Sp-Brn1/2/4 is the single class III POU-domain (Brn1/Brn2/Brn4/Oct6-related) homeodomain transcription factor of the purple sea urchin, with a POU-specific domain and a homeodomain that together confer sequence-specific DNA binding. It was cloned as the predicted gut-specific regulator of the endoderm differentiation gene endo16: zygotic brn1/2/4 transcription begins at low level in the 20-h blastula endoderm, rises abruptly about ten-fold as gastrulation starts, and is confined to the gut endoderm (described as midgut by Yuh et al. and as foregut by later fluorescent in situ hybridization), coincident with endo16. In the endoderm gene regulatory network it is activated in veg2-derived anterior endoderm cells by Blimp1 in the last hours before gastrulation, downstream of the post-gastrular gut specification genes, and it drives the late, high-rate phase of endo16 transcription through endo16 cis-regulatory Module B; blocking its translation abolishes Module B-driven expression while leaving Module A intact. Predicted Brn1/2/4 sites also occur in the midgut/hindgut cis-regulatory module of blimp1/krox1a. Independently of its gut role, Brn1/2/4 is expressed in scattered ectodermal neural precursors under SoxC control, is required for the differentiation of serotonergic and all synaptotagmin B-expressing neurons, and at low dose its misexpression produces excess neurons, placing it as a neurogenic regulator downstream of SoxC, a role shared with its vertebrate class III POU relatives.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000978 RNA polymerase II cis-regulatory region sequence-specific DNA binding | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of cis-regulatory sequence-specific DNA binding from the POU-domain family node (mouse Pou3f1-4 and Pou2f1, human POU2F1/POU3F2/POU3F3/POU3F4, fly ventral veins lacking). Sea urchin Brn1/2/4 acts through a defined cis-regulatory module of an RNA polymerase II gene, endo16 Module B, which contains its predicted binding site, and Brn1/2/4 sites are present in the blimp1/krox1a gut module. Reason: POU-specific domain plus homeodomain binding is family-wide and the target-specific morpholino result shows a module-specific requirement in endo16, i.e. the factor acts on a cis-regulatory region of an RNA polymerase II-transcribed gene. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. PMID:16798107 Its sequence contains binding sites for Brn1/2/4, Otx, and Blimp1/Krox itself, as predicted in a prior regulatory network analysis. |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of RNA polymerase II-specific DNA-binding transcription factor activity for the POU-domain family node. This is the core molecular function of Sp-Brn1/2/4, a POU-domain regulatory gene of the endoderm GRN whose knockdown abolishes expression driven by a specific cis-regulatory module of endo16 and which is also required in the neurogenic regulatory pathway. Reason: Directly supported on the target by morpholino knockdown and by its placement as a Blimp1-activated regulatory gene in the endoderm GRN. Supporting Evidence: PMID:15893979 The cDNA sequence reveals it to be a POU domain factor related closely to the mammalian factors Brain-1, -2, and -4. PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. PMID:21623371 Blimp1 then activates brn1/2/4 and tgif expression (Fig. 1f). |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the homeobox conserved site (IPR017970). Same term as the IBA row above and correct for this POU homeodomain transcription factor. Reason: Correct term, independently supported by the target-specific endo16 Module B knockdown result; redundant with, but consistent with, the PAINT row. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. |
| GO:0003677 DNA binding | IEA GO_REF:0000120 | MODIFY | Summary: Generic DNA binding from the homeodomain and POU-domain InterPro entries and the UniRule POU rule. Correct but uninformative; the cis-regulatory sequence-specific binding term already carried by the IBA row is the informative one. Reason: Too general. Brn1/2/4 binds predicted sites in cis-regulatory modules of RNA polymerase II genes (endo16 Module B, blimp1/krox1a 1a module), so GO:0000978 should be used. Proposed replacements: RNA polymerase II cis-regulatory region sequence-specific DNA binding Supporting Evidence: PMID:16798107 Its sequence contains binding sites for Brn1/2/4, Otx, and Blimp1/Krox itself, as predicted in a prior regulatory network analysis. |
| GO:0003700 DNA-binding transcription factor activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping of the POU domain (IPR000327) to generic DNA-binding transcription factor activity. Correct at the family level; the RNA polymerase II-specific child is the standard term for a metazoan developmental transcription factor and is already present. Reason: Sound but less specific than GO:0000981, which the IBA and InterPro rows already assert and which the target-specific evidence supports. Proposed replacements: DNA-binding transcription factor activity, RNA polymerase II-specific Supporting Evidence: PMID:15893979 The cDNA sequence reveals it to be a POU domain factor related closely to the mammalian factors Brain-1, -2, and -4. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Nuclear localization inferred from the homeobox/POU UniRule and ARBA rules. No direct localization data in sea urchin, but a POU homeodomain transcription factor that regulates endo16 through a cis-regulatory module necessarily acts in the nucleus. Reason: Correct cellular component for a sequence-specific transcription factor; consistent with its function as a cis-regulatory activator. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. |
| GO:0006351 DNA-templated transcription | IEA GO_REF:0000104 | MODIFY | Summary: UniRule transfer of the generic transcription process term. A sequence-specific transcription factor regulates transcription rather than carrying it out, and the regulatory term is already present on the IBA row. Reason: The process a transcription factor participates in is regulation of transcription by RNA polymerase II (GO:0006357), not the transcription reaction itself; the sign-specific child GO:0045944 is supported by the endo16 knockdown result. Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000120 | MODIFY | Summary: InterPro/UniRule mapping to generic regulation of transcription. Correct but the RNA polymerase II-specific term (already carried by the IBA row) and the positive-regulation child are better supported for this activator of endo16. Reason: Less specific than GO:0006357, which the PAINT row already asserts; the knockdown evidence shows positive regulation of an RNA polymerase II gene (endo16). Proposed replacements: positive regulation of transcription by RNA polymerase II Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. |
| GO:0006357 regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of regulation of transcription by RNA polymerase II from the POU-domain family node. Supported on the target by the endo16 knockdown result and by the gene's placement as a Blimp1-activated node of the veg2 endoderm GRN. Reason: Correct process for a sequence-specific transcription factor with demonstrated target-gene regulation in the sea urchin embryo. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. PMID:21623371 A few additional regulatory genes are activated in the final hours before gastrulation: brn1/2/4, gatae, dac and tgif are expressed in veg2-derived cells and hnf1 is expressed in the veg1 endoderm domain by 27 h after fertilization. |
| GO:0090575 RNA polymerase II transcription regulator complex | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference that the protein is part of an RNA polymerase II transcription regulator complex, seeded by mouse and human POU factors (including Oct-1, which forms well-characterized complexes with cofactors) and worm unc-86. POU factors can act as monomers, homodimers or with partners; no sea urchin data address whether Brn1/2/4 acts within a defined complex. Reason: Phylogenetically reasonable and not contradicted, but there is no target-specific evidence for a complex, and the informative description of Brn1/2/4 function is its sequence-specific transcription factor activity rather than complex membership. |
| GO:0001228 DNA-binding transcription activator activity, RNA polymerase II-specific | IMP PMID:15893979 Brn1/2/4, the predicted midgut regulator of the endo16 gene ... | NEW | Summary: Brn1/2/4 is a transcriptional activator. Morpholino arrest of its translation blocks the late, high-rate phase of endo16 expression and specifically abolishes the output of endo16 cis-regulatory Module B, the module whose dissection had predicted a midgut-specific activator, while Module A is unaffected; the kinetics show the late endo16 rise follows the rise of brn1/2/4 transcripts. Brn1/2/4 binding sites are also present in the gut cis-regulatory module of blimp1/krox1a. Reason: Module-specific loss of a positive output upon knockdown of a POU-domain DNA-binding factor whose site is in that module establishes the sign (activation) and the RNA polymerase II target; this is the activator variant of the GO:0000981 activity already carried by the IBA rows. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. PMID:15893979 Measured in the same embryos, the late rise in prevalence of endo16 transcripts follows that of brn1/2/4 transcripts. PMID:16798107 Its sequence contains binding sites for Brn1/2/4, Otx, and Blimp1/Krox itself, as predicted in a prior regulatory network analysis. |
| GO:0045944 positive regulation of transcription by RNA polymerase II | IMP PMID:15893979 Brn1/2/4, the predicted midgut regulator of the endo16 gene ... | NEW | Summary: Brn1/2/4 positively regulates transcription of the endoderm differentiation gene endo16 in the gut. Knockdown abolishes the late phase of endo16 transcription and the output of cis-regulatory Module B; each endo16 gene produces about one mRNA every two minutes once brn1/2/4 transcripts near their plateau. Reason: The gene product itself performs the regulatory step (it is the module B activator of an RNA polymerase II gene), so the positive-regulation process term is appropriate; this is the sign-specific child of the GO:0006357 IBA row. Supporting Evidence: PMID:15893979 Arrest of Brn1/2/4 translation by MASO treatment blocks the late phase of endo16 expression and specifically abolishes expression of cis-regulatory Module B of endo16, while not affecting Module A, also as predicted. PMID:15893979 The kinetics of accumulation of these transcripts indicates that the switch into the late phase of endo16 expression occurs when the brn1/2/4 transcript level nears its plateau (2000 molecules mRNA per embryo), after which each endo16 gene produces about 1 mRNA every 2 min (about 380 molecules mRNA per min in the whole embryo). |
| GO:0048565 digestive tract development | IMP PMID:15893979 Brn1/2/4, the predicted midgut regulator of the endo16 gene ... | NEW | Summary: Brn1/2/4 is a gut-endoderm regulatory gene activated in veg2-derived anterior endoderm by Blimp1 in the last hours before gastrulation, downstream of the genes executing post-gastrular specification of the midgut, and it drives the gut differentiation gene endo16. Its endodermal expression is gut-restricted (midgut per Yuh et al.; foregut per later FISH), so the broad digestive tract development term is used rather than a midgut- or foregut-specific one. Reason: The transcription factor performs a regulatory step of gut differentiation (activation of endo16) rather than being merely required for it, and its expression and network inputs are confined to the forming gut; the organ-level term is chosen because the cached literature disagrees on the gut subregion. Supporting Evidence: PMID:15893979 The brn1/2/4 gene lies downstream of the regulatory genes executing post-gastrular specification of the midgut, as shown by further gene expression perturbation experiments which provide an initial glimpse of the underlying network architecture. PMID:15893979 As predicted by the endo16 model, brn1/2/4 expression is confined perfectly to the midgut, coincident with the domain of endo16 expression. PMID:21623371 Blimp1 then activates brn1/2/4 and tgif expression (Fig. 1f). PMID:19250980 Endodermal expression of this gene is restricted to the foregut. |
| GO:0045666 positive regulation of neuron differentiation | IMP PMID:26657764 Neurogenic gene regulatory pathways in the sea urchin embryo... | NEW | Summary: Independently of its endodermal role, Brn1/2/4 acts downstream of SoxC in the neurogenic regulatory pathway. It is expressed in scattered ectodermal neural precursors in a SoxC-dependent manner (foregut expression is SoxC-independent), morpholino knockdown prevents the development of serotonergic and all synaptotagmin B-expressing neurons, and low-dose misexpression produces excess serotonergic neurons. This is the role of its vertebrate class III POU relatives (Brn1, Brn2, Brn4). Reason: Both loss-of-function (no differentiated neurons) and gain-of-function (excess neurons at low dose) for a transcription factor that sits in the neurogenic pathway between SoxC and terminal differentiation genes support a positive regulatory role in neuron differentiation. Non-endodermal and therefore a second, separate core function. Supporting Evidence: PMID:26657764 Using a combination of molecular screens and tests of gene function by morpholino-mediated knockdown, we identified SoxC and Brn1/2/4 , which function sequentially in the neurogenic regulatory pathway and are also required for the differentiation of all neurons. PMID:26657764 We conclude that Brn1/2/4 is required for the development of serotonergic and all SynB -expressing neurons and can directly or indirectly drive the production of excess neurons at low dose PMID:26657764 in SoxC morphants, labeling of individual ectoderm cells for Brn1/2/4 transcripts was dramatically reduced, whereas expression in the foregut was not detectably affected |
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Download this section (compressed HTML)Q: Is the endodermal expression domain of brn1/2/4 the midgut (Yuh et al. 2005, WMISH and endo16 coincidence) or the foregut (Cole and Arnone 2009, FISH)? The answer decides whether a midgut development or foregut-specific term should replace the organ-level digestive tract development annotation.
Q: Which endoderm GRN genes does Brn1/2/4 regulate besides endo16? Perturbation data in Peter and Davidson 2011 (Fig. 4 and supplementary tables) may define outputs such as gatae or foxa, but the cached text does not state them, so no such linkages were annotated.
Q: Is the Brn1/2/4 input into endo16 Module B direct (site mutation in Module B) or mediated by another Module B factor? The morpholino result is module-specific, consistent with the predicted site, but a Module B site mutation was not quotable from the abstract.
Experiment: Mutate the predicted Brn1/2/4 site in endo16 Module B in a GFP reporter and compare with the Brn1/2/4 morpholino phenotype to show direct activation.
Type: cis-regulatory analysis
Experiment: Double FISH of brn1/2/4 with foregut (foxa) and midgut (endo16, blimp1/krox1a) markers at gastrula and prism stages to settle the midgut versus foregut discrepancy.
Type: expression analysis
Experiment: Brn1/2/4 morpholino followed by NanoString or RNA-seq of the endoderm regulatory gene set at 27-48 h to define its outputs in the gut GRN and separate them from its neural targets.
Type: perturbation transcriptomics
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