Staphylococcus aureus pathogenicity island DNA is packaged in particles composed of phage proteins

Abstract

Staphylococcus aureus pathogenicity islands (SaPIs) have an intimate relationship with temperate staphylococcal phages. During phage growth, SaPIs are induced to replicate and are efficiently encapsidated into special small phage heads commensurate with their size. We have analyzed by amino acid sequencing and mass spectrometry the protein composition of the specific SaPI particles. This has enabled identification of major capsid and tail proteins and a putative portal protein. As expected, all these proteins were phage encoded. Additionally, these analyses suggested the existence of a protein required for the formation of functional phage but not SaPI particles. Mutational analysis demonstrated that the phage proteins identified were involved only in the formation and possibly the function of SaPI or phage particles, having no role in other SaPI or phage functions.

FIG. 1.

Protein compositions of the phage- and SaPI-specific particles.

FIG. 2.

Locations of genes encoding the proteins analyzed in this study. Arrows indicate predicted open reading frames, as annotated in the database entry (accession number AF424781 for φ11 and DQ517338 for 80α). Black arrows indicate genes deleted in this study. The open reading frame number for each gene is indicated.

FIG. 3.

Replication and encapsidation analysis of the different φ11 mutants. A Southern blot of the different φ11 mutant lysates carrying SaPIbov1 tst::tetM, obtained with samples taken 60 min after MC induction, separated on agarose, and blotted with a phage- or SaPIbov1-specific probe, is shown. The upper band is “bulk” DNA, including chromosomal, phage, and replicating SaPI; the lower band is SaPI linear monomers released from phage heads.

FIG. 4.

Electron micrographs of φ11 gene 54 mutant lysates. Note the presence of SaPIbov1 particles (lower panels), which have smaller heads. wt, wild type.