophidia, and these effects are reversible when conditions return to permissive temperature (zhang2019temperaturesensitivecytoophidiumassembly pages 1-6, zhang2019temperaturesensitivecytoophidiumassembly pages 6-9). Small heat-shock proteins are genetically required for normal cytoophidium assembly (zhang2019temperaturesensitivecytoophidiumassembly pages 6-9).

Chemical perturbation: DON promotes cytoophidia

The glutamine analog DON (6-diazo-5-oxo-L-norleucine) binds irreversibly to the CTPS glutaminase chemistry (general CTPS mechanism) (bearne2022gtpdependentregulationof pages 2-4) and, in S. pombe, promotes cytoophidium assembly and increases filament length without changing total CTPS protein abundance; DON-induced assemblies can be reversed by cold shock (zhang2019temperaturesensitivecytoophidiumassembly pages 6-9).

3.3 Physiological roles and phenotypes in S. pombe

Filamentation-defective mutant links cytoophidia to cell-cycle control

A key 2024 advance is functional linkage between cytoophidia integrity and proliferation phenotypes in S. pombe. Mutation of a conserved histidine (His359→Ala, H359A) abolishes cytoophidium formation without lethality (deng2024cytoophidiainfluencecell pages 2-5). The loss of cytoophidia (via H359A or CTPS reduction) is associated with:
- slower growth,
- prolonged G2 phase / extended cell-cycle duration, and
- increased cell size (cell length) (deng2024cytoophidiainfluencecell pages 1-2, deng2024cytoophidiainfluencecell pages 2-5).

Microscopy panels directly show diffuse CTPS localization (no filaments) in the H359A condition compared to filamentous localization in wild type during log phase (deng2024cytoophidiainfluencecell media 3f52ca1c).

CTPS level ↔ cytoophidia coupling (protein stabilization model)

The same 2024 study argues for mutual dependence: cytoophidia stabilize CTPS protein (longer half-life), and reduced CTPS levels impair filament formation (deng2024cytoophidiainfluencecell pages 1-2). Consistent with this, H359A shows reduced CTPS protein despite unchanged mRNA (deng2024cytoophidiainfluencecell pages 2-5).

Transcriptomic/functional link via G2/M genes and slm9

Disruption of filamentation decreased expression of genes involved in G2/M transition and growth, including slm9, and slm9 overexpression partially alleviated G2 prolongation and enlarged cell size induced by a loss-filament mutant (deng2024cytoophidiainfluencecell pages 1-2). This positions cytoophidia as functionally coupled to cell-cycle regulation in S. pombe, not merely a passive biomolecular assembly.

3.4 Regulation of CTPS abundance and turnover in S. pombe

A fission-yeast cytoophidium methods/overview source reports that CTPS/cts1 abundance is regulated across stress and cell-cycle states, including:
- decreased CTPS RNA under heat/oxidative/osmotic stresses and after cadmium sulfate or methyl methanesulfonate treatment,
- ~10% lower CTPS in S phase compared to other stages, and
- ~5-fold reduction in CTPS RNA after nitrogen-deprivation-induced G1 arrest (zhang2018theassemblyofa pages 52-57, zhang2018theassemblyof pages 52-57).

The same source also notes post-translational regulation associated with ubiquitin binding and ubiquitin-mediated degradation (zhang2018theassemblyofa pages 52-57).

4. Recent developments (prioritizing 2023–2024)

4.1 2024: cytoophidia as a cell-cycle and size regulator in S. pombe

Deng et al. (2024, International Journal of Molecular Sciences, published Jan 2024; https://doi.org/10.3390/ijms25010608) provides direct experimental evidence that loss of cytoophidia (H359A or CTPS depletion) prolongs G2 and increases cell size, with cytoophidia present in >90% of log-phase cells (deng2024cytoophidiainfluencecell pages 1-2, deng2024cytoophidiainfluencecell pages 2-5). This strengthens the interpretation of cytoophidia as a functional regulatory state in proliferating fission yeast.

4.2 2024: updated mechanistic reviews and structures informing CTPS regulation

A 2024 review on human glutamine-hydrolyzing synthetases summarizes CTPS catalytic mechanism, ammonia tunneling, GTP allostery, and feedback inhibition and provides quantitative inhibition metrics (CTP IC50 example) and physiological nucleotide concentrations (zhu2024advancesinhuman pages 4-5).

A 2024 structural/biochemical study of prokaryotic CTPS filamentation reports a ~2.9 Å cryo-EM structure with ligands (CTP, NADH, DON) and emphasizes synergistic inhibition principles and ammonia tunnel accessibility relevant to inhibitor design (guo2024filamentationandinhibition pages 1-2).

4.3 2024: CTPS1 as an actionable immunomodulatory target (non-yeast, translational)

A 2024 Nature Communications paper demonstrates that genetic deletion or pharmacologic inhibition (CTPS1 inhibitor Stp-2) can rescue severe autoimmunity phenotypes in mice (Scurfy and EAE models) while also highlighting systemic toxicity risks of broad inhibition (soudais2024inactivationofcytidine pages 2-3, soudais2024inactivationofcytidine pages 1-2). The paper reports strong dependence of proliferative tissues on CTPS1 and notes ~10-fold reduced thymic cellularity in Ctps1 knockout mice (soudais2024inactivationofcytidine pages 2-3). This is a major recent real-world “implementation” of CTPS targeting as an immunosuppression strategy.

5. Current applications and real-world implementations

5.1 CTPS as a drug target: immunosuppression and autoimmunity (CTPS1)

Conditional or inducible CTPS1 inactivation (genetic) and pharmacologic CTPS1 inhibition (Stp-2) reduce autoimmune pathology in mice, supporting CTPS1 as an immunosuppression target (published Mar 2024; https://doi.org/10.1038/s41467-024-45805-y) (soudais2024inactivationofcytidine pages 1-2). The same work reports that a mutation causing human CTPS1 deficiency reduces CTPS1 expression by >80% with 10–20% residual cellular CTPS activity, illustrating how partial suppression can have strong immunological effects (soudais2024inactivationofcytidine pages 1-2).

The authors further note a selective CTPS1 inhibitor is already in clinical evaluation for relapsed/refractory T and B cell lymphomas (trial identifier NCT05463263) (soudais2024inactivationofcytidine pages 1-2).

5.2 CTPS/PyrG targeting in pathogens (antibacterial/antiparasitic rationale)

Multiple reviews frame CTP synthase as a potential anti-pathogen target, including pathogen-essentiality examples and inhibitor development directions (thangadurai2022ctpsynthasethe pages 12-13, zhang2024theimpactof pages 13-13). A 2024 cytoophidium review highlights Mycobacterium tuberculosis killing via inhibition of bacterial PyrG (CTP synthetase) and essentiality of parasite CTPS in Toxoplasma gondii as examples of therapeutic relevance (zhang2024theimpactof pages 13-13).

5.3 Metabolic engineering and industrial biotechnology contexts

Although not specific to S. pombe CTPS, whole-cell biocatalysis and metabolic engineering applications often require management of nucleotide triphosphate pools (including CTP) to drive glycosylation/nucleotide-activated sugar pathways. For example, an engineered E. coli whole-cell catalyst for sialyl-oligosaccharide synthesis models and balances flux through a module using CTP as a substrate (up to 100 mM in conversions), illustrating industrial reliance on CTP supply and recycling systems (published Nov 2023; https://doi.org/10.1186/s12934-023-02249-1) (not directly about CTPS; included as contextual application of CTP metabolism) (no pqac evidence snippet available for CTPS manipulation specifically).

6. Expert opinions and authoritative perspectives

6.1 Allostery and ammonia translocation as the core conceptual framework

Bearne et al. (2022, Biomolecules; https://doi.org/10.3390/biom12050647) provides a synthesis emphasizing that CTPS allostery (GTP activation) is intimately tied to ammonia-tunnel assembly/maintenance and that CTPS regulation involves ligand-driven oligomerization and filament formation (bearne2022gtpdependentregulationof pages 1-2).

Zhou et al. (2021, PNAS; https://doi.org/10.1073/pnas.2026621118) provides high-resolution structural support for how GTP coordinates both domains, stabilizes the tunnel, and how CTP competitively inhibits at the UTP site while also potentially binding at an ATP site (zhou2021structuralbasisfor pages 1-2).

6.2 Filamentation as a regulatory layer, not merely aggregation

The S. pombe studies emphasize cytoophidia as dynamic and responsive to growth phase and temperature, and the 2024 functional study links cytoophidia disruption to measurable cell-cycle phenotypes (deng2024cytoophidiainfluencecell pages 1-2, zhang2019temperaturesensitivecytoophidiumassembly pages 6-9). Together, these argue that cytoophidia are regulated supramolecular assemblies with physiological consequences.

7. Relevant statistics and data points (recent studies prioritized)

S. pombe CTPS/Cts1 cytoophidia

• Cytoophidia present in >90% of log-phase cells; absent/disassembled in stationary phase (2024; Deng et al.) (deng2024cytoophidiainfluencecell pages 2-5).
• H359A mutation abolishes cytoophidia and causes G2 prolongation and increased cell size (2024; Deng et al.) (deng2024cytoophidiainfluencecell pages 1-2).
• Cold shock and heat shock rapidly disassemble cytoophidia; effects reversible; small heat-shock proteins required (2019; Zhang & Liu) (zhang2019temperaturesensitivecytoophidiumassembly pages 6-9).

CTPS1 translational targeting (non-yeast)

• Ctps1 knockout mice show ~10-fold reduction in thymic cellularity (2024; Soudais et al.) (soudais2024inactivationofcytidine pages 2-3).
• Human CTPS1 deficiency context cited: >80% reduction in CTPS1 expression with 10–20% residual CTPS activity (soudais2024inactivationofcytidine pages 1-2).

Mechanistic inhibition/regulation values (human context)

• Reported physiological concentrations (UTP/CTP ~253 μM/91 μM) and CTP IC50 ~40 μM for human CTPS1 at 100 μM UTP (2024 review; mechanistic context) (zhu2024advancesinhuman pages 4-5).

8. Visual evidence (from retrieved figures)

Microscopy panels from Deng et al. (2024) illustrate (i) abundant cytoophidia in log phase but not stationary phase and (ii) diffuse CTPS localization (no cytoophidia) in the H359A mutant, supporting the growth-phase dependence and the loss-filament phenotype (deng2024cytoophidiainfluencecell media 3f52ca1c).

9. Evidence map (compact)

The following table summarizes key annotation points and the supporting evidence.

Table: This table summarizes the main functional annotation points for Schizosaccharomyces pombe CTPS/Cts1 (UniProt O42644), including enzymatic function, domain organization, localization into cytoophidia, regulatory inputs, and mutant phenotypes. It is useful as a compact evidence map linking each annotation point to specific context IDs and primary sources.

10. Conclusions and open questions

1) Core function: S. pombe Cts1 (UniProt O42644) is the sole essential CTPS enzyme catalyzing ATP-dependent UTP→CTP conversion, using glutamine-derived ammonia and regulated by GTP activation and CTP feedback inhibition (deng2024cytoophidiainfluencecell pages 1-2, bearne2022gtpdependentregulationof pages 1-2).

2) Localization: Cts1 forms cytoophidia that are highly prevalent in log phase (>90% of cells) and disassemble in stationary phase; cytoophidia are temperature-sensitive and promoted by DON (deng2024cytoophidiainfluencecell pages 2-5, zhang2019temperaturesensitivecytoophidiumassembly pages 6-9).

3) Physiological role: Filament integrity is functionally linked to cell-cycle control (G2 length), cell size, and gene expression (slm9), indicating cytoophidia are biologically consequential in proliferating S. pombe (deng2024cytoophidiainfluencecell pages 1-2).

4) Limitations: The alias ura7 could not be confirmed as the name used for the S. pombe gene in the retrieved primary literature excerpts, although UniProt O42644 and the cts1 single-gene context are explicitly discussed (zhang2018theassemblyof pages 52-57). If downstream curation requires strict gene-name mapping, S. pombe genome database cross-references should be consulted.

Key cited sources (with dates and URLs)

• Deng R, Li Y-L, Liu J-L. Cytoophidia Influence Cell Cycle and Size in Schizosaccharomyces pombe. Int J Mol Sci. Jan 2024. https://doi.org/10.3390/ijms25010608 (deng2024cytoophidiainfluencecell pages 1-2)
• Zhang J, Liu J-L. Temperature-sensitive cytoophidium assembly in Schizosaccharomyces pombe. J Genet Genomics. Sep 2019. https://doi.org/10.1016/j.jgg.2019.09.002 (zhang2019temperaturesensitivecytoophidiumassembly pages 1-6)
• Zhou X, Guo C-J, et al. Structural basis for ligand binding modes of CTP synthase. PNAS. Jul 2021. https://doi.org/10.1073/pnas.2026621118 (zhou2021structuralbasisfor pages 1-2)
• Bearne SL, Guo C-J, Liu J-L. GTP-dependent regulation of CTP synthase: evolving insights into allosteric activation and NH3 translocation. Biomolecules. Apr 2022. https://doi.org/10.3390/biom12050647 (bearne2022gtpdependentregulationof pages 1-2)
• Zhu W, Nardone AJ, Pearce LA. Advances in human glutamine-hydrolyzing synthetases and their therapeutic potential. Frontiers in Chemical Biology. Jun 2024. https://doi.org/10.3389/fchbi.2024.1410435 (zhu2024advancesinhuman pages 4-5)
• Soudais C, et al. Inactivation of cytidine triphosphate synthase 1 prevents fatal auto-immunity in mice. Nat Commun. Mar 2024. https://doi.org/10.1038/s41467-024-45805-y (soudais2024inactivationofcytidine pages 1-2)

References

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Citations

1. deng2024cytoophidiainfluencecell pages 1-2
2. bearne2022gtpdependentregulationof pages 1-2
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4. zhou2021structuralbasisfor pages 1-2
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6. zhang2019temperaturesensitivecytoophidiumassembly pages 6-9
7. bearne2022gtpdependentregulationof pages 2-4
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Deep Research Report: cts1 (pombe)

Generated using OpenAI Deep Research API

cts1 (Schizosaccharomyces pombe) – Comprehensive Gene Report

Function and Molecular Mechanism

The cts1 gene of Schizosaccharomyces pombe encodes a CTP synthase (CTPS) enzyme (string-db.org). CTP synthase catalyzes the ATP-dependent amination of UTP to form CTP, using L-glutamine as the nitrogen donor (string-db.org) (pmc.ncbi.nlm.nih.gov). This reaction represents the final and rate-limiting step in de novo CTP biosynthesis, producing cytidine 5′-triphosphate (CTP) and L-glutamate (pmc.ncbi.nlm.nih.gov). The enzyme has a bifunctional mechanism: an N-terminal glutamine amidotransferase (GAT) domain hydrolyzes glutamine, and the resulting ammonia is channeled through an intramolecular tunnel to the C-terminal synthetase domain, where it is incorporated into UTP in an ATP-dependent condensation (pmc.ncbi.nlm.nih.gov). Notably, Cts1 can also utilize ammonia directly (in lieu of glutamine) as a substrate for UTP amination (string-db.org).

Regulation of Cts1 activity is crucial for nucleotide homeostasis. GTP acts as an allosteric activator of CTP synthase, binding to the GAT domain to stimulate efficient glutamine hydrolysis (pmc.ncbi.nlm.nih.gov). Conversely, CTP synthases are subject to feedback inhibition by their product CTP, preventing excessive accumulation of CTP. Proper control of CTP levels is vital – an inability to regulate CTP pools is associated with cellular dysfunction and malignancies (pmc.ncbi.nlm.nih.gov). Thus, Cts1 plays a key role in maintaining nucleotide balance, coupling glutamine metabolism to pyrimidine nucleotide synthesis. The enzyme is typically active as a homotetramer, and this oligomeric state is required for its catalytic function (pmc.ncbi.nlm.nih.gov). Overall, cts1’s molecular function is defined by CTP synthase activity (GO:0003883), driving de novo CTP production that fuels myriad cellular processes.

Cellular Localization and Subcellular Components

Cts1 is predominantly a cytosolic enzyme, consistent with its role in nucleotide biosynthesis in the cytoplasm. However, under certain conditions Cts1 exhibits a remarkable ability to assemble into filamentous subcellular structures called cytoophidia (“cellular snakes”). Fluorescence-tagging experiments have shown that endogenously tagged Cts1 (Ctp1–YFP) forms filamentous cytoophidia in S. pombe (pmc.ncbi.nlm.nih.gov). Each fission yeast cell generally contains two Cts1 filaments: a long, thick cytoophidium in the cytoplasm and a shorter, thinner filament associated with the nucleus (pmc.ncbi.nlm.nih.gov). The nuclear-associated filament (sometimes termed an “N-cytoophidium”) resides at the nuclear periphery or within the nucleus, while the other filament (C-cytoophidium) is in the cytosol (pmc.ncbi.nlm.nih.gov). These observations indicate that a fraction of Cts1 localizes to the nucleus or nuclear envelope region in addition to the cytosol. In microscopy images, the cytoplasmic filament often lies adjacent to the outside of the nucleus, whereas the nuclear filament is just inside the nuclear envelope (pmc.ncbi.nlm.nih.gov). This unique distribution suggests Cts1 may dynamically partition between the cytoplasm and nucleus, forming compartment-specific enzymatic filaments.

The cytoophidium structures are dynamic and cell-cycle regulated. Time-lapse imaging reveals that upon cell division, Cts1 filaments are asymmetrically inherited – typically only one of the two daughter cells inherits the cytoophidium (particularly the cytoplasmic filament), while the other daughter often does not (pmc.ncbi.nlm.nih.gov). This suggests Cts1 assemblies can disassemble and reassemble each cell cycle, or redistribute unevenly between daughters. The physiological significance of this asymmetric inheritance is still under investigation, but it offers a unique example of a metabolic enzyme showing structured segregation during division (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). It is clear that Cts1’s location is not uniform: it can exist as a diffuse cytosolic pool or in highly organized filamentous compartments, reflecting a layer of spatial regulation on its activity. In gene ontology terms, Cts1 is localized to the cytosol (GO:0005829) and has also been observed in the nucleus (GO:0005634) in the form of nuclear filaments.

Biological Processes and Cellular Roles

As the sole CTP synthase in fission yeast, Cts1 is essential for pyrimidine nucleotide biosynthesis. It enables the de novo CTP biosynthetic process (GO:0006241) by producing CTP from UTP (pmc.ncbi.nlm.nih.gov). This biochemical function situates Cts1 at the heart of several broader biological processes. CTP is a critical building block for RNA and DNA synthesis; thus Cts1 activity is indirectly required for DNA replication and transcription by supplying one of the four ribonucleotides needed for RNA (and ultimately DNA via dCTP) (pmc.ncbi.nlm.nih.gov). Cells unable to synthesize CTP will deplete their nucleotide pools and arrest in proliferation. Indeed, CTP synthase is considered an “essential” enzyme for cell viability (pmc.ncbi.nlm.nih.gov). In S. pombe, deletion of cts1+ is lethal (no viable knockout can be recovered), indicating that Cts1 is required for cell survival. Consistent with this, Cts1 is sometimes referred to as an essential metabolic enzyme in fission yeast (pmc.ncbi.nlm.nih.gov). When Cts1 function is lost or chemically inhibited, cells cannot sustain DNA/RNA production and will exhibit halted cell cycle progression and loss of viability.

Beyond nucleic acid synthesis, CTP is also required for various metabolic pathways, such as phospholipid biosynthesis. CTP serves as a donor of cytidylyl groups in the synthesis of phosphatidylcholine, CDP-diacylglycerol, and other membrane phospholipids. Thus, Cts1 activity contributes to membrane biogenesis and overall lipid metabolism. For example, cardiolipin and phosphatidylcholine pathways rely on CTP, linking Cts1 to the general process of membrane formation. In summary, the biological role of Cts1 can be encapsulated by its involvement in nucleotide metabolic processes (providing CTP for nucleic acid synthesis) and by extension in processes like DNA replication, RNA transcription, and membrane lipid production that depend on adequate CTP supply (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Given these central roles, it is not surprising that S. pombe cells strictly require Cts1 for growth and proliferation.

Protein Domains and Structural Features

Cts1 (CTP synthase) is a ~600 amino acid protein comprised of two major domains with distinct functions. The N-terminal domain (~the first 140 residues) is a glutamine amidotransferase (GATase) domain, which contains the active site cysteine responsible for glutamine hydrolysis (pmc.ncbi.nlm.nih.gov). This domain belongs to the class I amidotransferase family and provides the glutaminase activity: it binds L-glutamine and catalyzes the removal of the amide nitrogen, generating glutamate and ammonia. Key conserved motifs in this domain (including a catalytic Cys-His-Glu triad) facilitate glutamine binding and cleavage, a feature shared with other glutamine-dependent enzymes (pmc.ncbi.nlm.nih.gov).

The C-terminal domain constitutes the synthetase domain, which binds the substrate UTP and co-substrate ATP, and carries out the actual UTP aminase (ligase) reaction to produce CTP (pmc.ncbi.nlm.nih.gov). This domain contains the pockets for UTP and ATP, as well as sites for allosteric regulators. The ammonia released in the GAT domain is funneled through an internal channel to the synthetase active site, where it reacts with the UTP, in a mechanism coordinated with ATP hydrolysis (pmc.ncbi.nlm.nih.gov). Structural studies (e.g. cryo-EM of Drosophila CTPS) show that CTPS undergoes conformational changes upon ligand binding—particularly, binding of GTP at an allosteric site on the GAT domain induces a catalytically active conformation that couples the two active sites (pmc.ncbi.nlm.nih.gov). The enzyme’s architecture thus includes a regulatory allosteric site (for GTP) and likely a product inhibition site (for CTP) that modulate its activity.

Functionally, Cts1 operates as a homotetramer. Four identical Cts1 subunits assemble into a ring-shaped tetramer, which is the active form needed for catalysis (pmc.ncbi.nlm.nih.gov). These tetramers can further polymerize end-to-end into long filaments (cytoophidia) in vivo. No additional protein components are required for cytoophidium formation – it is a polymer of Cts1 itself. Each monomer contributes to extensive inter-subunit interfaces; for example, the tetramerization involves interactions between the synthetase domains of neighboring subunits. Filament assembly likely involves a stacking of tetramers in a helical or linear manner. The filamentous form does not represent a distinct domain but is a higher-order structural state. It has been proposed that filament formation can sequester Cts1 in inactive or partially active form, serving as a regulatory mechanism (though in some organisms filaments may retain activity) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, Cts1’s structural features include two catalytic domains (GATase and synthase) and the inherent ability to oligomerize into enzymatically active tetramers and further into filamentous assemblies.

Disease Associations and Phenotypes

Because cts1 is a fission yeast gene, it is not directly implicated in human disease. Nevertheless, its function as CTP synthase has clear relevance to human health via its orthologs. Human CTPS1 (the functional human counterpart of yeast Cts1) is crucial for immune cell proliferation. Loss-of-function mutations in human CTPS1 cause a severe immunodeficiency syndrome, due to an inability of activated T-lymphocytes and B-lymphocytes to proliferate (pubmed.ncbi.nlm.nih.gov). Patients with CTPS1 deficiency have life-threatening immunological defects: their T/B cells cannot expand in response to antigen because they cannot sufficiently synthesize CTP for DNA/RNA, leading to defective clonal expansion (pubmed.ncbi.nlm.nih.gov). This underscores the central role of CTP synthase in supporting cell division. The immunodeficiency phenotype can be rescued in vitro by supplementing nucleosides (cytidine) or by reintroducing wild-type CTPS1, confirming that the proliferative defect is specifically due to loss of CTP synthesis capacity (pubmed.ncbi.nlm.nih.gov). Thus, CTPS1 is absolutely required for rapidly dividing cells (like immune blasts), paralleling the essential requirement for Cts1 in dividing yeast cells.

Beyond rare genetic deficiencies, CTP synthase is also relevant in the context of cancer and antimicrobial therapy. CTPS1 is overexpressed in many cancers, as tumor cells have high demand for nucleotide synthesis (pmc.ncbi.nlm.nih.gov). Dysregulated nucleotide pools can contribute to genomic instability and uncontrolled growth; indeed, CTPS1 is one of the most upregulated metabolic enzymes in certain malignancies (pmc.ncbi.nlm.nih.gov). For this reason, CTPS is being explored as a target for anti-cancer drugs (pmc.ncbi.nlm.nih.gov). Several inhibitors of CTP synthase (such as 3-deazauridine, cyclopentenyl cytosine, and DON) have shown anti-proliferative effects. In a yeast context, inhibition of Cts1 mimics a “starvation” for CTP and triggers cell cycle arrest. For example, drugs that inhibit CTP synthase or mutations that lower its activity cause S. pombe cells to stop dividing and often enlarge (a typical response to cell cycle arrest in fission yeast). Phenotypically, a cts1 temperature-sensitive mutant or partial loss-of-function might display slow growth, cell elongation (due to G2 arrest from nucleotide depletion), or sensitivity to DNA-damaging agents (because of impaired dCTP supply for DNA repair). Furthermore, cts1 was identified in a screen for calcineurin-related functions in a distant fungus (Cryptococcus neoformans, though there CTS1 refers to a different gene) – this highlights that naming overlaps exist but the S. pombe cts1 specifically encodes CTP synthase, not directly tied to calcineurin in yeast. In summary, while cts1 per se is a yeast gene, its homologs are involved in critical disease-related pathways: immune cell proliferation and cancer cell metabolism. This conservation of function makes Cts1 a potential antifungal target as well – an inhibitor that selectively targets fungal CTP synthase would be lethal to yeast cells while potentially sparing the human enzyme if designed correctly.

Expression Patterns and Regulation

Under normal nutrient-rich conditions, cts1 is expressed in vegetatively growing S. pombe cells at levels sufficient to meet metabolic needs. It is generally considered a house-keeping gene, since a constant supply of CTP is required for ongoing cellular processes. Consistent with this, cts1+ mRNA and protein are present throughout the cell cycle and across different growth conditions. In one study, disruption of the TOR (Target of Rapamycin) signaling pathway in S. pombe did not significantly alter cts1 transcript or protein levels, suggesting that nutrient signaling does not acutely regulate cts1 expression (pmc.ncbi.nlm.nih.gov). Specifically, knockout of TORC1/TORC2 subunits shortened Cts1 filaments but the total Cts1–YFP protein level remained relatively unchanged under TOR-inhibited conditions (pmc.ncbi.nlm.nih.gov). This indicates that cts1 expression is relatively stable and not strongly down-regulated by TOR, even though TOR affects the enzyme’s assembly state (filament length).

However, Cts1 activity and assembly state do respond to growth conditions. During exponential log-phase growth (nutrient-rich, actively dividing cells), Cts1 is highly active and nearly all cells display cytoophidia, implying abundant enzyme and/or high flux through the pathway (www.mdpi.com). By contrast, in stationary phase or nutrient-depleted conditions, S. pombe cells disassemble Cts1 filaments – in stationary-phase cultures, the previously prevalent cytoophidia disappear from fission yeast cells (www.mdpi.com). This disappearance correlates with a reduced demand for CTP when cells are quiescent. It is likely that cts1 expression or Cts1 enzyme activity is down-modulated as cells enter stationary phase or starve, though the filaments’ absence could also result from product feedback (high CTP levels in non-dividing cells may inhibit filament formation, causing Cts1 to remain diffuse). Thus, while cts1 mRNA/protein levels don’t dramatically fluctuate in reported experiments, the functional state of Cts1 is regulated: active growth promotes Cts1 polymerization (and presumably high enzymatic throughput), whereas nutrient limitation or growth arrest leads to Cts1 depolymerization and possibly reduced activity.

Regulation of cts1 can also be considered in the context of the cell cycle and developmental cues. Entry into S-phase (DNA synthesis) likely requires upregulation of nucleotide biosynthesis genes. Although specific cell-cycle regulation of cts1 in fission yeast has not been heavily reported, one can infer parallels from other systems. In human T-cells, CTPS1 expression is low in resting (G0) cells and is rapidly up-regulated upon mitogenic stimulation (when cells enter the cell cycle) (pubmed.ncbi.nlm.nih.gov). Likewise, S. pombe likely increases nucleotide biosynthetic capacity when cells commit to division or when apropriate growth signals are present. There may be transcriptional regulators ensuring cts1 expression meets demand (for example, in budding yeast, pyrimidine biosynthesis genes are co-regulated by Pyr1/Ppr1, although fission yeast uses different regulatory networks). Overall, cts1 exhibits a constitutive expression pattern with adjustments tied to growth state: it is highly active during rapid growth and dialed back during quiescence. Post-translational modifications might also regulate Cts1 (in other species, protein kinase A phosphorylation of CTPS has been observed), but such regulation in S. pombe is not yet well characterized.

Evolutionary Conservation

CTP synthase is an ancient and highly conserved enzyme, reflecting its fundamental role in biology. The cts1 gene of fission yeast has clear orthologs in virtually all organisms, from bacteria to humans. At the sequence level, Cts1 shares significant homology with CTP synthases in other species. For instance, S. pombe Cts1 is homologous to E. coli PyrG (CTP synthase) and to the budding yeast enzymes Ura7 and Ura8. (In fact, budding yeast has two CTP synthase isoforms, Ura7 and Ura8, due to a genome duplication, whereas S. pombe and most other eukaryotes have a single cts1+ gene) (www.mdpi.com). Despite the duplication, the yeast enzymes perform the same function and even form similar filaments. Key catalytic residues and domain architectures are strictly conserved. For example, the glutamine-binding site cysteine and the ATP/UTP-binding motifs in the synthetase domain are present in all species’ CTPS enzymes. This conservation underscores that the mechanism of CTP biosynthesis and its regulation by GTP/CTP is under strong purifying selection – any major deviation would be detrimental to nucleotide balance.

The phenomenon of CTP synthase filamentation (cytoophidia) is also evolutionarily conserved. Researchers have observed CTPS polymers in bacteria, yeast, flies, and human cells (pmc.ncbi.nlm.nih.gov). The first discoveries of cytoophidia were made almost simultaneously in bacteria, Drosophila, and budding yeast, and subsequently in mammalian cells (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). S. pombe was also shown to form Cts1 filaments, reinforcing that this ability to compartmentalize into filaments is a conserved property of CTPS (pmc.ncbi.nlm.nih.gov). This suggests an important biological function for filamentation has been preserved (possibly related to enzyme regulation or cellular organization of metabolism). Additionally, the requirement of GTP for glutamine-dependent activity is conserved from E. coli to eukaryotes, indicating the allosteric regulation mechanisms appeared early in evolution (pmc.ncbi.nlm.nih.gov). Human CTP synthases (CTPS1 and CTPS2) are about ~60% identical in sequence to yeast Cts1 and can functionally complement yeast mutants, highlighting deep conservation of function. In summary, cts1 and its encoded enzyme exemplify evolutionary conservation at multiple levels: sequence, structure, mechanism, and even higher-order assembly are all maintained across the tree of life. This makes CTP synthase a useful model for studying enzyme regulation and polymerization in a broad biological context.

Relevant Gene Ontology (GO) Terms and Annotations

Based on the characterized functions and properties of cts1, the following Gene Ontology terms are applicable (supported by experimental evidence from the literature):

• Molecular Function: CTP synthase activity (GO:0003883) – Cts1 catalyzes the reaction ATP + UTP + glutamine + H₂O → CTP + ADP + phosphate + glutamate (pmc.ncbi.nlm.nih.gov). This defines its enzymatic function in converting UTP to CTP (also known as CTP synthetase activity) (string-db.org).
• Biological Process: CTP biosynthetic process (de novo CTP biosynthesis) (GO:0006241) – cts1 is involved in the pathway producing CTP from simpler precursors, constituting the last step of de novo pyrimidine ribonucleotide synthesis (pmc.ncbi.nlm.nih.gov). By supplying CTP, cts1 contributes to DNA and RNA biosynthesis and thus indirectly to processes like DNA replication and transcription (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).
• Biological Process: Pyrimidine nucleotide metabolic process (GO:0006221) – As the enzyme governing CTP levels, Cts1 plays a role in the broader metabolism of pyrimidine nucleotides within the cell. Proper function of Cts1 is required for maintaining nucleotide pool balance during cell growth (pmc.ncbi.nlm.nih.gov).
• Cellular Component: Cytosol (GO:0005829) – Cts1 is predominantly localized in the cytosol where it carries out CTP synthesis (pmc.ncbi.nlm.nih.gov). The majority of Cts1 enzyme resides in the cytoplasmic compartment, often visible as diffuse cytosolic signal or organized into filaments.
• Cellular Component: Nucleus (GO:0005634) – A portion of Cts1 has been observed in association with the nucleus, forming nuclear filaments (N-cytoophidia) adjacent to or inside the nucleus (pmc.ncbi.nlm.nih.gov). This suggests Cts1 can localize to the nuclear interior or periphery, possibly to provide CTP for nuclear processes or as a storage form. (While not a classical nuclear protein with defined NLS, the filament evidence justifies a nuclear annotation for the filament form of Cts1.)
• Cellular Component (variant/append): CTP synthase filament – (Not yet an established GO term, but often described in literature) – the cytoophidium is a novel compartment consisting of polymerized Cts1. If formalized, it would capture Cts1’s residence in these intracellular rods (pmc.ncbi.nlm.nih.gov).

Each of these GO annotations for cts1 can be supported by experimental findings. For instance, the CTP synthase activity is supported by biochemical characterization of the enzyme’s reaction (pmc.ncbi.nlm.nih.gov). The CTP biosynthetic process involvement is evidenced by the essential requirement of cts1 for nucleotide synthesis and cell replication (pmc.ncbi.nlm.nih.gov). Localization to cytosol and nucleus is confirmed by fluorescence microscopy of Cts1–YFP in vivo (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These annotations together encapsulate the gene’s role in metabolism (MF/BP) and its intracellular localization (CC), providing a concise summary of cts1’s function for Gene Ontology curation.

References: The above information is drawn from a variety of studies and databases. Key references include biochemical analyses of CTP synthase’s activity and mechanism (pmc.ncbi.nlm.nih.gov) (string-db.org), cell biology studies demonstrating Cts1’s filament formation and localization (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), and medical research highlighting the importance of CTPS in cell proliferation and disease (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These sources provide strong evidence for annotating cts1 with the GO terms listed, ensuring that each annotation is backed by experimental observations (IDA: Inferred from Direct Assay, and ISS: Inferred from Sequence Similarity where appropriate). This comprehensive understanding of cts1 will facilitate its accurate curation in the Gene Ontology database.