Nonidentity between plasmid and chromosomal copies of ISS 1-like sequences in Lactococcus lactis subsp. lactis CNRZ270 and their possible role in chromosomal integration of plasmid genes †

Abstract

The nucleotide sequence of an insertion sequence (IS) observed during mating experiments using the lactose-protease plasmid, pUCL22, of Lactococcus (Lc.) lactis subsp. lactis CNRZ270, was found to be similar to that of ISS 1 from Lc. lactis subsp. lactis ML3. The IS was named ISS 1RS. The chromosome of this strain contains several copies of ISS 1-like IS as assessed by hybridization. One of these copies was cloned and named ISS 1CH. Its sequence differs from that of the plasmid-borne copy, and appears to be more closely related to ISS 1N from Lc. lactis subsp. cremoris SK11. This suggests independent introduction of both ISS 1 elements. Moreover, the observation of plasmid genes integrated in the CNRZ270 chromosome near ISS 1CH suggests that their presence is the result of integration by a Campbell mechanism using both IS homologies. ISS 1-like sequences were also found on plasmids of numerous Lc. lactis strains, as well as one out of seven Lactobacillus (Lb.) casei and one out of three Lb. plantarum strains examined.