The site-specific Integration system of the temperate Streptococcus thermophiles Bacteriophage φSfi21.

The temperate bacteriophage fSfi21 integrates its DNA into the chromosome of Streptococcus thermophilus strains viasite-specific recombination. Nucleotide sequencing of the attachment sites identified a 40-bp identity region which surprisinglyoverlaps both the 18-terminal bp of the phage integrase gene and the 11-terminal bp of a host tRNAArg gene. A 2.4-kb phageDNA segment, covering attP, the phage integrase, and a likely immunity gene contained all the genetic information for faithfulintegration of a nonreplicative plasmid into the attB site. A deletion within the int gene led to the loss of integration proficiency.A number of spontaneous deletions were observed in plasmids containing the 2.4-kb phage DNA segment. The deletionsites were localized to the tRNA side of the identity region and to phage or vector DNA with 3- to 6-bp-long repeats fromthe border region. A similar type of deletion was previously observed in a spontaneous phage mutant