NOVEL ORGANIZATION OF THE SITE-SPECIFIC INTEGRATION AND EXCISION RECOMBINATION FUNCTIONS OF THE STAPHYLOCOCCUS-AUREUS SEROTYPE-F VIRULENCE-CONVERTING PHAGES PHI-13 AND PHI-42

Functions required for site-specific integration and excision of the S taphylococcus aureus serotype F virulence-converting phages phi 13 and phi 42 were localized and characterized. Like other temperate phages, integration of phi 13 and phi 42 sequences was found to require the p roduct of an int gene located close to the phage attP site. Both int g enes are almost identical, express proteins possessing characteristic features of the Int (integrase) family of recombinases, but share very little homology with previously described int genes, including those of the serotype B S. aureus phages L54a and phi 11. Nevertheless, all four S. aureus phages share an almost identical short sequence located immediately 5' to these distinct int genes, suggesting a common mecha nism of int gene regulation. Upstream from these common sequences, the sequences of phi 13 and phi 42 are quite distinct from each other, an d from the corresponding regions of phi 11 and L54a which encode the X is proteins that are required with Int to mediate site-specific excisi on of the latter phages. Surprisingly, phi 13 and phi 42 sequences enc ompassing the attP sites and int genes, but lacking either an adjacent or more distant phage excision protein gene, were sufficient to media te site-specific excision of integrated phage DNA sequences.