Activity of a Holin-Endolysin System in the Insecticidal Pathogenicity Island of Yersinia enterocolitica

Yersinia enterocolitica is a pathogen that causes gastroenteritis in humans. Because of its low-temperature-dependent insecticidal activity, it can oscillate between invertebrates and mammals as host organisms. The insecticidal activity of strain W22703 is associated with a pathogenicity island of 19 kb (Tc-PAI ), which carries regulators and genes encoding the toxin complex (Tc). The island also harbors four phage-related and highly conserved genes of unknown functions, which are polycistronically transcribed. Two open reading frames showed significant homologies to holins and endolysins and exhibited lytic activity in Escherichia coli cells upon overexpression. When a set of Yersinia strains was tested in an equivalent manner, highly diverse susceptibilities to lysis were observed, and some strains were resistant to lysis. If cell lysis occurred (as demonstrated by membrane staining), it was more pronounced when two accessory elements of the cassette coding for an i-spanin and an o-spanin were included in the overexpression construct. The pore-forming function of the putative holin, HolY, was demonstrated by complementation of the lysis defect of a phage λ S holin mutant. In experiments performed with membrane preparations, ElyY exhibited high specificity for W22703 peptidoglycan, with a cleavage activity resembling that of lysozyme. Although the functionality of the lysis cassette from Tc-PAI was demonstrated in this study, its biological role remains to be elucidated.IMPORTANCE The knowledge of how pathogens survive in the environment is pivotal for our understanding of bacterial virulence. The insecticidal and nematocidal activity of Yersinia spp., by which the bacteria gain access to nutrients and thus improve their environmental fitness, is conferred by the toxin complex (Tc) encoded on a highly conserved pathogenicity island termed Tc-PAI While the regulators and the toxin subunits of the island had been characterized in some detail, the role of phage-related genes within the island remained to be elucidated. Here, we demonstrate that this cassette encodes a holin, an endolysin, and two spanins that, at least upon overexpression, lyse Yersinia strains.