Molecular genetics of SaPl1 -: a mobile pathogenicity island in Staphylococcus aureus

The Staphylococcus aureus gene for toxic shock toxin (tst) is carried by a 15 kb mobile pathogenicity island, SaPI1, that has an intimate relationship with temperate staphylococcal phage 80 alpha. During phage growth, SaPI1 is excised from its unique chromosomal site, att(c), replicates autonomously, interferes with phage growth, and is efficiently encapsidated into special small phage heads commensurate with its size. Upon transfer to a recipient organism, SaPI1 integrates at att(c) by means of a self-coded integrase. One or more phage functions are required for excision, autonomous replication and encapsidation of the element and, thus, the overall relationship between SaPI1 and 80 alpha is similar to that between coliphages P4 and P2. Among other staphylococcal phages tested, only phi 13 interacts with SaPI1, inducing excision but not replication or transfer of the element.