CHARACTERIZATION OF THE UMU-COMPLEMENTING OPERON FROM R391

In addition to conferring resistances to antibiotics and heavy metals, certain R factors carry genes involved in mutagenic DNA repair. These plasmid encoded genes are structurally and functionally related to th e chromosomally encoded umuDC genes of Escherichia coli and Salmonella typhimurium. Three such plasmid operons, mucAB, impCAB, and samAB, ha ve been characterized at the molecular level. Recently, we have identi fied three additional umu-complementing operons from IncJ plasmid R391 and IncL/M plasmids R446b and R471a. We report here the molecular cha racterization of the R391 umu-complementing operon. The nucleotide seq uence of the minimal R plasmid umu-complementing (rum) region revealed an operon of two genes, rumA((R391)) and rumB((R391), with an upstrea m regulatory signal strongly resembling LexA-binding sites. Phylogenet ic analysis revealed that the RumAB((R391)) proteins are approximately equally diverged in sequence from the chromosomal UmuDC proteins and the other plasmid-encoded Umu-like proteins and represent a new subfam ily. Genetic characterization of the rumAB((R391)) operon revealed tha t in recA(+) and recA1730 backgrounds, the rumAB((R391)) operon was ph enotypically indistinguishable from mucAB. In contrast, however, the r umAB((R391)) operon gave levels of mutagenesis that were intermediate between those given by mucAB and umuDC in a recA430 strain. The latter phenotype was shown to correlate with the reduced posttranslational p rocessing of the RumA((R391)) protein to its mutagenically active form , RumA'((R391)). Thus, the rumAB((R391)) operon appears to possess cha racteristics that are reminiscent of both chromosome and plasmid-encod ed umu-like operons.