ANALYSIS OF THE MUTAGENIC PROPERTIES OF THE UMUDC, MUCAB AND RUMAB PROTEINS, USING A SITE-SPECIFIC ABASIC LESION

The mucAB and rumAB loci have been shown to promote mutagenesis to a g reater extent than the structurally and functionally homologous Escher ichia coli umuDC operon. We have analyzed the basis of this enhanced m utagenesis by comparing the influence of these operons, relative to um uDC, on the mutagenic properties of each of two abasic sites, specific ally located in a single-stranded vector. Experiments with these vecto rs are useful analytical tools because they provide independent estima tes of the efficiency of translesion synthesis and of the relative fre quencies of each type of nucleotide insertion or other kind of mutagen ic event. The umuDC, mucAB, and rumAB genes were expressed from their natural LexA-regulated promoter on low-copy-number plasmids in isogeni c strains carrying a umuDC deletion. In addition, plasmids expressing the UmuD'C, MucA'B, or RumA'B proteins were also used. Compared to umu DC, the chief effect of mucAB was to increase the efficiency of transl esion synthesis past the abasic site. The enhanced capacity of mucAB f or translesion synthesis depended about equally on an inherently great er capacity to promote this process and on a greater susceptibility of the MucA protein to proteolytic processing. The RumA protein also app eared to be more susceptible to proteolytic processing, but the inhere nt capacity of the Rum products for translesion synthesis was no great er than that of UmuDC. dAMP was inserted opposite one of the two abasi c sites studied at a somewhat greater frequency in strains expressing rum (82%) compared to those expressing umu (72%), which might result i n higher mutation frequencies in rumAB than in umuDC strains.