2 GROUPS OF BACTERIOPHAGES INFECTING STREPTOCOCCUS-THERMOPHILUS CAN BE DISTINGUISHED ON THE BASIS OF MODEL OF PACKAGING AND GENETIC-DETERMINANTS FOR MAJOR STRUCTURAL PROTEINS

A comparative study of 30 phages of Streptococcus thermophilus was per formed based an DNA restriction profiles, DNA homology, structural pro teins, packaging mechanisms, and host range data. All phages exhibited distinct DNA restriction profiles, with some phages displaying simila rly sized restriction fragments. DNA homology was shown to be present among all 30 phages. The phages could be divided into two groups on th e basis of their packaging mechanism as mas derived from the appearanc e of submolar DNA fragments In restriction enzyme digests and the pres ence (cos-containing phages) or absence (pac-containing phages) of coh esive genomic Extremities. Interestingly, the 19 identified cos-contai ning phages possessed two major structural proteins (32 and 26 kDa) in contrast to the remaining 11 pac-containing phages, which possessed t hree major structural proteins (41, 25, and 13 kDa). Southern hybridiz ation demonstrated that all pac-containing phages tested contain homol ogs of the genes encoding the three major structural proteins of the p ac-containing phage O1205, whereas all cos-containing phages tested ex hibit homology to the gene specifying, one of the structural component s of the cos-containing phage Phi 7201, Fifty-seven percent of the pha ges (both cos and pac containing) possessed the previously identified 2.2-kb EcoRI fragment of the temperate S. thermophilus phage Sfi18 (H. Brussow, A. Probst, M. Fremont, and J. Sidoti, Virology 200:854-857, 1994). No obvious correlation was detected between grouping based on p ackaging mechanism and host range data obtained with 39 industrial S. thermophilus strains.