MAL33 (systematic name YBR297W; synonym MAL3R) is a 468-residue Saccharomyces cerevisiae protein belonging to the fungal Zn2Cys6 (GAL4-type binuclear zinc cluster) family of transcription factors. It carries a canonical N-terminal Zn(2)-C6 fungal-type DNA-binding domain (residues 8-34), a predicted nuclear localization signal (residues 41-49), and a central fungal-transcription-factor middle homology region (Pfam Fungal_trans / IPR007219). On domain grounds the zinc cluster is expected to coordinate two structural zinc ions and to bind sequence-specific (CGG-triplet-containing) DNA, consistent with a role as a nuclear, RNA polymerase II sequence-specific DNA-binding transcription factor. MAL33 is one of the paralogous MAL-activator genes of the S. cerevisiae MAL multigene loci: each MAL locus is a complex of three genes encoding a MAL-activator, a maltase, and a maltose permease, and MAL33 is the activator gene of the telomere-associated MAL3 locus on chromosome II (together with the MAL31 permease and MAL32 maltase). The prototypical, well-characterized MAL-activator is Mal63p of the MAL6 locus, which mediates maltose-inducible transcription of the MAL structural genes through a transactivation region and a C-terminal maltose-responsive regulatory domain. MAL33 is 71% identical to Mal63p, but the MAL33 allele of the S288C reference strain is reported to be a non-functional MAL-activator (while UniProt annotates the S288C ORF as intact); consequently MAL33's own DNA targets, DNA-binding sites, inducing condition, and specific biological role have not been experimentally established, and its family-level maltose-regulon function is inferred from its functional paralogs rather than measured for MAL33 itself.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic transfer of RNA-Pol-II DNA-binding transcription factor activity from the Zn2Cys6/GAL4-type DNA-binding domain (IPR001138, IPR020448, IPR036864). Reason: MAL33 carries an intact N-terminal Zn(2)-C6 fungal-type DNA-binding domain (residues 8-34) and belongs to the fungal Zn2Cys6 MAL-activator family, so a sequence-specific RNA-Pol-II DNA-binding transcription factor activity is the most defensible molecular-function statement. This is a domain/family capability; the specific target genes and DNA sites of MAL33, and whether the non-functional S288C allele actually activates transcription, are recorded as knowledge gaps. Supporting Evidence: GO_REF:0000002 Gene Ontology annotation through association of InterPro records with GO terms |
| GO:0003677 DNA binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic transfer of the generic DNA binding term from the Zn2Cys6 DNA-binding domain (IPR007219, IPR020448). Reason: Correct but generic. DNA binding is a direct property of the Zn2Cys6 domain; it is the parent of the more informative sequence-specific / RNA-Pol-II DNA-binding transcription-factor terms also annotated. Retained as a valid, if non-specific, domain-level term. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Automated cellular-component annotation placing MAL33 in the nucleus (UniProtKB-SubCell SL-0191 plus InterPro), consistent with the predicted NLS and DNA-binding TF classification. Reason: A nuclear site of action is the parsimonious, domain-consistent location for a Zn2Cys6 DNA-binding transcription factor with a predicted nuclear localization signal (residues 41-49). There is no direct experimental localization for MAL33, but nucleus is the appropriate inferred compartment. |
| GO:0006351 DNA-templated transcription | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO electronic transfer of the generic DNA-templated transcription process from the fungal-TF domains (IPR007219). Reason: Broadly correct for a transcription factor but very generic (a high-level parent of the regulation-of-transcription terms). Retained as a non-core, domain-level process rather than a demonstrated MAL33-specific function. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO electronic transfer of a generic transcription-regulation process from the Zn2Cys6/GAL4-type and fungal-TF domains (IPR001138, IPR020448, IPR036864). Reason: Domain-appropriate and correct in kind, but generic (the DNA-templated parent of the RNA-Pol-II regulation term). No MAL33-specific target gene or regulatory event is demonstrated; kept as non-core. |
| GO:0006357 regulation of transcription by RNA polymerase II | IEA GO_REF:0000108 | KEEP AS NON CORE | Summary: Logical-inference (GO_REF:0000108) derivation of the RNA-Pol-II regulation process from the RNA-Pol-II DNA-binding transcription factor activity (GO:0000981). Reason: The most specific of the process terms and appropriate for a Zn2Cys6 RNA-Pol-II TF on family grounds, but still an inferred, generic capability: the specific genes MAL33 regulates, the inducing condition, and whether the non-functional S288C allele actually performs this role are undetermined. Retained as a non-core, domain-derived process; the maltose-regulon specificity is written up as the primary knowledge gap. |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic transfer of zinc ion binding from the Zn2Cys6 binuclear zinc-cluster domain (IPR001138, IPR007219, IPR020448, IPR036864). Reason: MAL33 carries a canonical fungal Zn(2)-C6 binuclear zinc cluster whose six cysteines coordinate two structural zinc ions; zinc binding is a robust, well-supported domain-level property. Supporting Evidence: GO_REF:0000002 Gene Ontology annotation through association of InterPro records with GO terms |
| GO:0008270 zinc ion binding | RCA PMID:30358795 The cellular economy of the Saccharomyces cerevisiae zinc pr... | ACCEPT | Summary: Reviewed-computational-analysis (RCA) inclusion of MAL33 in the S. cerevisiae zinc proteome (Wang et al. 2018), based on combined domain and motif searches for zinc-binding proteins. Reason: Independent computational support for zinc binding, concordant with the Zn2Cys6 domain and with the InterPro zinc-binding annotation. Not a direct MAL33 metal-binding measurement, but a reasonable, domain-consistent inference. Supporting Evidence: PMID:30358795 The yeast zinc proteome of 582 known or potential zinc-binding proteins was identified using a bioinformatics analysis that combined global domain searches with local motif searches. |
| GO:0003700 DNA-binding transcription factor activity | ISS PMID:10447589 Functional domain analysis of the Saccharomyces MAL-activato... | ACCEPT | Summary: SGD ISS (inferred from sequence similarity) annotation classifying MAL33 as a DNA-binding transcription factor, with_from SGD:S000029659 (MAL63), the functional prototype MAL-activator characterized in PMID:10447589. Reason: The molecular activity — a fungal Zn2Cys6 DNA-binding transcription factor — is well supported by the intact Zn(2)-C6 domain and the MAL-activator family membership, and is the most defensible MF statement for MAL33. It rests on sequence similarity to the functional Mal63p, not on a direct MAL33 assay; because the S288C MAL33 allele is a non-functional activator, the specific transcriptional-activation outcome for MAL33 itself remains a knowledge gap. Supporting Evidence: PMID:10447589 the sequence-specific DNA-binding domain of Mal63p is contained in residues 1-100 |
| GO:0005634 nucleus | ISS PMID:10447589 Functional domain analysis of the Saccharomyces MAL-activato... | ACCEPT | Summary: SGD ISS annotation that MAL33 is active in the nucleus, inferred by sequence similarity to the functional MAL-activator Mal63p (with_from SGD:S000029659). Reason: Consistent with MAL33's DNA-binding zinc cluster, predicted NLS, and MAL-activator family classification. Nuclear action is the expected, domain-consistent location; retained as a reasonable inferred compartment (no direct MAL33 localization assay exists). |
| GO:0006355 regulation of DNA-templated transcription | ISS PMID:10447589 Functional domain analysis of the Saccharomyces MAL-activato... | KEEP AS NON CORE | Summary: SGD ISS annotation that MAL33 is involved in regulation of DNA-templated transcription, inferred by sequence similarity to the functional MAL-activator Mal63p (with_from SGD:S000029659). Reason: Family- and domain-appropriate as a generic capability, but it transfers the MAL-activator role from the functional Mal63p to MAL33 by homology. The specific MAL structural genes MAL33 would regulate, the maltose-inducing condition, and whether the non-functional S288C MAL33 allele actually activates transcription are all undetermined. Retained as a non-core, inferred process; the maltose-regulon specificity is captured as the primary knowledge gap. |
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Download this section (compressed HTML)Q: Is the S288C reference-strain MAL33 allele a functional MAL-activator, and if not, which sequence changes relative to the functional Mal63p inactivate it?
Q: Which genes does MAL33 directly bind and regulate, at which MAL upstream activation sequences, and under what inducing condition (maltose)?
Q: How does MAL33 relate functionally to the other MAL-activator paralogs, including the functional Mal63p and the other non-functional reference-strain allele MAL13?
Experiment: Assay maltose-inducible transactivation by the S288C MAL33 allele using a MAL-UAS reporter and quantification of endogenous MAL31/MAL32 induction, with a functional MAL63 as positive control, to determine whether MAL33 has any activator function.
Experiment: ChIP-seq/ChIP-exo of epitope-tagged MAL33 (in maltose vs glucose) to identify direct in vivo binding sites and target genes, if any.
Experiment: Domain-swap and point-mutant analysis between MAL33 and the functional Mal63p to localize the sequence changes responsible for the reported loss of MAL33 function in the reference strain.
Experiment: Trans-complementation tests measuring whether MAL33 (alone or overexpressed) restores maltose fermentation to a MAL-activator-null strain, benchmarked against MAL63, to test residual/conditional activity and redundancy.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Whether the MAL33 allele of the S. cerevisiae S288C reference strain is a functional MAL-activator at all is the central unknown: it is reported to be non-functional, so its capacity to activate maltose-inducible transcription of the MAL structural genes has not been demonstrated for MAL33 itself. The direct DNA target genes, the specific MAL upstream activation sequence(s) MAL33 occupies, and the inducing condition are undetermined for MAL33.
OPEN BIOLOGY BP_DARK
What is known: MAL33 is a bona fide fungal Zn2Cys6 MAL-activator-family protein with an intact N-terminal Zn(2)-C6 DNA-binding domain (residues 8-34), a predicted NLS (41-49), and a fungal-TF middle homology region; on these grounds it can bind zinc, bind sequence-specific DNA, and act as a nuclear RNA-Pol-II transcription factor. It is the activator gene of the MAL3 locus (with MAL31 permease and MAL32 maltase). The functional, maltose-inducible activation mechanism — DNA-binding domain, transactivation region, inhibitory region, and C-terminal maltose-responsive domain — has been mapped only for the paralogous, functional Mal63p (MAL6 locus), from which all of MAL33's process and TF-activity annotations are transferred by sequence similarity (ISS) or domain inference (IEA). MAL33 is ~71% identical to Mal63p, yet the S288C allele does not confer maltose fermentation. No direct functional assay of MAL33 (DNA binding, target identification, transactivation) exists.
Significance: MAL33 exemplifies a gene whose family role (maltose-regulon activation) is textbook for its functional paralogs but whose own biological activity is unestablished and, in the reference strain, apparently lost. Resolving whether the S288C MAL33 allele retains any activator function — and if not, which sequence changes inactivate it relative to Mal63p — would clarify the extent of allelic MAL-activator degeneration in the reference genome and prevent over-attribution of a demonstrated maltose-regulon role to MAL33 based purely on homology.
What would resolve it: Directly test the S288C MAL33 allele: express tagged MAL33 and assay maltose-inducible transactivation of a MAL-UAS reporter and of the endogenous MAL31/MAL32 genes; ChIP of tagged MAL33 to map in vivo binding sites; and compare against a functional MAL63 control. If MAL33 is inactive, domain-swap / point-mutant analysis against Mal63p to localize the inactivating sequence changes.
Provenance (the field's own admissions):
Gap: The functional relationship of MAL33 to the other MAL-activator paralogs is uncharacterized: whether MAL33 is redundant with, distinct from, or (in S288C) simply a degenerate copy of the functional activators (Mal63p and relatives), and how it relates to MAL13 — the other non-functional MAL-activator allele of the reference strain — has not been tested directly for MAL33.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: S288C-derived strains carry two MAL loci (MAL1 and MAL3) whose activator genes (MAL13 and MAL33) are both reported as non-functional; the encoded proteins are ~71% identical to the functional Mal63p, and the maltose-non-inducible phenotype of an S288C-isogenic strain is complemented in trans by a plasmid-borne functional MAL63. MAL33 is thus known at the sequence and locus level, but no genetic or biochemical study has directly tested MAL33's redundancy with, or divergence from, the functional activators.
Significance: Understanding whether MAL33 is a spent duplicate or retains a conditional/partial activity would determine whether MAL33 should carry a maltose-regulon function at all in the reference strain, and would inform the broader question of how the tandemly arrayed, telomere-associated MAL loci evolve and degenerate.
What would resolve it: Combinatorial complementation/epistasis: test whether S288C MAL33 (and MAL13) can restore maltose fermentation to a mal-activator-null background alone or when overexpressed, and compare directly with functional MAL63; sequence/domain-swap analysis to map divergence responsible for loss of function relative to Mal63p.
Provenance (the field's own admissions):
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