title: An aerobic recA-, umuC-dependent pathway of spontaneous base-pair substitution mutagenesis in Escherichia coli.
abstract: Antimutator alleles indentify genes whose normal products are involved in spontaneous mutagenesis pathways. Mutant alleles of the recA and umuC genes of Escherichia coli, whose wild-type alleles are components of the inducible SOS response, were shown to cause a decrease in the level of spontaneous mutagenesis. Using a series of chromosomal mutant trp alleles, which detect point mutations, as a reversion assay, it was shown that the reduction in mutagenesis is limited to base-pair substitutions. Within the limited number of sites than could be examined, transversions at AT sites were the favored substitutions. Frameshift mutagenesis was slightly enhanced by a mutant recA allele and unchanged by a mutant umuC allele. The wild-type recA and umuC genes are involved in the same mutagenic base-pair substitution pathway, designated "SOS-dependent spontaneous mutagenesis" (SDSM), since a recAumuC strain showed the same degree and specificity of antimutator activity as either single mutant strain. The SDSM pathway is active only in the presence of oxygen, since wild-type, recA, and umuC strains all show the same levels of reduced spontaneous mutagenesis anaerobically. The SDSM pathway can function in starving/stationary cells and may, or may not, be operative in actively dividing cultures. We suggest that, in wild-type cells, SDSM results from basal levels of SOS activity during DNA synthesis. Mutations may result from synthesis past cryptic DNA lesions (targeted mutagenesis) and/or from mispairings during synthesis with a normal DNA template (untargeted mutagenesis). Since it occurs in chromosomal genes of wild-type cells, SDSM may be biologically significant for isolates of natural enteric bacterial populations where extended starvation is often a common mode of existence.
authors.affiliation: Department of Biological Sciences, San Jose State University, San Jose, CA 95192, USA.
authors.forename: S
authors.initials: S
authors.lastname: Bhamre
authors.forename: B B
authors.initials: BB
authors.lastname: Gadea
authors.forename: C A
authors.initials: CA
authors.lastname: Koyama
authors.forename: S J
authors.initials: SJ
authors.lastname: White
authors.forename: R G
authors.initials: RG
authors.lastname: Fowler
mesh_terms: D000332:Aerobiosis; D001426:Bacterial Proteins / Q000235:genetics*; D004252:DNA Mutational Analysis; D004269:DNA, Bacterial / Q000032:analysis / Q000235:genetics; D004259:DNA-Directed DNA Polymerase; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D016368:Frameshift Mutation; D016296:Mutagenesis; D020125:Mutation, Missense; D010100:Oxygen / Q000378:metabolism; D017354:Point Mutation; D011938:Rec A Recombinases / Q000235:genetics*; D013014:SOS Response, Genetics
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; C078202:UmuC protein, E coli; D011938:Rec A Recombinases; D004259:DNA-Directed DNA Polymerase; D010100:Oxygen