A SHORT NONCODING VIRAL-DNA ELEMENT SHOWING CHARACTERISTICS OF A REPLICATION ORIGIN CONFERS BACTERIOPHAGE RESISTANCE TO STREPTOCOCCUS-THERMOPHILUS

A 302-bp noncoding DNA fragment from the DNA replication module of pha ge phi Sfi21 was shown to protect the Streptococcus thermophilus strai n Sfi1 from infection by 17 of 25 phages. The phage-inhibitory DNA pos sesses two determinants, each of which individually mediated phage res istance. The phage-inhibitory activity was copy number dependent and o perates by blocking the accumulation of phage DNA. Furthermore, when c loned on a plasmid, the phi Sfi21 DNA acts as an origin of replication driven by phage infection. Protein or proteins in the phi Sfi21-infec ted cells were shown to interact with this phage-inhibitory DNA fragme nt, 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 phi Sfi19, a phage that is insensitive to the inhibitory activity of the phi Sfi21-derived DNA, led to the characterisation of a variant putati ve phage replication origin that differed in 14 of 302 nucleotides fro m that of phi Sfi21. The variant origin was cloned and exhibited an in hibitory activity toward phages that were insensitive to the phi Sfi21 -derived DNA. (C) 1998 Academic Press