Indirect selection for plasmid mutants: isolation of ColVBtrp mutants defective in self-maintenance in Escherichia coli.

An efficient method for isolation of a large number of plasmid mutants is described. It is based on the fact that N-methyl-N'-nitro-N-nitrosoguanidine induces a number of closely linked mutations within a short segment of the bacterial chromosome. Thus, selection for reversions of an auxotrophic marker located on the ColVBtrp plasmid yielded a large fraction (more than 50 percent) of mutants defective in some plasmid functions, including its own maintenance in the host bacteria. The results of preliminary characterization of strains carrying these mutated plasmids are presented.