INDUCTION OF ONLY ONE SOS OPERON, UMUDC, IS REQUIRED FOR SOS MUTAGENESIS IN ESCHERICHIA-COLI

The actions of UmuDC and RecA proteins, respectively in SOS mutagenesi s are studied here with the following experimental strategy. We used l exAl (Ind-) bacteria to maintain all SOS proteins at their basal conce ntrations and then selectively increased the concentration of either U muDC or RecA protein. For this purpose, we isolated operator-constitut ive mutations o(c) in the umuDC and umuDC operons and also used the o9 8c-recA mutation. The o1c-umuDC mutation prevents LexA repressor from binding to the operator and improves the Pribnow box consensus sequenc e. As a result, 5000 UmuD and 500 UmuC molecules per cell were produce d in lexAl bacteria. This concentration is sufficient to restore SOS m utagenesis. The level of RecA protein present in the repressed state p romoted full UmuD cleavage. Overproduction of RecA alone did not promo te SOS mutagenesis. Increasing the level of RecA in the presence of hi gh concentrations of UmuDC proteins has no further effect on SOS mutge nesis. We conclude that, after DNA damage, umuDC is the only SOS opero n that must be induced in Escherichia coli to promote SOS mutagenesis.