Oct-1 POU domain-DNA interactions: cooperative binding of isolated subdomains and effects of covalent linkage.

Structural and biochemical studies of Oct-1 POU domain-DNA interactions have raised important questions about cooperativity and the role of the linker connecting the POU-specific domain and the POU homeo domain. To analyze these interactions, we have studied binding of the isolated domains. Surprisingly, we find that two unlinked polypeptides corresponding to the POU-specific domain and the POU homeo domain bind cooperatively to the octamer site and have a coupling energy of 1.6 kcal/mole. We suggest that overlapping DNA contacts near the center of the octamer site may be the source of this cooperativity, as there are no protein-protein contacts between the domains in the crystal structure of the Oct-1 POU domain-DNA complex. These studies also have allowed us to describe the thermodynamic contribution of the linker (present in the intact POU domain) in terms of an effective concentration (3.6 mM). The broader implications for understanding cooperativity in protein-DNA recognition and gene regulation are discussed.