Summary: The presence in recipient strains of 12 of the plasmid R46 greatly reduced the yield of recombinants from crosses with several Hfr strains and virtually abolished the formation of recombinants by P1 transduction without, however, significantly affecting the transfer of the F prime from a strain carrying F. The R46 plasmid had paradoxical effects on mutability: it appeared to enhance the yield of mutants following irradiation with ultraviolet light but it reduced the number of mutants detectable in unirradiated cultures. The effects of this plasmid on ultraviolet light survival of the wild type and several mutants defective for recombination and repair have been measured and the results, in the main, confirm similar observations by Tweats (1976). Not only is the survival of the strain harbouring R46 greater than that of the parent strain in all the cases studied, but the survival of ultraviolet irradiated bacteriophage Δ is also greater.


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