The Lysis cassette of jumbophage PhiKZ.

Pseudomonas jumbophage phiKZ is prominent because of the proteinaceous nuclear structure that is formed during the infection cycle, protecting the replicating phage DNA from host nucleases. Progress has been made in deciphering the molecular basis of nucleus formation and other unique aspects of phiKZ biology, but the understanding of how it causes host lysis remains minimal. Here we present bioinformatic, physiological and molecular evidence for a "lysis cassette", the expression of which is necessary and sufficient for the temporally regulated disruption of the host envelope. This cluster contains genes for the usual components of an MGL (multigene lysis) system, including the holin, endolysin, i-spanin and o-spanin. In addition, a fifth gene in the cluster, encoding a cytoplasmic protein, was found to accelerate the timing of holin-mediated lethality when expressed in trans. Evidence is provided that suggests this lysis regulator protein interacts with the cytoplasmic domain of the phiKZ class III holin. In support of this notion, alpha-fold analysis generated a high-confidence structure of a conserved holin-lysis regulator heterodimer complex. Infections at high multiplicity resulted in slower bulk culture lysis profiles than at low multiplicity, suggesting that phiKZ might have a lysis-inhibition system.