Mutations in the right operator of bacteriophage lambda: evidence for operator-promoter interpenetration

A set of virulent mutants of bacteriophage lambda have been selected from λv2 v3. The sites of mutation form two microclusters, both close to v3. Some of the mutants, selected for their ability to grow on a λ-lysogen, can also grow on a λdv carrier strain. They are called “supervirulent” and a mutation conferring super-virulence is called vs. The sites of mutation to vs lie between the presumed promoter mutants (x3, x7) and x13, implying that the operator and promoter interpenetrate each other. The relative affinities of λ repressor for binding, in vitro, to λv⁺, λv3, λvs326, and λv3 vs327 DNA were 1, $\text{1}\text{4}$, $\text{1}\text{20}$, and $\text{1}\text{4000}$, respectively. We suggest that two separate mutations in the right operator are needed to confer virulence because promoter sites lie within the operator.