Binding of lac repressor headpiece to poly[d(A-T)]. A thermal denaturation study.

The binding of the lac repressor headpiece to poly[d(A-T)] has been investigated using thermal denaturation experiments. The binding of the headpiece leads to a stabilization of the double-stranded structure. Competition experiments using circular dichroism measurements confirm that the headpiece binds stronger on the double-stranded poly[d(A-T)] than on a single-stranded nucleic acid like poly(A). Theoretical analyses of the melting curves allow the determination of the binding constant and of the site size on the poly[d(A-T)]. This latter value is found to be 4 base pairs, in good agreement with that determined by other experimental approaches, and is much smaller than the values previously found for the lac repressor.