Genetics of Virulence

In 1954, Jacob and Wollman described a virulent mutant of λ , called λvir, that is insensitive to repressor; it grows in λ lysogens (Jacob and Wollman, 1954). Jacob and Wollman concluded that at least four mutations are required for the virulent phenotype, and they mapped three of these. In 1968 we repeated and extended Jacob and Wollman’s experiments. Our results can be summarized as follows: (1) All of the mutations in λvir required for virulence are located in the immunity region. (2) Lambda vir contains three mutations necessary for virulence; one located to the left of c I, the other two just to the right of c I. It is helpful to summarize Jacob and Wollman’s findings before presenting our own. They described a strain, λ 11, which forms partly turbid plaques (“ centre ”) on sensitive cells. They found that stocks of λ 11 contain, at a frequency of about 10 −6 , phage ( λ virulents) which form clear plaques on λ lysogens. Since no other λ tested gave rise to virulent mutants, Jacob and Wollman assumed that several mutations must be required for virulence, and that λ 11 possesses all but one of them. The mutation causing λ 11 to become virulent they called v 1. When λvir was crossed with λ wild type, two nonvirulent recombinants were found among the progeny. One formed clear plaques on sensitive cells, and stocks of this phage contained virulents at a frequency of about 10 −5 ; the other formed turbid plaques and did not mutate to virulence. Crossing these two nonvirulent phages produced...