AN ALTERED-SPECIFICITY MUTATION IN A HUMAN POU DOMAIN DEMONSTRATES FUNCTIONAL ANALOGY BETWEEN THE POU-SPECIFIC SUBDOMAIN AND PHAGE-LAMBDA REPRESSOR

The POU motif, conserved among a family of eukaryotic transcription factors, contains two DNA-binding domains: an N-terminal POU specific domain (POUs) and a C-terminal homeodomain (POUHD) Surprisingly, POUs is similar in structure to the helix-turn-helix domains of bacteriophage repressor and Cro proteins. Such similarity predicts a common mechanism of DNA recognition. To test this prediction, we have studied the DNA-binding properties of the human Oct-2 POU domain by combined application of chemical synthesis and site-directed mutagenesis. The POUs footprint of DNA contacts, identified by use of modified bases, is analogous to those of bacteriophage repressor-operator complexes. Moreover, a loss-of-contact substitution in the putative POUs recognition alpha-helix leads to relaxed specificity at one position in the DNA target site. The implied side chain-base contact is identical to that of bacteriophage repressor and Cro proteins. These results establish a functional analogy between the POUs and prokaryotic helix-turn-helix elements and suggest that their DNA specificities may be governed by a shared set of rules.