Genetic control of AAF-induced mutagenesis at alternating GC sequences: An additional role for RecA

Abstract

In a previous study, the forward mutation spectrum induced by the chemical carcinogen N-acetoxy-N-2-acetylaminofluorene was determined (Koffel-Schwartz et al. 1984). It was found that 90% of the induced mutations are frameshift mutations located within specific sequences (mutation hot spots). Two classes of mutation hot spots were found: (i) — 1 frameshift mutations occuring within runs of guanines (i.e. GGGG→GGG); (ii) — 2 frameshift mutations occurring within the NarI recognition sequence (GGCGCC→GGCC). In the present work, we further investigate the genetic requirements of these frameshift events by using specific reversion assays. Like UV-induced mutagenesis, frameshift mutations occurring within runs of G's (also referred to as the “slippage pathway”) require the activated form of the RecA protein (RecA^*). On the other hand, frameshift mutations occuring at the NarI site (the “NarI mutation pathway”) require a LexA-controlled function(s) that is not UmuDC. The LexA-controlled gene(s) that is (are) involved in this pathway remain to be identified. Moreover, this pathway does not require RecA^* for the proteolytic processing of a protein other than LexA (like the cleavage of UmuD in UV-induced mutagenesis). An “additional” role of RecA can be defined as follows: The non-activated form of the RecA protein acts as an inhibitor in the NarI mutation pathway. This inhibition is relieved upon activation of RecA by UV irradiation of the bacteria. A recA deletion mutant is totally proficient in the NarI mutation pathway provided the SOS system is derepressed [lexA (Def) allele]. Therefore, RecA does not actively participate in the fixation of the mutation. A molecular model for this “additional” role of RecA is proposed.