An altered-specificity mutation in a human POU domain demonstrates functional analogy between the POU-specifc 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 (POU_S) and a C-terminal homeodomain (POU_HD) Surprisingly, POU_S 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 POU_s footprint of DNA contacts, identified by use of modified bases, is analogous to those of bacteriophage represser-operator complexes. Moreover, a loss-of-contact substitution in the putative POU8 recognition 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 POUs and prokaryotic helix-turn-helix elements and suggest that their DNA specificities may be governed by a shared set of rules.