Different interactions of Cro repressor dimer with the left and right halves of OR3 operator DNA.

lambda Cro repressor protein is titrated with two half-operator DNA duplexes comprising the right and left halves of the major binding site on phage lambda DNA, the OR3 operator. The comparison of binding strengths and the conformation of Cro repressor in the two protein-DNA complexes shows that base pair differences between the two halves of the OR3 operator affect the binding of Cro repressor protein. Some 1H NMR resonances are assigned for both protein and DNA in the Cro-operator DNA complexes which are then used to highlight differences between Cro right half and Cro left half protein-operator DNA interactions. These differences are compared to the asymmetry found in the lambda C1 repressor-operator DNA complex. Mechanisms for the recognition of the Cro transcriptional regulatory protein have considered only interactions between a single Cro monomer and a consensus half-operator site with the assumption that the interactions in the remaining half-site are related by the 2-fold symmetry of the complex. A revised model is suggested that allows asymmetry in the two halves of the protein-DNA complex. Methods are proposed to avoid problems in the general use of 1H NMR spectroscopy to study protein-DNA interaction such as intermediate exchange behavior and sample aggregation.