Characterization of the lysogeny DNA module from the temperate Streptococcus thermophilus bacteriophage φSfi21

Abstract

Phage φSfi21, the only temperate Streptococcus thermophilus phage from our phage collection, showed extensive DNA homology with virulent phages from lytic group I Southern blot hybridizations demonstrated that the φSfi21- specific DNA was clustered in an approximately 6.6-kb-long region, the putative lysogeny module. Sequence analysis and database research identified an integrase within this module; orf 203 with homology to an anonymous off 258 from the tempera'e lactococcal phage BK5-T; orf 127 and off 122 with weak homology to the N- and C-terminal parts, respectively, of the cl-like repressor from lactococcal phages Tuc2009 and BK5-T; orf 75 with homology to a repressor protein from lambdoid phage 434 and an anti repressor ant with homology to phage P1. The molecular arrangement of the predicted orfs in phage φSfi21 was very similar to that of the lactococcal phage BKS-T. The transition from φSfi21-specific DNA into DNA shared with virulent phages was abrupt and flanked at one side by notable DNA repeats. Sequence analysis identified a holin protein to the left of the lysogeny module. A site- specific deletion of 2.4 kb, which reproducibly transformed φSfi21 into a lytic phage, was localized in the lysogeny module. It was flanked at both sides by conspicuous DNA repeats. One repeat region reflected the DNA around the attP site, while the other reflected the putative genetic switch region between repressor and anti-repressor genes S. thermophilus host Sfi 1 transformed with a plasmid containing int and off 203 showed resistant e to superinfection by heterologous phages, but not by the homologous φSfi21. Part of the int gene could be deleted without loss of this activity, while a deletion in orf 203 resulted in loss of the phage resistance. We speculate on the possibility of a bipartite immunity system for the control of lysogeny in φSfi21.