Identifying the components of the Shewanella phage LambdaSo lysis system

Phage-induced lysis of Gram-negative bacterial hosts usually requires a set of phage lysis proteins, a holin, an endopeptidase and a spanin system, to disrupt each of the three cell envelope layers. Genome annotations and previous studies identified a gene region in the Shewanella oneidensis prophage LambdaSo, which comprises potential holin- and endolysin-encoding genes but lacks an obvious spanin system. By a combination of candidate approaches, mutant screening, characterization and microscopy we found that LambdaSo uses a pinholin/signal-anchor-release (SAR) endolysin system to induce proton-leakage and degradation of the cell wall. Between the corresponding genes we found that two extensively nested open reading frames encode a two-component spanin module Rz/Rz1. Unexpectedly, we identified another factor strictly required for LambdaSo-induced cell lysis, the phage protein Lcc6. Lcc6 is a transmembrane protein of 65 amino acid residues with hitherto unknown function, which acts at the level of holin in the cytoplasmic membrane to allow endolysin release. Thus, LambdaSo-mediated cell lysis requires at least four protein factors (pinholin, SAR-endolysin, spanin, Lcc6). The findings further extend the known repertoire of phage proteins involved in host lysis and phage egress. SignificanceFor the release of the assembled virions, phages have to breach the cell envelope. For Gram-negatives, this requires the disruption of three layers, the outer and inner membrane and the cell wall. In most cases, the lysis systems of phages infecting Gram-negatives comprises holins to disrupt or depolarize the membrane, thereby releasing or activating endolysins, which then degrade the cell wall. This, in turn, allows the spanins to become active and fuse outer and inner membrane, completing cell envelope disruption and allowing phage egress. Here we show that the presence of these three components may not be sufficient to allow cell lysis, implicating that also in known phages further factors may be required.