SITE-SPECIFIC SPONTANEOUS DELETIONS IN 3 GENOME REGIONS OF A TEMPERATE STREPTOCOCCUS-THERMOPHILUS PHAGE

Site-specific spontaneous deletions were observed with high frequency in three regions of the genome of the temperate Streptococcus thermoph ilus phage phi SFi21. Deletion sizes were 750 bp (type 1), 2.7 kb (typ e 2), and 1 kb(type 3). Combinations of types 1 and 3 and 2 and 3 were observed. The mutants grew lytically although with reduced burst size s. Type 2 mutants lost the capacity to lysogenize host cells. Upon ser ial passage, the deletion mutants overgrew the wild-type phage. No dir ect or inverted DNA repeats were associated with type 1 or 2 deletion sites. Several independent phage isolates showed deletions at identica l nucleotide positions, suggesting a site-specific recombination syste m. Sequencing of an Xbal restriction fragment covering the type 1 dele tion predicted a single long open reading frame (ORF) showing a high d egree of amino acid similarity with two proteins from bacteriophage P1 implicated in its immunity control (KilA, Ant). Type 1 deletion leads to a loss of the conserved C-terminal part of this ORF. (C) 1996 Acad emic Press, Inc.