flbB encodes a 426-residue basic-leucine-zipper (bZIP) transcription factor of filamentous ascomycetes. It has an N-terminal bZIP DNA-binding and dimerization domain, preceded by a bipartite nuclear localization signal, and a C-terminal region related to the redox-responsive cysteine-rich domain of Yap1/Pap1-type AP-1 factors. FlbB is one of the upstream developmental activators of asexual sporulation (conidiation) in Aspergillus nidulans. Null mutants form aconidial "fluffy" colonies that do not express brlA, the master regulator of conidiophore development. The protein has two pools. One accumulates at the hyphal tip, where FlbE binds its bZIP domain and makes it transcriptionally competent; the other moves retrogradely, in a dynein-dependent manner, to apical nuclei, where its accumulation depends on FlbD. In the nucleus FlbB activates flbD, and FlbB and FlbD then bind the brlA promoter cooperatively and induce brlA. Cys382 in the C-terminal region and the bZIP domain are both needed for tip localization and for conidiation. Transcriptome analysis of the null mutant shows further roles: repression of the dba secondary-metabolite cluster, and activation of urdA, a repressor of premature sexual development. flbB mutants also show altered branching and autolysis under osmotic stress.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000976 transcription cis-regulatory region binding | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted. Cis-regulatory DNA binding, propagated from the bZIP ancestor PTN008082960 and supported in A. nidulans by ChIP binding of FlbB at the brlA promoter. Reason: flbB sits in PANTHER PTHR40621:SF6. The IBA comes from PAINT node PTN008082960, the shared ancestor of the FlbB clade and the Yap1/Pap1/Cap1 clade. Its donors (Yap1, Pap1, Cap1) belong to the second clade, but cis-regulatory DNA binding is a function of the conserved bZIP basic region. PTN008082960 is the ancestor of both clades, so propagating the term to flbB is phylogenetically sound. The term also has direct support in this species: chromatin immunoprecipitation shows FlbB on the brlA promoter, where binding depends on FlbD. That is binding to a cis-regulatory region. It is not by itself proof of sequence-specific recognition, so the term is kept at this level and is not refined to a sequence-specific child. Supporting Evidence: PMID:31065746 Both participate in the control of conidiation through cooperative binding to the promoter of brlA ( brlA p ). Furthermore, chromatin immunoprecipitation assays showed that FlbB cannot bind brlA p in the absence of FlbD file:NEUCR/NCU07379/NCU07379-bioinformatics/RESULTS-rereview.md Both clades descend from PAINT node PTN008082960, so the general bZIP DNA-binding transcription activator terms propagated from that node apply. |
| GO:0001228 DNA-binding transcription activator activity, RNA polymerase II-specific | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the core molecular function. FlbB is a Pol II-specific DNA-binding activator of flbD and brlA. Reason: This is the most specific supported molecular function for FlbB. The IBA comes from PTN008082960, the bZIP node shared by the FlbB and Yap1 clades, and the term is general to bZIP activators rather than specific to the Yap1 clade. The A. nidulans data give independent support. FlbB activates expression of flbD and brlA, both protein-coding (Pol II) genes, and the ΞflbB transcriptome confirms that it activates them. FlbB occupies the brlA promoter (ChIP), and loss of FlbE abolishes its transcriptional activity. PAINT propagated the generic activator terms from this node but not the oxidative-stress terms of the Yap1/Pap1 donors, and that omission is respected here. Supporting Evidence: PMID:25701285 Results confirmed the activating role of FlbB on downstream regulators of conidiation such as flbD and brlA. PMID:20132447 we show that bZip transcription factor FlbB, which has been attributed to participate in transducing the aerial stimulus signal, activates the expression of c-Myb transcription factor FlbD. Both factors then jointly activate brlA PMID:26256571 flbE deletion also abrogates transcriptional activity indicating that FlbB gains transcriptional competence from interactions with FlbE at the tip. |
| GO:0090575 RNA polymerase II transcription regulator complex | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Kept as non-core. A plausible family-level complex inference; no FlbB-containing Pol II regulator complex has been isolated. Reason: bZIP factors act as dimers, and in A. nidulans FlbB binds the brlA promoter cooperatively with FlbD (it cannot bind without FlbD). A promoter-bound transcription-regulator assembly is therefore plausible. The original paper also proposes that FlbB acts in a heteromeric complex. No complex has been purified or defined, however, so this stays a non-core inference and should not be read as evidence that a complex exists. Supporting Evidence: PMID:17993569 Like many bZIP proteins ( 45 ), FlbB may form a heteromeric complex. |
| GO:0000976 transcription cis-regulatory region binding | IEA GO_REF:0000002 | ACCEPT | Summary: Accepted. The same claim as the IBA row, reached here from the AP-1-like InterPro signature. Reason: The InterPro AP-1-like signature (IPR050936) agrees with the IBA and with the ChIP evidence for FlbB on the brlA promoter. It is concordant with the IBA row and is accepted the same way. Supporting Evidence: PMID:31065746 Both participate in the control of conidiation through cooperative binding to the promoter of brlA ( brlA p ). |
| GO:0003700 DNA-binding transcription factor activity | IEA GO_REF:0000002 | MODIFY | Summary: Correct but general. Refine to the Pol II-specific activator term that the gene already carries. Reason: FlbB is a DNA-binding transcription factor, but the evidence supports the more specific descendant GO:0001228 (DNA-binding transcription activator activity, RNA polymerase II-specific), which the gene already carries by IBA. FlbB activates the Pol II genes flbD and brlA. Proposed replacements: DNA-binding transcription activator activity, RNA polymerase II-specific Supporting Evidence: PMID:25701285 Results confirmed the activating role of FlbB on downstream regulators of conidiation such as flbD and brlA. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Accepted. FlbB's nuclear pool is shown directly in several A. nidulans studies. Reason: This row comes from an ARBA-derived UniProt subcellular-location statement, which is not evidence in itself. The claim is still correct: a bipartite NLS is needed for FlbB to accumulate in nuclei, and that nuclear pool executes its transcriptional function. Supporting Evidence: PMID:26256571 Finally, a bipartite nuclear localization signal is required for nuclear localization of FlbB. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | MODIFY | Summary: Correct but general. Refine to positive regulation of transcription by RNA polymerase II. Reason: FlbB activates, rather than represses, Pol II transcription of flbD and brlA. The specific descendant GO:0045944 is already on the gene (IEA from GO_REF:0000108) and is the supported refinement. Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:25701285 Results confirmed the activating role of FlbB on downstream regulators of conidiation such as flbD and brlA. |
| GO:0045944 positive regulation of transcription by RNA polymerase II | IEA GO_REF:0000108 | ACCEPT | Summary: Accepted as the core process. FlbB positively regulates Pol II transcription of flbD and brlA. Reason: This row is a logical inference from the activator MF. The underlying claim is directly supported in A. nidulans: FlbB is required for brlA expression, induces flbD, and binds the brlA promoter together with FlbD. Supporting Evidence: PMID:20132447 we show that bZip transcription factor FlbB, which has been attributed to participate in transducing the aerial stimulus signal, activates the expression of c-Myb transcription factor FlbD. Both factors then jointly activate brlA PMID:17993569 no brlA expression was detected in the flbB -null mutant, showing that FlbB is required, directly or indirectly, for proper brlA transcription regulation |
| GO:0048315 conidium formation | IMP PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | ACCEPT | Summary: Accepted. Loss of flbB blocks conidiation (fluffy phenotype), and the acts_upstream_of_or_within qualifier fits a regulator that sits upstream of brlA. Reason: Null and point mutants of flbB are aconidial or strongly delayed in conidiation. FlbB does not carry out conidiophore morphogenesis itself; it triggers the program by inducing flbD and brlA. The acts_upstream_of_or_within qualifier states this correctly. The regulatory role is captured more precisely by GO:0075307 (positive regulation of conidium formation), which is used in core_functions. Supporting Evidence: PMID:17993569 proper temporal and quantitative expression of FlbB is a prerequisite for brlA transcription, and misscheduled overexpression inhibits conidiation. |
| GO:0048315 conidium formation | IMP PMID:21423749 Elucidation of functional markers from Aspergillus nidulans ... | ACCEPT | Summary: Accepted. A C382A point mutant made in this study is fluffy, giving a separate mutational line of evidence. Reason: PMID:21423749 is mostly a bioinformatic analysis, but it includes a wet-lab test: a FlbB C382A strain was built and compared with the null and the flbB102 truncation, and it was fluffy with sparse conidiation. That is genuine IMP evidence from this paper, alongside the null phenotype it cites from earlier work. Supporting Evidence: PMID:21423749 flbB-C382A exhibited a fluffy phenotype with sparse conidiation thus confirming that C382 is critical for FlbB function in Anidu. PMID:21423749 Deletion of either protein results in a distinctive phenotype characterized by the formation of cottony colonies (βfluffyβ phenotype) with a broad delay in the timing of conidiation and a substantial reduction in the number of conidiophores with respect to the wild type strain |
| GO:0006357 regulation of transcription by RNA polymerase II | IMP PMID:21624998 Regulation of conidiation by light in Aspergillus nidulans. | MODIFY | Summary: Correct but general. FlbB is needed for light induction of brlA, which is a positive effect on Pol II transcription. Reason: The light study (abstract only in the cache) reports that light induction of brlA needs flbA-C, flbB included. The effect is activation, so the signed descendant GO:0045944, already on the gene, is the right refinement. The curator's experimental call is kept; only its specificity changes. Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:21624998 We have found that light induction of brlA required the photoreceptor complex composed of a phytochrome FphA, and the white-collar homologs LreA and LreB, and the fluffy genes flbA-C. |
| GO:0075307 positive regulation of conidium formation | IMP PMID:21423749 Elucidation of functional markers from Aspergillus nidulans ... | ACCEPT | Summary: Accepted as the core process. FlbB is a positive regulator of conidiation. Reason: This signed, process-specific term best describes FlbB's developmental role. Loss-of-function alleles, including the C382A point mutant made in this study, abolish or strongly delay conidiation. Supporting Evidence: PMID:21423749 flbB-C382A exhibited a fluffy phenotype with sparse conidiation thus confirming that C382 is critical for FlbB function in Anidu. |
| GO:0005634 nucleus | IDA PMID:20132447 The concerted action of bZip and cMyb transcription factors ... | ACCEPT | Summary: Accepted. Direct localization of FlbB in nuclei. Reason: The cached record is abstract-only, and the curator's IDA is accepted. Later work (PMID:31065746) shows that FlbB's nuclear accumulation depends on FlbD, which matches the concerted FlbB/FlbD action described in this paper. Supporting Evidence: PMID:31065746 FlbD controls the nuclear accumulation of FlbB through a cMyb domain and a C-terminal LxxLL motif. |
| GO:0001411 hyphal tip | IDA PMID:19656299 The bZIP-type transcription factor FlbB regulates distinct m... | ACCEPT | Summary: Accepted. FlbB localizes to the tips of growing metulae as well as vegetative hyphal tips. Reason: The cached abstract states the tip localization directly. The apical pool is a defining and functionally required feature of FlbB. Supporting Evidence: PMID:19656299 FlbB was located at the tip of growing metulae, following a similar pattern as described in vegetative hyphae. |
| GO:0003677 DNA binding | IDA PMID:19656299 The bZIP-type transcription factor FlbB regulates distinct m... | MODIFY | Summary: Correct but general. FlbB binds a cis-regulatory region (the brlA promoter), so refine to the cis-regulatory binding term the gene already carries. Reason: The cached record for PMID:19656299 is abstract-only, so the binding assay behind this IDA cannot be seen here, and the curator's call that FlbB binds DNA is accepted. The binding that has been characterized in A. nidulans is FlbB occupancy of the brlA promoter by ChIP. That supports the descendant GO:0000976 (transcription cis-regulatory region binding), which the gene already carries by IBA and IEA. Bare DNA binding is less informative than the evidence allows. Proposed replacements: transcription cis-regulatory region binding Supporting Evidence: PMID:31065746 Furthermore, chromatin immunoprecipitation assays showed that FlbB cannot bind brlA p in the absence of FlbD |
| GO:0005634 nucleus | IDA PMID:19656299 The bZIP-type transcription factor FlbB regulates distinct m... | ACCEPT | Summary: Accepted. Direct nuclear localization of FlbB. Reason: The record is abstract-only and the curator's IDA is accepted. It agrees with the other localization studies, which show FlbB in apical nuclei. |
| GO:0001411 hyphal tip | IDA PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | ACCEPT | Summary: Accepted. FlbB accumulates at the hyphal apex in vegetative hyphae. Reason: This is the first report of FlbB's apical pool. Later work showed that tip localization needs Cys382, the bZIP domain and FlbE, and that it is required for FlbB's transcriptional competence. Supporting Evidence: PMID:17993569 FlbB accumulates at the hyphal apex during early vegetative growth but is later found in apical nuclei, suggesting that an activating modification triggers nuclear import. PMID:26256571 Tip localization of FlbB depends on Cys382 in the C-terminus and the bZIP DNA-binding domain in the N-terminus. |
| GO:0001411 hyphal tip | IDA PMID:19007409 Aspergillus nidulans FlbE is an upstream developmental activ... | ACCEPT | Summary: Accepted. FlbB colocalizes with FlbE at the hyphal tip. Reason: The abstract reports colocalization of FlbB and FlbE at the hypha tip, which depends on the actin cytoskeleton. Supporting Evidence: PMID:19007409 Both UDAs are functionally interdependent, and colocalize at the hypha tip in an actin cytoskeleton-dependent manner. |
| GO:0001411 hyphal tip | IPI PMID:19007409 Aspergillus nidulans FlbE is an upstream developmental activ... | ACCEPT | Summary: Accepted. Tip localization tied to the in vivo FlbB-FlbE interaction (BiFC). Reason: IPI is an unusual code for a location, but the claim is correct. FlbB and FlbE interact in vivo (BiFC), and FlbE is required for FlbB to localize at the tip. Supporting Evidence: PMID:19007409 Moreover, bimolecular fluorescence studies show that they physically interact in vivo. PMID:26256571 Furthermore, the absence of FlbE results in loss of tip localization but not nuclear accumulation. |
| GO:0003700 DNA-binding transcription factor activity | ISM PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | MODIFY | Summary: Correct but general. Refine to the Pol II-specific activator term. Reason: The sequence-model call (bZIP domain) is sound. As with the other GO:0003700 rows, the activator and Pol II specificity are supported, so the refinement is GO:0001228, which the gene already carries. Proposed replacements: DNA-binding transcription activator activity, RNA polymerase II-specific Supporting Evidence: PMID:17993569 In this report we describe FlbB as a putative basic-zipper-type transcription factor restricted to filamentous fungi. |
| GO:0003700 DNA-binding transcription factor activity | IDA PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | MODIFY | Summary: Correct but general. Refine to the Pol II-specific activator term. Reason: FlbB is a transcription factor required for brlA expression, and later work shows that it activates flbD and brlA and occupies the brlA promoter. The curator's experimental call is kept. Its informative form is GO:0001228, already carried by IBA. Proposed replacements: DNA-binding transcription activator activity, RNA polymerase II-specific Supporting Evidence: PMID:17993569 proper temporal and quantitative expression of FlbB is a prerequisite for brlA transcription, and misscheduled overexpression inhibits conidiation PMID:20132447 Both factors then jointly activate brlA |
| GO:0005634 nucleus | IDA PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | ACCEPT | Summary: Accepted. FlbB is found in apical nuclei. Reason: This is the original report of the nuclear pool. Later work showed that FlbB reaches the nucleus by dynein-dependent retrograde transport from the tip. Supporting Evidence: PMID:17993569 FlbB accumulates at the hyphal apex during early vegetative growth but is later found in apical nuclei, suggesting that an activating modification triggers nuclear import. |
| GO:0006357 regulation of transcription by RNA polymerase II | ISM PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | MODIFY | Summary: Correct but general. Refine to positive regulation of transcription by RNA polymerase II. Reason: The bZIP architecture supports a role in Pol II transcription, and the experiments show that the role is activation. The refinement is GO:0045944, already on the gene. Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:17993569 In this report we describe FlbB as a putative basic-zipper-type transcription factor restricted to filamentous fungi. |
| GO:0006357 regulation of transcription by RNA polymerase II | IDA PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | MODIFY | Summary: Correct but general. FlbB is required for brlA transcription, so refine to positive regulation. Reason: With no brlA expression in the null mutant, the effect is positive. The signed descendant GO:0045944 is the supported refinement. Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:17993569 no brlA expression was detected in the flbB -null mutant, showing that FlbB is required, directly or indirectly, for proper brlA transcription regulation |
| GO:0075307 positive regulation of conidium formation | IMP PMID:17993569 Basic-zipper-type transcription factor FlbB controls asexual... | ACCEPT | Summary: Accepted as the core process. FlbB positively regulates conidium formation. Reason: flbB loss-of-function mutants fail to induce brlA and are aconidial. FlbB acts as an activator of the conidiation program upstream of brlA. Supporting Evidence: PMID:17993569 proper temporal and quantitative expression of FlbB is a prerequisite for brlA transcription, and misscheduled overexpression inhibits conidiation |
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Download this section (compressed HTML)Q: Which DNA sequence does FlbB recognize in the brlA and flbD promoters, and does the conserved H19 histidine in its basic region change its specificity from that of the canonical AP-1 (TGA[C/G]TCA) bZIPs?
Q: The conidiation role is established only in Aspergillus. The N. crassa ortholog NCU07379 (same FlbB clade) is reported to conidiate normally when deleted. Which parts of FlbB function (DNA binding, activation, tip localization) should be treated as conserved across the FlbB clade, and which as Eurotiales-specific?
Q: Does Cys382, which aligns with the redox-sensing cysteines of Yap1/Pap1, sense an oxidative signal at the tip, or is it needed only structurally for FlbE binding and tip retention?
Experiment: Run EMSA or DAP-seq with recombinant FlbB bZIP domain, alone and with FlbD, on the brlA and flbD promoters, and do genome-wide ChIP-seq of FlbB in wild-type and ΞflbD backgrounds. This would define direct targets and the binding motif, and test whether FlbD is needed for binding or only for nuclear retention.
Experiment: Express A. nidulans flbB and the N. crassa ortholog NCU07379 reciprocally in ΞflbB A. nidulans, and in N. crassa under conidiation-inducing conditions, to test whether the activator function is conserved even though the N. crassa knockout phenotype differs.
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