LACTOBACILLUS-HILGARDII PLASMID PLAB1000 CONSISTS OF 2 FUNCTIONAL CASSETTES COMMONLY FOUND IN OTHER GRAM-POSITIVE ORGANISMS

A Lactobacillus hilgardii plasmid, pLAB1000, was studied to understand the organization of autonomous replicons from lactobacilli. Two cassettes could be identified. First, the replication region consisted of a sequence coding for a replication protein (Rep) and its corresponding target site, similar to those from plasmids pUB110, pC194 (Staphylococcus aureus), pFTB14, pBAA1 (Bacillus sp.), and pLP1 (Lactobacillus sp.). Sequence analysis indicated the possible synthesis of an antisense RNA that might regulate Rep production. The results also suggested that pLAB1000 replicated via a single-stranded DNA intermediate, and a putative lagging-strand initiation site was found that had similarities to those of α3, St-1, and G4 isometric bacteriophages. The second cassette of pLAB1000 consisted of a sequence coding for a putative mobilization protein (Mob) and its corresponding RS(A) site. This cassette was similar to those found in pT181, pUB110, pE194 (S. aureus), and pGI2 (Bacillus sp.), and it was found to be conserved among different Lactobacillus plasmid replicons. The origins and evolution of these functional cassettes are also discussed.