Duplication of the β-galactosidase gene in some Lactobacillus plantarum strains

Curing of a plasmid that encoded a β-galactosidase gene (β-gal) from the Lactobacillus plantarum strain of dairy origin LL441 was not accompanied by complete loss of the lactose utilization phenotype. DNA–DNA hybridization, using an internal fragment of the β-gal gene as a probe, revealed a second determinant located on the chromosome of the cured derivatives. The chromosomal copy was present in all of a series of β-Gal+ L. plantarum and Lactobacillus pentosus strains from different origins. In addition, four other L. plantarum strains harboured plasmid encoded β-gal genes as well. Since both sequences cross-hybridized and present a similar genetic organization, it is postulated that the plasmid copy was generated through gene duplication and, probably, selected by growth of the strains in lactose rich environments.