Zebrafish pou[c]: a divergent POU family gene ubiquitously expressed during embryogenesis.

We report the isolation and characterization of cDNA for a novel zebrafish (Brachyodanio rerio) POU domain gene, pou[c], which is ubiquitously expressed during embryonic development. This gene encodes a 610 amino acids long protein with a 149 amino acid POU domain ending only 8 residues before the C terminus. The 453 amino acids long region N-terminal to the POU domain contains several features typical of transcriptional activation domains such as an acidic region with a putative amphipathic alpha-helix, a glutamine-rich region, and short threonine- and/or serine-rich regions. Comparison of the POU domain of pou[c] to other known POU sequences clearly show that pou[c] has the most divergent POU domain sequence reported to date. Thus, we suggest that pou[c] should be placed as the presently sole member of a new, sixth class of POU proteins. DNA-binding studies revealed that pou[c] is not an octamer-binding transcription factor like the Oct proteins described from mammals, chicken and Xenopus. Rather, pou[c] binds with high affinity to the TAATGARAT motif found in the promoters of the herpes simplex virus immediate early genes and to degenerate octamer-TAATGA motifs. Circular permutation analyses also show that pou[c] induces DNA bending upon sequence-specific binding.