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

During the SOS response, LexA repressor is inactivated. by specific cl eavage. Although cleavage requires RecA protein in vivo, RecA acts ind irectly as a coprotease by stimulating an inherent self-cleavage activ ity of LexA. In lambda lysogens, cleavage of lambda CI repressor in a similar but far slower reaction results in prophage induction. We desc ribe an intermolecular cleavage reaction in which the C-terminal fragm ent of LexA acted as an enzyme to cleave other molecules of LexA. The C-terminal fragment of lambda repressor cleaved the LexA substrates ab out as efficiently as did the LexA enzyme, suggesting that the slow ra te of CI self-cleavage results from a weak interaction between its cle avage site and the active site.