The site-specific recombination system of actinophage TG1

Actinophage TG1 forms stable lysogens by integrating at a unique site on chromosomes of Streptomyces strains. The phage (attP(TG1)) and bacterial (attB(TG1)) attachment sites for TG1 were deduced from comparative genomic studies on the TG1-lysogen and nonlysogen of Streptomyces avermitilis. The attB(TG1) was located within the 46-bp region in the dapC gene (SAV4517) encoding the putative N-succinyldiaminopimelate aminotransferase. TG1-lysogens of S. avermitilis, however, did not demand either lysine or diaminopimelate for growth, indicating that the dapC annotation of S. avermitilis requires reconsideration. A bioinformatic survey of DNA databases using the fasta program for the attB(TG1) sequence extracted possible integration sites from varied streptomycete genomes, including Streptomyces coelicolor A3(2) and Streptomyces griseus. The gene encoding the putative TG1 integrase (int(TG1)) was located adjacent to the attP(TG1) site. TG1 integrase deduced from the int(TG1) gene was a protein of 619 amino acids having a high sequence similarity to C31 integrase, especially at the N-terminal catalytic region. By contrast, sequence similarities at the C-terminal regions crucial for the recognition of attachment sites were moderate or low. The site-specific recombination systems based on TG1 integrase were shown to work efficiently not only in Streptomyces strains but also in heterologous Escherichia coli.