Isogenic Escherichia coli strains carrying single DNA-repair mutations were compared for their capacity for (i) the repair of X-ray-induced DNA double-strand breaks (DSB) as measured using neutral sucrose gradients; (ii) medium-dependent resistance, i.e., a recA-dependent X-ray survival phenomenon that correlates closely with the capacity for repairing DSB; and (iii) the growth medium-dependent, recA-dependent repair of X-ray-induced DNA single-strand breaks (SSB) as measured using alkaline sucrose gradients (about 80% of these SSB are actually parts of DSB). These three capacities were measured to quantitate more accurately the involvement of the various genes in the repair of DSB over a wide dose range. The mutations tested were grouped into five classes according to their effect on the repair of X-ray-induced DSB: (I) the recA, recB, recC, and lexA mutants were completely deficient; (II) the radB and recN mutants were about 90% deficient; (III) the recF and recJ mutants were about 70% deficient; (IV) the radA and uvrD mutants were about 30% deficient; and (V) the umuC mutant resembled the wild-type strains in its capacity for the repair of DSB.

We report a simple and efficient colorimetric method to screen large numbers of bacterial strains for UV- and X-radiation sensitivity. We used reference radiation-sensitive and control strains of Escherichia coli K-12 to compare our colorimetric method to a standard clonogenic plating method. Our colorimetric method was as accurate as the standard method and was superior in terms of savings in supplies and man-hours.

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Escherichia coli strains carrying radA100 or sms mutations were identical in their sensitivities to either methyl methanesulfonate or UV radiation treatment and in their plasmid complementation patterns for UV radiation survival. DNA sequencing analysis of the radA mutant and radA ؉ strains and comparison of their sequences with the published sms gene sequence showed the radA mutant to differ only by a G-to-A transition mutation, which is predicted to change a cysteine in a zinc-finger motif to tyrosine. The sms gene is concluded to be identical to the previously described radA gene.The radA100 mutation (named for its role in radiation resistance) was isolated because it made Escherichia coli cells sensitive to gamma radiation (3). This mutation maps close to the serB locus (3) and has been placed at 99.5 min on the E. coli K-12 linkage map (1). Log-phase radA mutant cells show increased sensitivity to X rays, methyl methanesulfonate (MMS), and UV radiation when grown in rich medium but not when grown in minimal medium (3). Rich medium-grown radA100 cells are also 30% deficient in their ability to repair X-rayinduced DNA double-strand breaks compared with radA ϩ cells (15).Ten years after the description of the radA gene, Neuwald et al. (11) described the sms gene (named for sensitivity to MMS), which is cotranscribed with the serB gene but is not required for serine biosynthesis. The sms gene is 1,380 bp long, and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis shows a 55-kDa product (11). It is of general interest that certain Sms protein domains show substantial similarity to different motifs of Lon protease and RecA protein (8, 11), two proteins that have long been known for their roles in gene regulation and DNA repair (e.g., reference 6). The purpose of this work was to test the hypothesis that the radA and sms genes are identical.(This paper represents partial fulfillment by Y. Song of the requirements for a Master's thesis from Truman State University, Kirksville, Mo., 1996.) Effects of radA100 and sms mutations on survival. Earlier studies of the radA and sms mutations involved distantly related E. coli strains. In this study, undesired strain differences were minimized by transducing both the radA and sms mutations into the same parental uvr ϩ and uvrA6 strains for phenotype comparison studies (all strains and plasmids used in this study are listed in Table 1). Although the original sms-1 gene disruption mutant (AN1) has apparently been lost, George V. Stauffer kindly provided a similarly constructed mutant whose sms mutation was used in our studies. The uvrA radA and uvrA sms strains showed essentially identical sensitivities to MMS, and they required only 66 and 56% as much MMS treatment, respectively, as the uvrA control strain to yield a surviving fraction of 10%; i.e., their respective D 10 ratios were 0.66 and 0.56 (Fig. 1a).The radA mutation has been reported to sensitize a wildtype (uvrA ϩ ) strain to UV radiation (3), while the sms mutation reportedly has no effect (11). W...