Summary: ØvML3 is a virulent prolate-headed bacteriophage that attacks a genetically well-studied strain family including subsp. strains 712, C2 and ML3. A restriction map was constructed for ØvML3 using a wide variety of restriction endonucleases. The DNA was highly refractory to restriction; this is a common feature of lytic lactococcal phages. Genome size was estimated as 23 kb, which is similar to the sizes of other phages of the same morphological group. The presence of heat-dissociable DNA fragments was noted in gel electrophoresis of restriction enzyme digests. Cohesive ends were confirmed by pretreating the DNA with T4 ligase, which rendered the composite fragment insensitive to heat. The phage ØvML3 genome thus consists of linear and non-permuted double-stranded DNA with complementary cohesive ends. The lysin gene from this phage acts as a specific lysis agent against the lactic streptococci. This gene has been cloned and sequenced previously. Further specific subcloning of RV fragments of ØvML3 DNA has located the lysin gene in the central region of the genome, orientated from right to left. This location was confirmed by hybridization of a lysin gene probe to ØvML3 DNA. The lysin gene probe showed homology to a number of other prolate-headed phages including P001. The lysin gene of P001 was shown to be located in the central region of its genome. However, the isometric phage P107 did not hybridize, in spite of encoding its own lysin gene.


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