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-PAIYe), 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, a highly diverse susceptibility to lysis was observed, and some strains were resistant towards lysis. If cell lysis occurred, which was 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 holing mutant. Neither HolY nor the putative endolysin, ElyY, were able to lyse W22703 cells when their genes were overexpressed separately. Using membrane preparations, ElyY exhibited high specificity for W22703 peptidoglycane with a cleavage activity resembling that of lysozyme. Although the functionality of the lysis cassette from Tc-PAIYe was demonstrated in this study, its biological role remains to be elucidated.
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