Edinburgh Napier University

  A 302-bp noncoding DNA fragment from the DNA replication module of phage φSfi21 was shown to protect theStreptococcus thermophilusstrainSfi1 from infection by 17 of 25 phages. The phage-inhibitory DNA possesses two determinants, each of which individually mediated phage resistance. The phage-inhibitory activity was copy number dependent and operates by blocking the accumulation of phage DNA. Furthermore, when cloned on a plasmid, the φSfi21 DNA acts as an origin of replication driven by phage infection. Protein or proteins in the φSfi21-infected cells were shown to interact with this phage-inhibitory DNA fragment, forming a retarded protein–DNA complex in gel retardation assays. A model in which phage proteins interact with the inhibitory DNA such that they are no longer available for phage propagation can be used to explain the observed bacteriophage resistance. Genome analysis of φSfi19, a phage that is insensitive to the inhibitory activity of the φSfi21-derived DNA, led to the characterisation of a variant putative phage replication origin that differed in 14 of 302 nucleotides from that of φSfi21. The variant origin was cloned and exhibited an inhibitory activity toward phages that were insensitive to the φSfi21-derived DNA.