An Altered-Specificity Mutation in a Human POU Domain Demonstrates Functional Analogy Between the POU-Specific Subdomain and Phage λ 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, POUSis 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 POUSfoot-print 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 POUSrecognition α-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 POUSand prokaryotic helix-turn-helix elements and suggest that their DNA specificities may be governed by a shared set of rules.