CHARACTERIZATION OF A TEMPERATE STREPTOCOCCUS-THERMOPHILUS BACTERIOPHAGE AND ITS GENETIC-RELATIONSHIP WITH LYTIC PHAGES

The temperate Streptococcus thermophilus bacteriophage phi SF121 showed an 38-kb-long double-stranded DNA genome with cohesive ends. A single integration site was used in lysogens established in three different S. thermophilus strains. The attP and attB sites were localized on the restriction map of phage DNA and by hybridization on pulsed field separated bacterial DNA. All laboratory-established lysogens showed in addition to integrated prophage DNA unintegrated monomer phage DNA with unligated cos sites. The genetic relatedness of phi SFi21 DNA with DNA from lytic phages was studied in dot blot and Southern blot hybridization by using individual restriction fragments of phi SFi21 DNA as probes. Lytic group I phages hybridized with fragments of the central and the right part of the phi SFi21 genome but failed to hybridize with a fragment joining both parts. Lytic group II phages showed hybridization with the right half of the phi SFi21 genome. In lytic group IV phages, biologically a heterogeneous group, many different combinations of cross hybridization were detected in accordance with the hypothesis of the modular evolution of phage genomes. (C) 1995 Academic Press, Inc.