Effect of R factors and other plasmids on ultraviolet susceptibility and host cell reactivation property of Escherichia coli.

Some R factors, like some colicin factors, confer partial protection against the bactericidal effect of ultraviolet (UV) irradiation. Of 31 plasmids (17 R, 3 col, and 11 R-col factors) tested in Escherichia coli K-12, 15 protected, 11 had little or no effect, and 5 caused increased UV susceptibility. The effect of representative plasmids was qualitatively the same in K-12 of wild-type UV sensitivity, lambda-lysogenic or non-lysogenic, and in UV-sensitive mutants of classes uvrA, uvrB, uvrC, and recA (except that a sensitizing factor did not increase the sensitivity of two recA hosts). It is inferred that the UV-protecting effect of some plasmids does not result from their specifying enzymes similar to those deficient in such mutants. UV killing of multiply auxotrophic K-12, of wild-type sensitivity or recA or uvrC, was reduced by deprivation of required amino acids for 2 hr before irradiation, and further reduced if "starvation" was continued for 2 hr after irradiation. The plasmids tested in these conditions produced qualitatively the same effects as in nonstarved cells-except that in K-12 of wild-type UV sensitivity the effect of protecting plasmids was reversed (i.e. they caused decreased survival) when the cells were starved after irradiation. Two UV-protecting R factors reduced the ability of HCR(+) K-12 to support growth of irradiated phage T1.