Mechanism of specific LexA cleavage: autodigestion and the role of RecA coprotease.

Specific LexA cleavage can occur under two different conditions: RecA-mediated cleavage requires an activated form of RecA, while an intramolecular self-cleavage termed autodigestion proceeds spontaneously at high pH and does not involve RecA. The two cleavage reactions are closely related. We postulate that RecA stimulates autodigestion rather than acting as a typical protease, and it is proposed to term this activity 'RecA coprotease' to emphasize this indirect role. The mechanism of autodigestion is similar to that of a serine protease, and RecA appears to act by reducing the pKa of a critical lysine residue LexA. A new class of mutants, termed lexA (IndS), is described; these mutations increase the rate of LexA cleavage.