Mutant LexA Proteins with Specific Defects in Autodigestion

In self-processing biochemical reactions, a protein or RNA molecule specifically modifies its own structure. Many such reactions are regulated in response to the needs of the cell by an interaction with another effector molecule. In the system we study here, specific cleavage of the Escherichia coli LexA repressor, LexA cleaves itself in vitro at a slow rate, but in vivo cleavage requires interaction with an activated form of RecA protein. RecA acts indirectly as a coprotease to stimulate LexA autodigestion. We describe here a new class of lexA mutants, lexA (Adg-; for autodigestion-defective) mutants, termed Adg- for brevity. Adg- mutants specifically interfered with the ability of LexA to autodigest but left intact its ability to undergo RecA-mediated cleavage. The data are consistent with a conformational model in which RecA favors a reactive conformation capable of undergoing cleavage. To our knowledge, this is the first example of a mutation in a regulated self-processing reaction that impairs the rate of self-processing without markedly affecting the stimulated reaction. Had wild-type lexA carried such a substitution, discovery of its self-processing would have been difficult; we suggest that, in other systems, a slow rate of self-processing has prevented recognition that a reaction is of this nature.