Figure 7-8: The bacterial SOS system and its relation to induction of lytic development. DNA damage causes the disintegration of some DNA to smaller degradation products, of which single stranded DNA is especially effective. ssDNA activates the RecA protein to a form RecA* whose coprotease activity accelerates the spontaneous cleavage of the LexA protein into two inactive polypeptides. The normal function of LexA is to repress (either directly or indirectly) the transcription of damage-inducible (din) genes of the host, many of which function in DNA repair. The repressor proteins of many phages (including lambda) mimic LexA, and their cleavage is likewise stimulated, inducing lytic development.