The multisite character of host-range mutations in bacteriophage λ

Mapping of the h and hh* host-range mutations in phage λ by two-point crosses with reference J^- point mutations, and with λgal deleted for part of J, locates these mutations in the promoter-distal portion of the J cistron. Analysis of phenotypic h⁺ recombinants, formed in crosses of the type h or hh* × J^t-, or h⁺ revertants of h, hh*, and J_def mutants, indicates that such phenotypic h⁺ particles often retain cryptic h determinants. Similar determinants are also present in some common laboratory strains of λ. These h⁺ recombinants and revertants carry a variety of different h markers, since recombination analysis allows several classes of particles carrying cryptic h markers to be distinguished. These genetic data suggest that the extended host-range phenotype in λ is due to multiple rather than single, mutations in the distal region of gene J, although the number of sites involved and their arrangement remain uncertain. The genetic location of the h and hh* mutations is confirmed at the physical level by comparing the tryptic peptide maps of the J proteins purified from lysates of cells infected with different h⁺, h, hh*, J_am, and J⁴³⁴ phage and from purified λh⁺ virions. Examination of these peptide maps shows there are several methionine-containing peptides altered in the h and hh* maps. Some of these altered peptides are derived from the C-terminal 5-10% of the J polypeptide in the region of nonhomology between λ and 434.