Characterization of plasmid pVT745 isolated from Actinobacillus actinomycetemcomitans

A 25.1-kb plasmid, pVT745, was isolated from Actinobacillus actinomycetemcomitans strain VT745. This plasmid hybridized under stringent conditions to chromosomal DNA from 15 A. actinomycetemcomitans isolates obtained from geographically diverse regions of the United States. Southern blot analyses of HincII-digested A. actinomycetemcomitans genomic DNA revealed five strain-specific patterns of hybridization, which clearly indicated that intact pVT745 was not inserted into any of the genomes. Plasmid pVT745 was digested with BamHI and PstI into three, non-overlapping fragments of approximately 7.0, 8.0, and 10.0 kb. The fragments were cloned in Escherichia coli on the low copy number vector pGB2. Although these three fragments exhibited no cross-hybridization, genomic DNA from isolates representing each of the strain-specific patterns hybridized with two or three of the cloned fragments. The results suggest that two or more unique sequences within pVT745 also may be present in genomic DNA obtained from various strains of A. actinomycetemcomitans.