TRANSCRIPTIONAL ACTIVATION BY OCT-3 - EVIDENCE FOR A SPECIFIC ROLE OFTHE POU-SPECIFIC DOMAIN IN MEDIATING FUNCTIONAL INTERACTION WITH OCT-1

Oct-3, a member of the POU family of transcription factors, is express ed in pluripotent cells of early mammalian embryos and in undifferenti ated embryonal carcinoma cell lines. Using a variety of Oct-3 mutants, we have identified two different domains of Oct-3 which activate tran scription in transfected mammalian cells. One of these domains, locate d in the C-terminal part of the protein, plays a major role in transcr iptional activation when Oct-3 is bound to its cognate site, the octam er motif. An Oct-3 mutant containing a single amino acid substitution in the POU homeodomain is unable to bind the octamer target in vitro, yet is still able to activate transcription in an octamer-dependent ma nner. We provide evidence that transactivation by this mutant involves protein-protein interactions with the ubiquitous octamer binding fact or Oct-1. This interaction requires the POU-specific domain of Oct-3 a nd allows recruitment of Oct-3 to the target promoter even in the abse nce of Oct-3 DNA binding.