CrtB is the phytoene synthase of the carotenoid-producing enterobacterium Pantoea ananatis (historically Erwinia uredovora), a 309-residue soluble enzyme of the phytoene/squalene synthase family. It catalyses the committed, pathway-defining step of C40 carotenoid biosynthesis: the head-to-head (1'-1) condensation of two molecules of the C20 isoprenoid geranylgeranyl diphosphate into the cyclopropylcarbinyl intermediate prephytoene diphosphate, followed by rearrangement of that intermediate with loss of a proton to give the colourless C40 hydrocarbon 15-cis-phytoene. The enzyme requires ATP, which is not hydrolysed during turnover, together with either Mn2+ or Mg2+, and is inhibited by phosphate and by squalestatin. Because it eliminates a proton rather than reducing the intermediate, it needs no nicotinamide cofactor, which is the chemical step that distinguishes it from its squalene synthase relatives. CrtB acts inside the bacterial cell on the geranylgeranyl diphosphate pool supplied by CrtE, and its product is handed on to the desaturase CrtI, the cyclase CrtY and the hydroxylase CrtZ that build lycopene, beta-carotene and zeaxanthin. By controlling entry of isoprenoid carbon into the carotenoid branch, CrtB sets the flux for the pigments that give the organism its yellow colouration, and its compact, self-contained activity has made it a standard entry enzyme for reconstructing carotenoid pathways in heterologous hosts.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004311 geranylgeranyl diphosphate synthase activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro2GO assigns GGPP synthase activity from IPR044843, a fold-level trans-isoprenyl diphosphate synthase signature. CrtB does not synthesise GGPP; it consumes GGPP as its substrate, and the GGPP synthase of this organism is the separate gene crtE. Reason: GO:0004311 is defined as the head-to-tail condensation of farnesyl diphosphate with isopentenyl diphosphate to give geranylgeranyl diphosphate, verified against QuickGO. That is the opposite of what CrtB does. Expressing CrtB alone in E. coli depleted the host GGPP pool and suppressed growth, and the experimental remedy was to co-express crtE, the actual Erwinia GGPP synthase, to raise GGPP levels; an enzyme cannot both deplete GGPP and be the enzyme that makes it. Misawa and colleagues had already shown crtE and crtB to be distinct open reading frames of the same cluster in this organism, so the activity being transferred belongs to a paralogous neighbour rather than to this protein. Mechanistically the two are different reaction types: GGPP synthase is a 1'-4 head-to-tail prenyltransferase, whereas CrtB performs a 1'-1 head-to-head condensation. IPR044843 is a bacterial-type trans-isoprenyl diphosphate synthase entry capturing the shared all-alpha terpenoid synthase fold, which the same UniProt record also matches as Gene3D "Farnesyl Diphosphate Synthase" and SUPFAM "Terpenoid synthases"; a fold-level match cannot discriminate chain elongation from head-to-head condensation. This is a paralog over-annotation from an InterPro2GO mapping, and it is contradicted by direct experiment rather than merely unsupported. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG GRANULARITY MISMATCH Sources checked: InterPro:IPR044843 · Trans_IPPS_bact-type SOURCE BAD Fold-level bacterial trans-isoprenyl diphosphate synthase entry shared by head-to-tail elongases and head-to-head condensing enzymes; it cannot license a specific chain-elongation activity. Supporting Evidence: PMID:9593819 Presumably inhibition of growth arose from the depletion of the substrate geranylgeranyl pyrophosphate (GGPP) PMID:9593819 GGPP levels were increased by co-expressing the isoprenoid biosynthetic genes crtE and idi, encoding the Erwinia GGPP synthase and Rhodobacter isopentenyl pyrophosphate isomerase, respectively. PMID:2254247 Six open reading frames were found and designated the crtE, crtX, crtY, crtI, crtB, and crtZ genes in reference to the carotenoid biosynthesis genes of a photosynthetic bacterium, Rhodobacter capsulatus file:PANAN/crtB/crtB-uniprot.txt InterPro; IPR044843; Trans_IPPS_bact-type. file:PANAN/crtB/crtB-deep-research-falcon.md GGPP is the physiologically established substrate and 15-cis-phytoene is the product. file:PANAN/crtB/crtB-deep-research-falcon.md CrtB occupies the entry point from general isoprenoid metabolism into the carotenoid branch |
| GO:0016117 carotenoid biosynthetic process | IDA PMID:9593819 Expression of an active phytoene synthase from Erwinia uredo... | ACCEPT | Summary: Direct assay of the purified enzyme converting GGPP to phytoene, the first C40 carotenoid and the committed intermediate of the pathway. CrtB catalyses a step of this process, so it participates rather than merely being required for it. Reason: This is the core biological process of the gene. The supporting study developed a non-radioactive assay for the GGPP-to-phytoene conversion using the overexpressed Erwinia uredovora enzyme, and identified the product by co-chromatography against authentic standards with matching spectra. The participation test is satisfied in its simplest form: CrtB itself performs the condensation that creates the first carotenoid, so this is catalysis of a pathway step and not necessity evidence standing in for participation. UniProt independently records the pathway assignment for this entry. Supporting Evidence: PMID:9593819 A non-radioactive assay was developed that enabled the conversion of GGPP to phytoene. PMID:9593819 The reaction product was identified by co-chromatography with authentic standards on HPLC systems and comparison of spectral characteristics. file:PANAN/crtB/crtB-uniprot.txt Carotenoid biosynthesis; phytoene biosynthesis. |
| GO:0016117 carotenoid biosynthetic process | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning assignment of the same process term that the gene already holds by IDA. The electronic call agrees with the direct experimental evidence. Reason: The rule reaches the correct process at the correct level of specificity for a phytoene synthase, and is fully concordant with the IDA row from PMID:9593819 and with the UniProt pathway assignment. Duplicate GO identifiers across different evidence codes are expected and carry independent provenance, so this row is retained alongside the experimental one rather than collapsed into it. Supporting Evidence: file:PANAN/crtB/crtB-uniprot.txt Carotenoid biosynthesis; phytoene biosynthesis. PMID:9593819 A non-radioactive assay was developed that enabled the conversion of GGPP to phytoene. |
| GO:0016120 carotene biosynthetic process | IEA GO_REF:0000108 | ACCEPT | Summary: Inter-ontology logical inference from GO:0046905, which this gene holds with IDA. The product 15-cis-phytoene is a hydrocarbon carotenoid, so the inferred process term is chemically correct. Reason: GO:0016120 is defined as the formation of carotenes, that is hydrocarbon carotenoids, verified against QuickGO. 15-cis-phytoene is precisely such a C40 hydrocarbon and is the precursor of every downstream carotene in this organism, so inferring the process from the verified 15-cis-phytoene synthase activity is sound. Note for the record that GO:0016120 is not currently a descendant of GO:0016117 in the ontology, its ancestors running instead through carotene metabolic process and isoprenoid biosynthetic process; the gene therefore legitimately holds both process terms and neither is redundant with the other. No ontology change is proposed here. Supporting Evidence: file:PANAN/crtB/crtB-uniprot.txt The enzyme produces 15-cis-phytoene. PMID:9593819 A non-radioactive assay was developed that enabled the conversion of GGPP to phytoene. |
| GO:0016765 transferase activity, transferring alkyl or aryl (other than methyl) groups | IEA GO_REF:0000120 | MODIFY | Summary: A true but uninformative grandparent of the activity this gene actually has. GO:0046905 is a verified descendant of GO:0016765, and the gene holds GO:0046905 with direct experimental evidence, so the generic term should be replaced by the specific one. Reason: A QuickGO ancestor query confirms GO:0046905 descends from GO:0016765 via prenyltransferase activity GO:0004659, so this row is not wrong, only coarse. Its supporting signature IPR019845 is a squalene/phytoene synthase conserved-site pattern shared across the whole family, and a family-wide site pattern can only license a family-wide statement, which is why the automatic pipeline stops at this level. For this protein, however, the precise reaction is established by direct assay and matched to EC 2.5.1.32 and RHEA:34475, so a term conveying only that some non-methyl alkyl group is transferred to some acceptor adds nothing over what the gene already carries. Broader IEA terms can often simply be accepted, but this one sits two levels above even prenyltransferase activity and names neither substrate, product, nor pathway, so it is too general to retain. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: InterPro:IPR019845 · Squalene/phytoene_synthase_CS SUPPORTS SOURCE BUT NOT TARGET Family-wide conserved-site pattern; correctly supports a generic transferase statement but cannot reach the specific activity that direct assay establishes for this protein. ARBA:ARBA00026321 · ARBA:ARBA00026321 SUPPORTS SOURCE BUT NOT TARGET Automatic rule reaching only the generic transferase level for the phytoene/squalene synthase family. Proposed replacements: 15-cis-phytoene synthase activity Supporting Evidence: file:PANAN/crtB/crtB-uniprot.txt InterPro; IPR019845; Squalene/phytoene_synthase_CS. file:PANAN/crtB/crtB-uniprot.txt EC=2.5.1.32 {ECO:0000269|PubMed:9593819}; PMID:9593819 A non-radioactive assay was developed that enabled the conversion of GGPP to phytoene. |
| GO:0046905 15-cis-phytoene synthase activity | IDA PMID:9593819 Expression of an active phytoene synthase from Erwinia uredo... | ACCEPT | Summary: The defining molecular function, established by direct in-vitro assay of the overexpressed enzyme from this organism with product identification against authentic standards. Reason: This is as clean as an IDA for an enzymatic activity gets: the crtB product was overexpressed to about a fifth of total cellular protein, partially purified, assayed for the GGPP-to-phytoene conversion, and the product confirmed by co-chromatography and spectral comparison. Kinetic and cofactor parameters were determined on the same preparation, giving a Km of 41 microM for GGPP and a requirement for ATP with either Mn2+ or Mg2+. The 15-cis specificity in the term is correct: although the in-vitro material contained both isomers, UniProt records that the enzymatic product is 15-cis-phytoene and that the all-trans form arises by photoisomerisation after the fact. This term is the gene's core function and matches the required function recorded for this protein in the carotene backbone biosynthesis module. Supporting Evidence: PMID:9593819 The crtB gene encoding phytoene synthase from the carotenogenic enterobacterium Erwinia uredovora was overexpressed to about 20% of the total cellular protein in Escherichia coli. PMID:9593819 The reaction product was identified by co-chromatography with authentic standards on HPLC systems and comparison of spectral characteristics. PMID:9593819 The Km value for GGPP was determined as 41 microM. file:PANAN/crtB/crtB-uniprot.txt The enzyme produces 15-cis-phytoene. |
| GO:0046905 15-cis-phytoene synthase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automatic assignment grounded in RHEA:34475 and EC 2.5.1.32, which are the same reaction that UniProt records for this entry with experimental evidence. Concordant with the IDA row for the identical term. Reason: The cited Rhea reaction and EC number correspond exactly to the catalytic activity UniProt attributes to this protein on experimental grounds from PMID:9593819, so the electronic route reaches the right answer by the right chemistry. Retained alongside the IDA row for the same term, since duplicate identifiers under different evidence codes represent independent provenance. Supporting Evidence: file:PANAN/crtB/crtB-uniprot.txt Reaction=2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 file:PANAN/crtB/crtB-uniprot.txt EC=2.5.1.32 {ECO:0000269|PubMed:9593819}; |
| GO:0051996 squalene synthase [NAD(P)H] activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro2GO assigns the paralogous squalene synthase activity from IPR033904, the head-to-head trans-IPPS domain that phytoene synthases and squalene synthases share by construction. CrtB uses a different substrate, makes a different product, and requires no NAD(P)H. Reason: GO:0051996 is defined as the NAD(P)H-dependent condensation of two farnesyl diphosphate molecules to squalene, verified against QuickGO, and every component of that definition is wrong for CrtB: the substrate is the C20 geranylgeranyl diphosphate rather than the C15 farnesyl diphosphate, and the product is the C40 phytoene rather than the C30 squalene. The nicotinamide requirement is the decisive and most informative discrepancy. Squalene synthase and phytoene synthase are genuine homologues and both form a cyclopropylcarbinyl diphosphate intermediate by head-to-head condensation, which for this protein UniProt records explicitly as prephytoene diphosphate. The families diverge at the second half-step: squalene synthase reductively opens the intermediate using NADPH, whereas phytoene synthase instead eliminates a proton, which is exactly why the product carries the central 15-cis double bond and why no nicotinamide cofactor is needed. The experimentally determined cofactor set for this enzyme is ATP with Mn2+ or Mg2+ and contains no NAD(P)H, so the single feature that names GO:0051996 is the step CrtB demonstrably does not perform. IPR033904 is the shared head-to-head domain, matched on the same UniProt record by CDD cd00683, and therefore cannot discriminate between the two families. One contrary observation was weighed and rejected: CrtB is inhibited by squalestatin, a squalene synthase inhibitor, but inhibitor cross-reactivity reports active-site similarity, which the shared fold already explains and which is the very reason this signature matches; being inhibited by a drug aimed at a homologue is not evidence of catalysing the homologue's reaction. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG FUNCTIONAL DIVERGENCE Sources checked: InterPro:IPR033904 · Trans_IPPS_HH SOURCE BAD Head-to-head trans-IPPS domain common to squalene synthases and phytoene synthases; shared by construction, so it cannot assign the squalene-specific, NAD(P)H-dependent activity to a phytoene synthase. Supporting Evidence: PMID:9593819 The essential cofactors required were ATP in combinations with either Mn2+ or Mg2+. PMID:9593819 Phytoene synthesis was inhibited by phosphate ions and squalestatin. file:PANAN/crtB/crtB-uniprot.txt InterPro; IPR033904; Trans_IPPS_HH. file:PANAN/crtB/crtB-uniprot.txt The enzyme produces 15-cis-phytoene. |
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Download this section (compressed HTML)Q: Does CrtB function as a soluble cytoplasmic enzyme, or is it recruited to the cytoplasmic face of the inner membrane where its hydrophobic product accumulates and where the downstream desaturase CrtI must act?
Q: Which residues of CrtB coordinate the required Mn2+ or Mg2+, and does the metal bind the protein directly or principally chelate the diphosphate leaving groups of the two substrate molecules?
Q: What is the role of ATP, given that it is essential for activity but is reported not to be hydrolysed during the reaction?
Q: Do CrtB and the downstream enzymes CrtI, CrtY and CrtZ form a physical or kinetically coupled assembly that channels the insoluble intermediates, or are phytoene and lycopene released into the lipid phase between steps?
Q: Which residues set the C40 versus C30 boundary that separates phytoene synthases from their squalene synthase relatives within the same fold?
Experiment: Fractionate Pantoea ananatis into soluble and membrane fractions with validated marker controls and locate CrtB by immunoblot, in parallel with protease-accessibility assays on inverted and right-side-out vesicles and with fluorescent-fusion imaging of a functional tagged allele. Repeat in a crtI deletion background to test whether downstream pathway activity influences the distribution.
Hypothesis: CrtB associates with the cytoplasmic face of the inner membrane rather than being freely soluble, positioning it to deliver phytoene to CrtI.
Type: subcellular fractionation and imaging
Experiment: Solve the structure of CrtB alone and with a non-hydrolysable GGPP analogue and ATP to identify the metal and substrate sites, then test candidate residues by alanine substitution, measuring the effect on kinetics, metal dependence and product stereochemistry. This would place binding-site annotations on an entry that currently carries none, and would test directly whether the InterPro signatures shared with squalene synthase correspond to conserved or diverged active-site chemistry.
Hypothesis: CrtB binds its divalent metal through an aspartate-rich motif of the terpenoid synthase fold, and distinct residues govern the proton elimination that yields the 15-cis product instead of the NADPH-dependent reduction used by squalene synthases.
Type: structure determination and site-directed mutagenesis
Experiment: Assay the purified enzyme against a graded series of prenyl diphosphates, including farnesyl diphosphate, geranylfarnesyl diphosphate and the natural geranylgeranyl diphosphate, determining steady-state kinetics for each and identifying products by mass spectrometry, with and without added NADPH to confirm that no reductive route operates.
Hypothesis: CrtB is strictly selective for the C20 geranylgeranyl diphosphate and will not productively condense shorter prenyl diphosphates.
Type: substrate specificity panel
Experiment: Compare initial rates with ATP against non-hydrolysable analogues and with other nucleotides, quantify any ADP or phosphate release stoichiometrically against turnover, and test for nucleotide binding by isothermal titration calorimetry or thermal shift.
Hypothesis: ATP acts as a non-consumed allosteric or structural effector of CrtB rather than as a phosphoryl donor.
Type: ATP dependence dissection
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Where in the bacterial cell CrtB carries out catalysis is undetermined: it is not known whether the enzyme is freely soluble in the cytoplasm or associates with the cytoplasmic face of the inner membrane where its hydrophobic product partitions.
OPEN BIOLOGY CC_DARK
What is known: The reaction, substrate, product stereochemistry, cofactor requirement and kinetics are all firmly established by direct assay of the purified enzyme, and the sequence carries no signal peptide and no transmembrane segment. What is missing is any positive localization measurement.
Significance: Whether phytoene synthase is recruited to a membrane surface bears on how the insoluble C40 product is handed to the downstream desaturase CrtI, and hence on whether the pathway operates as a membrane-associated assembly or through free intermediates.
What would resolve it: Cell fractionation with membrane and soluble marker controls, protease accessibility, and fluorescent-fusion imaging in the native organism, ideally with and without an active downstream pathway.
Provenance (the field's own admissions):
Gap: The substrate specificity of CrtB has not been mapped: no panel of alternative prenyl diphosphates has been tested, so it is unknown whether the enzyme is strictly selective for the C20 geranylgeranyl diphosphate or will accept shorter or longer chains.
OPEN BIOLOGY
What is known: Geranylgeranyl diphosphate is established as the physiological substrate, with a measured Km of 41 microM, and the physiological product is established as 15-cis-phytoene. The unknown is the breadth of acceptance around that substrate, not the identity of the substrate itself.
Significance: Chain-length selectivity determines whether CrtB can be redirected to non-natural carotenoid backbones in engineered hosts, and would locate the residues that set the C40 versus C30 boundary against its squalene synthase relatives.
What would resolve it: Steady-state kinetics of the purified enzyme against a graded series of prenyl diphosphates, with products identified by mass spectrometry.
Provenance (the field's own admissions):
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