SUMMARY: The properties of a plaque-type variant, or ‘star’ mutant, of coliphage T2 are investigated. This mutant forms sectored plaques which are composed of approximately equal numbers of the parental star phage particles and of rapid-lysing () mutants. The star mutant is shown to be a one-step mutant of wild-type, possessing a mutation at a locus, , that is close to the locus of phage T2. The mutants occurring in the star plaques are all double-mutants, containing as well as the mutation, a mutation at some further locus. Rapid-lysing mutants isolated from different star plaques possess mutations at different loci. The high proportion of mutants occurring in the star plaques is due to selection of mutants that are formed spontaneously during replication of the star phage. This selection occurs during phage multiplication in that is no longer multiplying exponentially (old bacteria), but not in exponentially growing bacteria. The slow multiplication of star compared to phage in old cultures of is not due to an inability of the old bacteria to synthesize star phage particles at the same rate as phage particles, but is caused by two, probably related, factors. First, the star phage has a prolonged latent period in old bacteria, and secondly, the star phage particles that are eventually released when the old cells lyse re-adsorb very slowly to old bacteria in the culture. During multiplication in old bacteria the phage therefore goes through many more cycles of growth than the phage, and thus accumulates more rapidly.


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