Deletions within a hinge region of a specific DNA-binding protein.

Many proteins are organized as a set of compact functional domains connected by flexible, exposed segments of the polypeptide chain. To study one of these connector regions, we isolated a series of functional in-frame deletions in the central portion of a specific DNA-binding protein, the LexA repressor of Escherichia coli. These mutant proteins fell into two main classes: those with small deletions of two to eight amino acids functioned as repressor about as well as did wild type, while those with large deletions of 17-22 amino acids functioned well only at considerably higher concentrations. The mutant proteins were resistant to the specific cleavage reaction that triggers the SOS response. These data suggest that the conformation of the hinge region in LexA protein is important for cleavage. By contrast, the hinge plays a topological role in repressor function, connecting the two functional halves of the protein; in the SOS response, this function of the hinge is inactivated by cleavage, leading to inactivation of the repressor.