Bacteriophage T12 of Streptococcus pyogenes integrates into the gene encoding a serine tRNA

The region of temperate bacteriophage T12 responsible for integration into the chromosome of Streptococcus pyogenes has been identified. The integrase gene (int)and the phage attachment site (attP) are found immediately upstream of the gene for speA, the latter of which is known to be responsible for the production of erythrogenic toxin A (also known as pyrogenic exotoxin A). The integrase gene has a coding capacity for a protein of 41 457 Da, and the C-terminus of the deduced protein is similar to other conserved C-terminal regions typical of phage integrases. Upstream of int is a second open reading frame, which is capable of encoding an acidic protein of 72 amino acids (8744 Da); the position of this region in relation to int suggests it to be the phage excisionase gene (xis). The arms flanking the integrated prophage (attL and attR) were identified, allowing determination of the sequences of the phage (attP) and bacterial (attB) attachment sites. A fragment containing the integrase gene and attP was cloned into a streptococcal suicide vector; when introduced into S. pyogenes by electrotransformation, this plasmid stably integrated into the bacterial chromosome at attB. The insertion site for the phage into the S. pyogenes chromosome was found to be in the anticodon loop of a putative type II gene for a serine tRNA. attP and attB share a region of identity that is 96 bp in length; this region of identity corresponds to the 3' end of the tRNA-gene such that the coding sequence remains intact after integration of the prophage. The symmetry of the core region of att may set this region apart from previously described phage attachment sites (Campbell, 1992), and may play a role in the biology of this medically important bacteriophage.