Bacteriophage T4 lysis and lysis inhibition: A molecular perspective

For phage-induced lysis, dsDNA bacteriophages require two proteins: an "endolysin", a soluble enzyme which degrades the cell wall, and a "holin", which at programmed time allows endolysin to pass through the cytoplasmic membrane. More than 100 holin genes have been identified and characterized by common structural motifs. The sole exception is the t gene of bacteriophage T4, which by genetic and physiological criteria encodes a holin but lacks similarities with this large protein family. T4 gene t is also essential for a lysis-related phenomenon---lysis inhibition (LIN). T-even phages have the unique ability to delay the onset of lysis for hours in response to the presence of free T-even phage particles in the culture medium. Although LIN was described more than 50 years ago, and LIN mutants of bacteriophage T4 were widely used as genetic tools, the mechanism of lysis inhibition remains unknown. A hybrid phage, lambdat, was constructed by inserting the phage T4 t gene in place of the lambda holin gene, S. Induction of lambdat, resulted in abrupt lysis, indicating that t is indeed a holin gene. Induction of lysogens carrying both S and t was shown to result in earlier triggering of lysis than with either holin gene alone. This result suggests that the two very dissimilar holins contribute additively to the physiology of the timing mechanism or interact to form one mass-action pool. In either case, these results imply a common pathway for holin timing and function. Gene fusion analysis indicates that T has unique among holins transmembrane topology with a single transmembrane domain near the N-terminus, with the bulk of the protein disposed in the periplasm. Deletion analysis, mapping and characterization of forty-two t missense mutants indicate that the C-terminal periplasmic domain is essential of the timing mechanism and LIN, but not essential for hole-formation. We were able to impose LIN on inductions of the T hybrid lambdat phage, using superinfection by LIN-competent T4. The imposition of the LIN state depends on the allelic state of the rI and t genes of the superinfecting T4 phage, indicating that the LIN-sensitive state of the T holin is transitory. Moreover, it was shown that a cloned rI gene can impose a T-specific LIN in the context of lambdat, in the absence of other T4 genes and without secondary infection by T4. We show by cross-linking experiments that a T-RI heterodimer is formed during LIN, demonstrating that RI belongs to the functional class of antiholins, like the S107 protein of lambda. It was also shown that addition of Ni++ ions to the medium can block lysis by a T protein hexahistidine-tagged at its C-terminus, suggesting that liganding of the periplasmic domain is sufficient to impose lysis inhibition. These findings lay the foundation for further studies of lysis and lysis inhibition of bacteriophage T4 on molecular level.