Isolation of ultravirulent mutants of phage lambda.

Ultravirulent mutants of phage λ able to grow in the presence of high cellular levels of repressor, have been isolated from λν2ν3. These phages can be divided into three groups of increasing virulence reflecting the number of mutational steps required for their isolation. Multiple mutations in the operator regions leading to a decrease in the affinity of the λ repressor for the operator sites seen to be responsible for the ultravirulent phenotype.