Contacts between the LexA repressor--or its DNA-binding domain--and the backbone of the recA operator DNA.

Using hydroxyl radical footprinting and ethylation interference experiments, we have determined the backbone contacts made by the entire LexA repressor and its amino-terminal fragment with the recA operator DNA. These techniques reveal essentially the same contacts between both proteins and one side of the DNA helix if one assumes that the DNA stays in the normal B-conformation. This result is somewhat unexpected because protection of guanine bases against methylation suggested a somewhat twisted recognition surface. The backbone contacts revealed by both methods are symmetrically disposed with respect to the center of the operator, providing further evidence that the operator binds two LexA monomers. Each half-operator contains seven interfering phosphates. These phosphates are found on both sides of the 5'-CTGT sequence that is believed to be the principal recognition target. On the side close to the center of the operator are found two phosphates, whereas the other five are clustered on the side apart from the dyad axis. We are not aware of such an extended cluster of interfering phosphates for any other DNA-binding protein. A quantification of the hydroxyl radical footprints allowed us to compare further the affinity of the LexA repressor for the recA operator with that of its isolated DNA binding domain. We find an only 13-fold higher binding constant for LexA than for its amino-terminal domain, which is in good agreement with our earlier results for the uvrA operator using a completely different binding assay.