Electron microscopic heteroduplex study and restriction endonuclease cleavage analysis of the DNA genomes of three lactic streptococcal bacteriophages.

Three lactic streptococcal bacteriophages were compared with one another by electron microscopic analysis of heteroduplex DNA molecules. The phages were almost identical in morphology and had been isolated over a period of 10 years on different strains of Streptococcus cremoris from cheese plants situated in different parts of New Zealand. There was a high degree of homology between the DNAs, in agreement with Southern blot hybridization data reported earlier. There were, however, distinct regions of nonhomology, mostly between 0.45 and 1.71 kilobases in length, suggestive of the occurrence of block recombination events. A deletion of 2.23 kilobases in the two more recently isolated phages, or an insertion in the first isolate, was found. All three phage DNAs showed differences in restriction endonuclease cleavage sites. Alignment of the restriction endonuclease maps with the heteroduplex maps showed that differences in cleavage sites occurred most frequently in regions of nonhomology. However, differences in cleavage sites in regions of apparent homology were also detected, indicating that point mutations may have occurred in addition to block recombination events.