Construction of a new system for separate expression of mutagenesis proteins: The abilities to promote UV mutagenesis and interchangeability of MucA', MucB, SamA' and SamB proteins in Salmonella typhimurium

The two distinct mucAB and samAB operons originally isolated from the plasmids of Salmonella typhimurium encode proteins engaged in induced mutagenesis. They represent two extreme cases among these far characterized members of the enterobacterial umuDC family in respect to both the strength and the specificity of their effect. It is suggested that the MucA and SamA proteins are post-translationally processed to MucA' and SamA', respectively, which lack the N-terminal 25 amino acids and are the active species in mutagenesis. For the purpose of characterizing the individual activities of these proteins, we developed a new system for their SOS-independent separate and controllable expression in enterobacteria. Besides the matured forms of MucA', SamA' as well as MucB and SamB proteins we also expressed hybrid HisTag-MucA' and HisTag-SamA' proteins in which a synthetic 24 amino acid HisTag region replaces the natural 25 amino acid N-terminal leader present in the MucA and SamA precursors. In this study, we analyzed the effect of the mutagenesis proteins on the UV mutability of S. typhimurium YG5144. None of the proteins, if expressed alone, promoted UV mutagenesis. Different combinations of the proteins promoted mutagenesis to different extents in the order MucA' + MucB > SamA' + SamB greater than or equal to HisTag-MucA' + MucB greater than or equal to SamA' + MucB > MucA' + SamB > HisTag-SamA' + SamB. The mutagenesis enhancing potential of the combinations with MucB protein decreased as the expression of the proteins increased while the mutagenesis enhancing potential of the combinations with SamB protein increased together with the increase in the expression. The artificially expressed MucA' + MucB proteins were as active as their MucAB counterparts expressed from the plasmid pKM101 in promoting UV mutagenesis, but they were remarkably more efficient than their pKM101-born counterparts in promoting spontaneous mutagenesis. We conclude that the MucA'B and SamA'B proteins are partly interchangeable and the functionality of the resulting A' + B complex is largely dependent on the appropriate B-protein.