The Floating (Pathogenicity) Island: A Genomic Dessert

Among the prokaryotic genomic islands (GIs) involved in horizontal gene transfer (HGT) are the classical pathogenicity islands, including the integrative and conjugative elements (ICEs), the gene-transfer agents (GTAs), and the staphylococcal pathogenicity islands (SaPIs), the primary focus of this review. While the ICEs and GTAs mediate HGT autonomously, the SaPIs are dependent on specific phages. The ICEs transfer primarily their own DNA, the GTAs exclusively transfer unlinked host DNA, and the SaPIs combine the capabilities of both. Thus the SaPIs derive their importance from the genes they carry (their genetic cargo) and the genes they move. They act not only as versatile high-frequency mobilizers but also as mediators of phage interference and consequently are major benefactors of their host bacteria.

TrendsThree major types of horizontal gene transfer systems use radically different strategies for gene transfer at widely varying frequencies. Integrative and conjugative elements (ICEs) initiate their own transfer autonomously, staphylococcal pathogenicity islands (SaPIs) depend on helper phages, and gene-transfer agents (GTAs) generate and release tiny DNA-containing particles.GTAs and SaPIs were probably derived from prophages by complementary deletions. ICEs were probably derived from integrated conjugative plasmids by ‘trading’ replication for integration/excision.SaPIs interfere with their helper phages by three carefully regulated strategies to ensure high-frequency transfer.SaPIs mediate generalized transduction while in situ independently of their self-transfer.SaPIs and their relatives in Gram-positive cocci exist as cohesive and widespread families, as shown by orthology analysis of their hypothetical proteins, a method newly described for this review.