Abstract
Campylobacter phages are divided into two genera; Fletchervirus and Firehammervirus, showing only limited intergenus homology. Here, we aim to identify the lytic genes of both genera using two representative phages (F352 and F379) from our collection. We performed a detailed in silico analysis searching for conserved protein domains and found that the predicted lytic genes are not organized into lysis cassettes but are conserved within each genus. To verify the function of selected lytic genes, the proteins were expressed in E. coli, followed by lytic assays. Our results show that Fletchervirus phages encode a typical signal peptide (SP) endolysin dependent on the Sec-pathway for translocation and a holin for activation. In contrast, Firehammervirus phages encode a novel endolysin that does not belong to currently described endolysin groups. This endolysin also uses the Sec-pathway for translocation but induces lysis of E. coli after overexpression. Interestingly, co-expression of this endolysin with an overlapping gene delayed and limited cell lysis, suggesting that this gene functions as a lysis inhibitor. These results indicate that Firehammervirus phages regulate lysis timing by a yet undescribed mechanism. In conclusion, we found that the two Campylobacter phage genera control lysis by two distinct mechanisms.
Highlights
Bacteriophages have evolved diverse lysis mechanisms to liberate progeny phages from infected host cells
The main lytic proteins belong to three functional classes: endolysins, holins and spanins, that often are encoded by genes organized into a gene cluster, known as a lysis cassette [1,2]
Translocation across the cytoplasmic membrane and activation of endolysins in the periplasm are some of the mechanisms that phages utilize to control the timing of lysis [5,6]
Summary
Bacteriophages have evolved diverse lysis mechanisms to liberate progeny phages from infected host cells. Phage-induced lysis of bacterial cells relies on the osmotic imbalance caused by degradation of the cell wall after a coordinated action of bacteriophage-encoded lytic proteins. The main lytic proteins belong to three functional classes: endolysins, holins and spanins, that often are encoded by genes organized into a gene cluster, known as a lysis cassette [1,2]. In order for endolysins to target peptidoglycan, access to the periplasmic space is a prerequisite. Translocation across the cytoplasmic membrane and activation of endolysins in the periplasm are some of the mechanisms that phages utilize to control the timing of lysis [5,6]. Holins ensure that endolysins gain access to the periplasm [7].
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