LEXA AND LAMBDA-CI-REPRESSORS AS ENZYMES - SPECIFIC CLEAVAGE IN AN INTERMOLECULAR REACTION

During the SOS response, LexA repressor is inactivated. by specific cleavage. Although cleavage requires RecA protein in vivo, RecA acts indirectly as a coprotease by stimulating an inherent self-cleavage activity of LexA. In lambda lysogens, cleavage of lambda CI repressor in a similar but far slower reaction results in prophage induction. We describe an intermolecular cleavage reaction in which the C-terminal fragment of LexA acted as an enzyme to cleave other molecules of LexA. The C-terminal fragment of lambda repressor cleaved the LexA substrates about as efficiently as did the LexA enzyme, suggesting that the slow rate of CI self-cleavage results from a weak interaction between its cleavage site and the active site.