Involvement of umuDCST genes in nitropyrene-induced -CG frameshift mutagenesis at the repetitive CG sequence in the hisD3052 allele of Salmonella typhimurium

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons, umuDC _ST on the chromosome and samAB on a 60-MDa cryptic plasmid. The roles of theumuDC-like operons in chemically induced frameshift mutagenesis of the hisD3052 allele of S. typhimurium were investigated. Introduction of a pBR322-derived plasmid carrying umuDCST increased the rate of reversion of hisD3052, following treatment with 1-nitropyrene (1-NP) or 1,8-dinitropyrene (1,-8DNP) tenfold and fivefold, respectively, whereas it did not substantially increase the rate of reversion induced by other frameshift mutagens, i.e. 2-nitrofluorene (2NF) and 2-amino- 3-methyldipyrido1,2-a:3 ,2-d]imi-dazole (Glu-P-1). Introduction of a pBR322-derived plasmid carrying samAB did not increase the incidence of reversion of hisD3052 observed with any of the mutagens examined. Deletion of umuDC _STSubstantially lowered the reversion rate induced by l-NP or 1,8-DNP, but it did not affect reversion induced by 2-NF, Glu-P-1 or N-hydroxyacetylaminofluorene (N-OH-AAF). Deletion of samAB had little impact on reversion incidence induced by any of the five frameshift mutagens. DNA amplification using the polymerase chain reaction technique followed by restriction enzyme analysis using BssHII, suggested that the mutations induced by the five frameshift mutagens were all CG deletions at the CGCGCGCG sequence in hisD3052. These results suggest that umuDCST, but not samAB, is involved in the -2 frameshift mutagenesis induced by l-NP and 1,8-DNP at the repetitive CG sequence, whereas neither operon participates in induction of the same type of mutations by 2-NF, Glu-P-1 or N-OH-AAF.