The DNA binding specificity of the bipartite POU domain and its subdomains.

The POU domain is a conserved DNA binding region of approximately 160 amino acids present in a family of eukaryotic transcription factors that play regulatory roles in development. The POU domain consists of two subdomains, the POU-specific (POUS) domain and a POU-type homeodomain (POUHD). We show here that, like the POUHD, the Oct-1 POUS domain can bind autonomously to DNA but with low affinity. DNA binding studies and in vitro binding site selection revealed that the POU subdomains each have a different sequence specificity. The binding consensus of the POUS domain [gAATAT(G/T)CA] and POUHD (RTAATNA) respectively overlap the 'left half' and right half' of the POU domain recognition sequence [a(a/t)TATGC(A/T) AAT(t/a)t]. In addition to the core sequence, which is very similar to the octamer motif (ATGCAAAT), the flanking bases make a significant contribution to the binding affinity of the POU domain. Interestingly, at some positions the sequence preferences of the isolated POU subdomains are distinct from those of the POU domain, suggesting that the POU domain binding site is more than a simple juxtaposition of the POUS and POUHD target sequences. In addition, analysis of the binding kinetics of the POU domain and POUHD indicates that the POUS domain enhances the binding affinity by reducing the dissociation rate. Our results show that the POU domain proteins have DNA binding properties distinct from those of classic homeodomain proteins. We suggest a model for the way in which an additional conserved domain adds further specificity to DNA recognition by homeodomain proteins.

POU proteins bend DNA via the POU-specific domain.

POU proteins constitute a family of ubiquitous as well as cell type-specific transcription factors that share the conserved POU DNA binding domain. This domain consists of two distinct subdomains, a POU-specific domain and a POU homeodomain, that are both required for high affinity sequence-specific DNA binding. In a circular permutation assay, several POU proteins, including Oct-1, Oct-2A, Oct-6 and Pit-1, demonstrated a position dependent mobility of the protein-DNA complexes, suggesting induction of DNA bending. This was confirmed by detection of relative bend direction, using pre-bent DNA, and by enhanced ligase mediated cyclization. Bending was caused by interaction with the POU domain. By contrast, binding of the POU homeodomain did not distort the DNA structure, indicating that the POU-specific domain confers DNA bending.