SUMMARY: A mutant strain of K12, R2721, has been shown to differ from its parent strain, S491, in four associated phenotypic characters as a result of a single mutation. This strain did not give recombinants with DNA transduced by bacterio-phage P1 or bacteriophage Mu, nor transformants after exposure to R factor DNA: lysates of bacteriophage P1 grown on this strain did not appear to contain any transducing particles when tested on normal recipients. Moreover, the reversion rates, both spontaneous and ultraviolet-induced, for two auxotrophic markers were reduced. The frequency of revertants was at least two orders of magnitude lower in cultures of R2721 than in cultures of S491. Many of the rare revertants for one or other of the auxotrophic markers were found to have regained normal reversion frequencies for the other marker and for the capacity to be transduced. In all other respects, recombination in R2721 appeared normal, the frequency of chromosomal mobilization by an F” factor was unaffected and normal yields of recombinants were obtained from matings with Hfr strains. The only circumstance in which transduction of R2721 was observed was when the capacity to ferment galactose was selected and P1 had been grown on a strain carrying λdgal when, presumably, integration was effected by the phage-coded gene products. The mutation has been located on the chromosome map between and and has been given the symbol ( inhibition). Double mutants, () and (, have been isolated and show no unexpected properties.


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