Twin of I-POU: a two amino acid difference in the I-POU homeodomain distinguishes an activator from an inhibitor of transcription.

I-POU, a POU domain nuclear protein that lacks two conserved basic amino acids of the POU homeodomain is coexpressed in the developing Drosophila nervous system with a second POU domain transcription factor, Cf1-a. I-POU does not bind to DNA but forms a POU domain-mediated, high affinity heterodimer with Cf1-a, inhibiting its ability to bind and activate the dopa decarboxylase gene. The I-POU/Cf1-a dimerization interface encompasses only the N-terminal basic region and helices 1 and 2 of the POU homeodomains with precise amino acid and alpha-helical requirements. twin of I-POU, an alternatively spliced transcript of the I-POU gene, encodes a protein containing the two basic amino acid residues absent in I-POU. Twin of I-POU is incapable of dimerizing with Cf1-a, but can act as a positive transcription factor on targets distinct from those regulated by Cf1-a. These findings suggest that the I-POU genomic locus simultaneously generates both a specific activator and inhibitor of gene transcription, capable of modulating two distinct regulatory programs during neural development.