Roles of the mutagenesis proteins SamA'B and MucA'B in chemically induced frameshift mutagenesis in Salmonella typhimurium hisD3052.

The mutagenesis induced by ultraviolet light and many chemicals in Escherichia coli is largely dependent upon the proteins encoded by the umuDC operon and their analogs. In Salmonella typhimurium, there are two sets of umuDC-like operons: the umuDC(ST) operon in the chromosome and the samAB operon located on the 60-MDa cryptic plasmid. The former operon, but not the latter, confers UV mutability on S. typhimurium. Nevertheless, the samAB operon, when carried on high-copy-number plasmids, can efficiently promote UV mutagenesis. In order to characterize the function of samAB in greater detail, we have compared the abilities of MucA'B and a putative activated form of SamAB, i.e. SamA'B, to promote chemically induced frameshift mutagenesis in S. typhimurium hisD3052. MucAB is an activated form of the products of mucAB, which is the most potent umuDC analog characterized so far. We have used four plasmids, each carrying samA', samB, mucA' or mucB with a lac promoter instead of their own promoters. The results indicated that under the conditions of elevated expression, SamA'B can promote chemically induced frameshift mutagenesis by furylfuramide, aflatoxin B1, 1-nitropyrene, and 1,8-dinitropyrene, with efficiencies comparable to, or even better than, MucA'B. Increase of the levels of expression enhanced the ability of SamA'B to promote the mutagenesis, while it decreased that of MucA'B. Surprisingly, the elevated expression of MucB alone significantly enhanced the frameshift mutagenesis induced by 1-nitropyrene and 1,8-dinitropyrene, whereas the elevated expression of SamB, MucA' and SamA' did not enhance it. These results suggest that the abilities of SamA'B and MucA'B to promote mutagenesis strongly depend on their levels of expression. The possible roles of these mutagenesis proteins in chemically induced frameshift mutagenesis are discussed.