Cooperative interaction of oestrogen receptor 'zinc finger' domain polypeptides on DNA binding

The consensus oestrogen response element (ERE) contains two inverted copies of an AGGTCA consensus hexameric half-site, spaced by three base pairs. It differs from many other hormone response elements, such as consensus thyroid (TREp) and retinoic acid (DR-5 RARE) response elements, only in the relative spacing and orientation of these sequences. In the present study we report values for cooperativity (omega) of an oestrogen receptor DNA-binding domain polypeptide upon binding to these sequences. The polypeptide binds with negative cooperativity, or without cooperativity to retinoic acid and thyroid response elements respectively, but with high cooperativity to the ERE. We have also examined cooperativity upon binding of the polypeptide to an ERE variant. Since naturally occurring EREs commonly contain one hexamer which is considerably more degenerate than the other, we designed a hybrid response element in which one hexamer is a consensus ERE, while specific mutations were introduced into the other. We chose to mutate the second half-site to a glucocorticoid response element (GRE) half-site sequence (AGAACA), since normally no binding of the DNA-binding domain polypeptide to a GRE hexamer alone can be detected. In the hybrid response element, however, the GRE half-site is recognized with relatively high affinity, although binding to this sequence is dependent on the previous binding of a polypeptide to the ERE hexamer. Thus, cooperative interactions are capable of mediating the recognition of ERE sequence degeneracy. The ability of protein-protein interactions to mediate recognition of DNA sequence degeneracy may also have implications for transcription factors in general.