Sequence determinants for DNA packaging specificity in the S. aureus pathogenicity island SaPI1

• We mapped packaging cleavage sites for phage 80α and pathogenicity island SaPI1.
• Packaging directed by 80α TerS initiates within the 80α terS coding sequence.
• SaPI1 TerS-specific packaging initiates in an intergenic region.
• The SaPI1 pac site determinants were mapped using a plasmid-based assay.
• Deletion of the pac site from SaPI1 eliminates high frequency transduction.

The SaPIs and their relatives are a family of genomic islands that exploit helper phages for high frequency horizontal transfer. One of the mechanisms used by SaPIs to accomplish this molecular piracy is the redirection of the helper phage DNA packaging machinery. SaPIs encode a small terminase subunit that can be substituted for that of the phage. In this study we have determined the initial packaging cleavage sites for helper phage 80α, which uses the phage-encoded small terminase subunit, and for SaPI1, which uses the SaPI-encoded small terminase subunit. We have identified a 19 nt SaPI1 sequence that is necessary and sufficient to allow high frequency 80α transduction of a plasmid by a terminase carrying the SaPI1-encoded small subunit. We also show that the hybrid enzyme with the SaPI1 small terminase subunit is capable of generalized transduction.