Abstract
Staphylococcus aureus pathogenicity islands (SaPIs) are molecular parasites that hijack helper phages for their transfer. SaPIbov5, the prototypical member of a family of cos type SaPIs, redirects the assembly of ϕ12 helper capsids from prolate to isometric. This size and shape shift is dependent on the SaPIbov5-encoded protein Ccm, a homolog of the ϕ12 capsid protein (CP). Using cryo-electron microscopy, we have determined structures of prolate ϕ12 procapsids and isometric SaPIbov5 procapsids. ϕ12 procapsids have icosahedral end caps with Tend = 4 architecture and a Tmid = 14 cylindrical midsection, whereas SaPIbov5 procapsids have T = 4 icosahedral architecture. We built atomic models for CP and Ccm, and show that Ccm occupies the pentameric capsomers in the isometric SaPIbov5 procapsids, suggesting that preferential incorporation of Ccm pentamers prevents the cylindrical midsection from forming. Our results highlight that pirate elements have evolved diverse mechanisms to suppress phage multiplication, including the acquisition of phage capsid protein homologs.
Highlights
Staphylococcus aureus pathogenicity islands (SaPIs) are molecular parasites that hijack helper phages for their transfer
Structure of the prolate φ12 procapsid. φ12 procapsids were produced by mitomycin C induction of the S. aureus lysogen JP10942, which contains a φ12 terSam prophage with an amber stop codon in the terS gene (Supplementary Table 1)
The φ12 procapsids were imaged by cryo-electron microscopy (cryo-EM), which showed oblong cylindrical particles, about 40 nm wide and 82 nm long, with rounded end caps and the characteristic thick-walled, serrated appearance of phage procapsids (Fig. 1a)
Summary
Staphylococcus aureus pathogenicity islands (SaPIs) are molecular parasites that hijack helper phages for their transfer. SaPIbov[5], the prototypical member of a family of cos type SaPIs, redirects the assembly of φ12 helper capsids from prolate to isometric. This size and shape shift is dependent on the SaPIbov5-encoded protein Ccm, a homolog of the φ12 capsid protein (CP). S. aureus encodes a wide array of virulence factors that increase bacterial survival in the human host and that may increase the severity of the disease Many of these virulence factors are encoded on mobile genetic elements (MGEs)—including plasmids, bacteriophages, and genomic islands—that are transferred horizontally between hosts[2]. As long as the capsid is full, the DNA is cleaved at the cos site, resulting in exact units of DNA with sticky ends being packaged into each phage particle
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