Lysis and lysis inhibition in bacteriophage T4: rV mutations reside in the holin t gene

Abstract

Upon infecting populations of susceptible host cells, T-even bacteriophages maximize their yield by switching from lysis at about 25 to 35 min at 37 degrees C after infection by a single phage particle to long-delayed lysis (lysis inhibition) under conditions of sequential infection occurring when free phages outnumber host cells. The timing of lysis depends upon gene t and upon one or more rapid-lysis (r) genes whose inactivation prevents lysis inhibition. t encodes a holin that mediates the movement of the T4 endolysin though the inner cell membrane to its target, the cell wall. The rI protein has been proposed to sense superinfection. Of the five reasonably well characterized r genes, only two, rI and rV, are clearly obligatory for lysis inhibition. We show here that rV mutations are alleles of t that probably render the t protein unable to respond to the lysis inhibition signal. The tr alleles cluster in the 5' third of t and produce a strong r phenotype, whereas conditional-lethal t alleles produce the classical t phenotype (inability to lyse) and other t alleles produce additional, still poorly understood phenotypes. tr mutations are dominant to t+, a result that suggests specific ways to probe T4 holin function.

FIG. 1

The t gene (GenBank accession no. Y00408), with its derived protein sequence and mutations. t is flanked upstream by its late promoter (P_l) and by gene 38 and downstream by asiA and its early promoter (P_e); arrows indicate directions of transcription. The two blocks of sequence in boldface are the putative transmembrane residues (32). Numbers below the sequence identify mutated residues or reference residues. Entries above the sequence identify, in order, amino acid changes (am, amber), then base sequence changes in the complementary (noncoding) strand, and, in italics, names of alleles (alleles producing an r phenotype being shown in boldface).

FIG. 2

One-step growth curves of strains grown in H broth and plated with H agars (A) or grown in LB broth and plated with Drake agars (B).

FIG. 3

Lysis profiles determined after primary infection followed by superinfection with phage particles of various genotypes. Values in parentheses are the MOIs (displayed as a ratio of the MOI of the original infection to the MOI of the superinfection).