F' plasmids from HfrH and HfrC in recA- Escherichia coli.

We have isolated and characterized a number of clones resulting from matings of HfrH and HfrC cultures of Escherichia coli with auxotrophic recA- E. coli. As in Low's (1968) experiments, the recA- marker prevented integration of F' episomes into the vegetative chromosomes of the host. Both F'H F'C plasmids contained a great variety of non-selected nutritional markers. However, more F'H plasmids seemed to have expressed F+ characteristics than did F'C plasmids. These characteristics include (i) the presence of F-pili as determined by susceptibility to male-specific phages; (ii) fertility as determined by the merozygote's ability to transfer nutritional markers to an auxotrophic F- strain of E. coli; and (iii) a high degree of inheritability as estimated by the proportion of F' bacteria to F- bacteria in clones grown in a non-selective medium like broth. This proportion is seen to be affected by both factors that determine the probabilities that daughters of F' bacteria inherit the episome and from physiological factors that determine the rates of growth of F' and F- bacteria.