Cryptic plasmids from Lactobacillus helveticus and their evolutionary relationship.

Three different cryptic plasmids from Lactobacillus helveticus have been identified and their DNA sequences determined. Analysis and comparisons of their primary structures revealed stretches of DNA with considerable homology. Thus, large portions of the plasmid non-coding sequences were conserved at 80-90% identity between the different plasmids identified so far in L. helveticus. Nevertheless, different plasmids found in a same host strain utilise different genes of replication, probably acquired during evolution from different replicons from Gram-positive bacterial origins. A remnant structure of such a possible genetic integration of a foreign replication gene into one of the plasmids of L. helveticus was identified.