Complete genomic sequence of the lytic bacteriophage DT1 of Streptococcus thermophilus

Streptococcus thermophilus lytic bacteriophage DT1, isolated from a mozzarella whey, was characterized at the microbiological and molecular levels. Phage DT1 had an isometric head of 60 nm and a noncontractile tail of 260 x 8 nm, two major structural proteins of 26 and 32 kDa, and a linear double-stranded DNA genome with cohesive ends at its extremities. The host range of phage DT1 was limited to 5 of the 21 S. thermophilus strains tested. Using S. thermophilus SMQ-301 as a host, phage DT1 had a burst size of 276 +/- 36 and a latent period of 25 min. The genome of phage DT1 contained 34,820 bp with a GC content of 39.1%. Forty-six open reading frames (ORFs) of more than 40 codons were found and putative functions were assigned to 20 ORFs, mostly in the late region of phage DT1. Comparative genomic analysis of DT1 with the completely sequenced S. thermophilus temperate phage O1205 revealed two large homologous regions interspersed by two heterologous segments. The homologous regions consisted of the early replication genes, the late morphogenesis genes, and the lysis cassette. The divergent segments contained the DNA packaging machinery, the major structural proteins, and remnants of a lysogeny module.