Complete Sequence of Plasmid pLH1 from Lactobacillus helveticus ATCC15009: Analysis Reveals the Presence of Regions Homologous to Other Native Plasmids from the Host Strain

The complete sequence for plasmid pLH1 from Lactobacillus helveticus ATCC15009 has beendetermined. Analysis of the 19,360-bp primary sequence revealed a putative replication origin andinitiation protein, information that could provide the basis for the construction of cloning vectors forL. helveticus. Evidence that pLH1 is theta-replicating could be deduced from the plasmid size, fromthe homology to the replication protein of the Bacillus natto theta-replicating plasmid pLS32, andfrom the identification of a putative resolvase gene (orf-195). Although 14 open reading framescapable of encoding polypeptides longer than 100 amino acids were identified, none, on the basis ofhomology with known sequences, appeared to encode a well-characterized trait relevant to milkfermentation. Plasmid pLH1 revealed regions of identity with the smaller cryptic plasmids (pLH2 andpLH3) from the same strain and with other tracts of DNA, including insertion sequence elements,from a variety of other lactic acid bacteria. The presence of such regions provides a basis fordeveloping an explanation of the phenotypic variability observed in these bacteria. The plasmid alsoappears to possess a number of genetic elements present in other lactic acid bacterial plasmids,conservation of which would be consistent with an important functional or evolutionary role. It couldbe argued that the plasmid complement of L. helveticus ATCC15009 consists of parasitic entitiesconcerned only with their own replication and survival