Effect of phosphate buffer agar on the number of UV-induced mutations to streptomycin resistance in Escherichia coli strains.

The number of UV-induced mutations to streptomycin resistance in Escherichia coli B/r cells depends on the post-irradiation incubation medium. When cells are incubated on phosphate buffer agar (PBA) as opposed to brain heart infusion agar (BHI), there is an irreversible loss in the number of potential streptomycin resistant mutants which develop. This decrease in mutant numbers is known to occur without a corresponding loss in overall cell viability and cannot be explained in terms of the kinetics of dimer excision. Recent studies have indicated that pyrimidine dimers are the substrate for this repair on PBA. The genetics of PBA repair was investigated by observing the influence of the post-irradiation incubation medium (PBA vs BHI) on the number of streptomycin resistant mutants which develop in E. coli strains deficient in various repair pathways. A recB and recC strain (WP3 recB trp-; WP7 recC trp-) showed repair of potential streptomycin resistant mutants similar to that of B/r when incubated on PBA following irradiation. The medium had no effect on the number of mutants expressed in an excision deficient uvrA- strain (WP2 uvrA trp-) or a polymerase I mutant (P3478 polA thy-). This was interpreted to mean that the loss of streptomycin resistant mutants on PBA involves the excision repair pathway and is dependent on the polA gene product, polymerase I.