HP1 Bacteriophage Encodes a Lytic Cassette with a Pinholin and a Signal-Arrest-Release Endolysin.

HP1 is a temperate bacteriophage, belonging to the family and infecting Rd. By in silico analysis and molecular cloning, we characterized and gene products, present in the previously proposed lytic module of HP1 phage. The amino acid sequence of the gene product revealed the presence of signal-arrest-release (SAR) and muraminidase domains, characteristic for some endolysins. HP1 endolysin was able to induce lysis on its own when cloned and expressed in , but the new phage release from infected cells was suppressed by inhibition of the secretion () pathway. Protein encoded by gene is a transmembrane protein, with unusual C-out and N-in topology, when overexpressed/activated. Its overexpression in did not allow the formation of large pores (lack of leakage of β-galactosidase), but caused cell death (decrease in viable cell count) without lysis (turbidity remained constant). These data suggest that gene encodes a SAR-endolysin and that the gene product is a pinholin. HP1 SAR-endolysin is responsible for cell lysis and HP1 pinholin seems to regulate the cell lysis and the phage progeny release from cells, as new phage release from the natural host was inhibited by deletion of the gene.