Supervirulent Mutants and the Structure of Operator and Promoter

Abstract

Bacteria carrying the plasmid λdv (Matsubara and Kaiser, 1968) restrict the growth of λvir. However, it is possible to isolate mutants of λ that can grow on carriers of λdv. The purpose of this report is to describe such mutants. The new locus of mutation is called vs, for supervirulence. Mutants having the genotype v2v3vs are supervirulent in the sense that they can grow on nonlysogenic bacteria, on λ lysogens, and on λdv carriers. Experiments with these mutants suggest that the promoter may penetrate the operator in the right-hand operon of λ.

Stocks of λv2v3 contain mutants that form plaques on a lysogen at a frequency of about 10⁻⁶. One-tenth of these are supervirulent because they can also make plaques on a λdv carrier. The remainder are ordinary virulent mutants, which cannot make plaques on λdv carriers. This second class I shall denote vr, for restricted virulent. Jacob and Wollman’s (1954) v1 is thus analogous to a vr mutation. Sixteen independent vs and nineteen independent vr mutants were isolated by plating on a lysogen separate stocks grown from single plaques of λv2v3.

MAPPING
The 35 mutants were crossed with 2 reference mutants. λvs326 and λvs387. (All crosses were between λv2v3vs or λv2v3vr and λv2v3vs326 or λv2v3vs387.) Representative results are presented in Table 1. The supervirulent mutants fall into 2 classes: mutants of class I recombine with λvs387 but not with λvs326; mutants of class II recombine with λvs326 but not with λvs387. All of the λvr mutants belong to class...