Regulation of immunoglobulin gene transcription in a teleost fish: identification, expression and functional properties of E2A in the channel catfish

The function of the transcriptional enhancer, Eμ3′, of the IgH locus of the channel catfish, Ictalurus punctatus, involves the interaction of E-protein and Oct family transcription factors. The E-proteins [class I basic helix–loop–helix (bHLH) family] are encoded in mammals by three genes: E2A (of which E12/E47 are alternatively spliced products), HEB, and E2-2. An E2A homologue has been identified in a catfish B-cell cDNA library and contains regions homologous to the bHLH and activation domains of mammalian and other vertebrate E2A proteins. E2A message is widely expressed, being readily detected in catfish B cells, T cells, kidney, spleen, brain, and muscle. Its expression is lower than that previously observed for TF12/CFEB, the catfish homologue of HEB. E2A strongly activated transcription of a μE5 motif-dependent construct in catfish B cells, and also activated transcription from the core region of the catfish IgH enhancer (Eμ3′) in a manner dependent on the presence of the μE5 site. Catfish E2A, expressed in vitro, bound the μE5 motif present in the core region of Eμ3′. These results document the conservation of structure and function in vertebrate E2A and suggest a potential role of E2A in driving expression of the IgH locus at the phylogenetic level of a teleost fish.