Molecular characterization of the nisin resistance region of Lactococcus lactis subsp. lactis biovar diacetylactis DRC3.
The nisin resistance determinant of Lactococcus lactis subsp. lactis biovar diacetylactis DRC3 was localized onto a 1.3-kb EcoRI-NdeI fragment by subcloning and interrupting the NdeI site by cloning random NdeI fragments into it; the nisin resistance determinant was then sequenced. The nucleotide sequence revealed a large open reading frame containing 318 codons. Putative transcription and translation signal sequences were located directly upstream from the initiation codon. Immediately downstream of the termination codon was a palindromic region resembling a rho-independent termination sequence. This 957-nucleotide open reading frame and its associated transcription and translation signal sequences were cloned into plasmid-free L. lactis subsp. lactis LM0230 and conferred an MIC of 160 IU of nisin per ml. This level of nisin resistance is equivalent to that of the initial nisin-resistant subclone, pFM011, used for further subcloning in this study. The inferred amino acid sequence would result in a protein with a molecular mass of 35,035 Da. This value was in agreement with the molecular mass of a protein detected after in vitro transcription and translation of DNA encoding the nisin resistance gene, nsr. This protein contained a hydrophobic region at the N terminus that was predicted to be membrane associated but did not contain a typical signal sequence cleavage site. No significant homology was detected when the DNA sequence of the nsr gene and the amino acid sequence of its putative product were compared with other available sequences. When subjected to Southern hybridization, a 1.2-kb DraI fragment encoding the nsr gene did not hybridize with the genomic DNA of the nisin-producing strain L. lactis subsp. lactis 11454.