YMC-2011, a Temperate Phage of Streptococcus salivarius 57.I

is an abundant isolate of the oral cavity. The genome of 57.I consists of a 2-Mb chromosome and a 40,758-bp circular molecule, designated YMC-2011. Annotation of YMC-2011 revealed 55 open reading frames, most of them associated with phage production, although plaque formation is not observed in 57.I after lytic induction using mitomycin C. Results from Southern hybridization and quantitative real-time PCR confirmed that YMC-2011 exists extrachromosomally, with an estimated copy number of 3 to 4. Phage particles were isolated from the supernatant of mitomycin C-treated 57.I cultures, and transmission electron microscopic examination indicated that YMC-2011 belongs to the family. Phylogenetic analysis suggests that phage YMC-2011 and the -type phages of originated from a common ancestor. An extended -10 element (p ) and a σ -like promoter (p ) were mapped 5' to Ssal_phage00013 (encoding a CI-like repressor) and Ssal_phage00014 (encoding a hypothetical protein), respectively, using 5' rapid amplification of cDNA ends, indicating that YMC-2011 transcribes at least two mRNAs in opposite orientations. Studies using promoter-chloramphenicol acetyltransferase reporter gene fusions revealed that p , but not p , was sensitive to mitomycin C induction, suggesting that the switch from lysogenic growth to lytic growth was controlled mainly by the activity of these two promoters. In conclusion, a lysogenic state is maintained in 57.I, presumably by the repression of genes encoding proteins for lytic growth. The movement of mobile genetic elements such as bacteriophages and the establishment of lysogens may have profound effects on the balance of microbial ecology where lysogenic bacteria reside. The discovery of phage YMC-2011 from 57.I suggests that YMC-2011 and -infecting phages share an ancestor. Although and are close phylogenetically, is a natural inhabitant of the human mouth, whereas is commonly found in the mammary mucosa of bovine species. Thus, the identification of YMC-2011 suggests that horizontal gene transfer via phage infection could take place between species from different ecological niches.