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 λ gal 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 J 434 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.