Thymineless elimination of N group plasmids is Res−(RII)-dependent and determined by a different gene than the Uvp plasmid phenotype

Abstract

SUMMARYThe wild-type N group plasmid RN3, which is phenotypically Res+Mod+(RII) is not eliminated by thymine starvation of its bacterial host. Derivatives of RN3 selected for the Res−phenotype are eliminated. The presence or absence of the RII modification specificity does not affect thymineless elimination of RN3 Res−plasmids. A Res−Mod(am) RN3 mutant is not eliminated by thymine starvation from either amber suppressing or non-suppressing hosts, suggesting that it carries a cryptic mutation in a novel genetic locus required for elimination. Thymineless elimination is shown to berecA+-dependent and the presence of the X group plasmid R6K significantly inhibits elimination of RN3 Res−Mod+. However, since R6K has no effect on two other plasmid-mediated functions of UV protection and UV-induced mutagenesis, which are alsorecA+-dependent, it would appear that elimination is determined by a separate plasmid gene than that encoding the UV functions. This is confirmed using derivatives of another N group plasmid R390, which eliminate but which have lost the ability to increase UV-induced mutagenesis in their host.