Salmonella typhimurium has two homologous but different umuDC operons: cloning of a new umuDC-like operon (samAB) present in a 60-megadalton cryptic plasmid of S. typhimurium

Abstract

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The DNA which can restore UV mutability to a umuD44 strain and to a umuC122::Tn5 strain of E. coli has been cloned from Salmonella typhimurium TA1538. DNA sequence analysis indicated that the cloned DNA potentially encoded proteins with calculated molecular weights of 15,523 and 47,726 and was an analog of the E. coli umuDC operon. We have termed this cloned DNA the samAB (for Salmonella mutagenesis) operon and tentatively referred to the umuDC operon of S. typhimurium LT2 (C. M. Smith, W. H. Koch, S. B. Franklin, P. L. Foster, T. A. Cebula, and E. Eisenstadt, J. Bacteriol. 172:4964-4978, 1990; S. M. Thomas, H. M.Crowne, S. C. Pidsley, and S. G. Sedgwick, J. Bacteriol. 172:4979-4987, 1990) as the umuDCST operon. The samAB operon is 40% diverged from the umuDCST operon at the nucleotide level. Among five umuDC-like operons so far sequenced, i.e., the samAB, umuDCST, mucAB, impAB, and E. coli umuDC operons, the samAB operon shows the highest similarity to the impAB operon of TP110 plasmid while the umuDCST operon shows the highest similarity to the E. coli umuDC operon. Southern hybridization experiments indicated that (i) S. typhimurium LT2 and TA1538 had both the samAB and the umuDCST operons and (ii) the samAB operon was located in a 60-MDa cryptic plasmid. The umuDCsT operon is present in the chromosome. The presence of the two homologous but different umuDC operons may be involved in the poor mutability of S. typhimurium by UV and chemical mutagens.

Urinary tract infection drives genome instability in uropathogenicEscherichia coliand necessitates translesion synthesis DNA polymerase IV for virulence

“…In Salmonella typhimurium, a plasmid-borne homolog of Pol V called SamAB has been described. 47 A BLAST analysis of the UTI89 genome for Pol V homologs suggested such a factor, annotated as encoded by the open reading frames impCA, may be present on the megaplasmid. We did not find any in vitro and in vivo genetic evidence for an active role of Pol V or ImpC/ImpA proteins in pathogenicity and mutability of UPEC UTI89 strain.…”

Mutagenic DNA repair in enterobacteria

“…Furthermore, introduction of E. coli recA into S. typhimiariiam did not increase induced mutability, suggesting that poor mutagenesis was not based on a limitation of S. typhimurium RecA action (unpublished observation). Different mutagenic potencies are also seen when cloned examples of uimu-like genes are introduced into E. coli (4,30,37) or when miucAB enhanced the low levels of mutagenesis of many of the species examined here (35). Thus, lama operons from different sources may have different mutagenic potencies inherent in their sequences.…”

“…Nevertheless, mutagenesis relying on SOS induction was demonstrated in S. typhimurium (31), and it was ultimately shown that S. typhimurium does indeed have a chromosomal umuDC operon despite all the previous indications to the contrary (44,45,48,49). In fact, S. typhimurium LT2 contains two umuDC-like operons; the second, designated samAB, is encoded by the cryptic LT2 plasmid (30).…”

“…It is possible that in some cases sensitization and loss of mutations occurs through events like lysogenic induction. However, in E. coli such induction does not exclude mutagenesis in the fraction of cells surviv- (30). Furthermore, the environs of the iamla genes in E. coli, E. alkalescens, E. dispar, and S. typhimiurium have been found to be polymorphic (38) and may on occasion produce rearrangements in the lma genes.…”

“…To date, five umuDC-like operons have been characterized at the molecular level (8,20,30,32,40,45,48). All share certain structural similarities that suggest that they originated from a common ancestor.…”