Peptide Utilization Encoded by Lactococcus lactis subsp. lactis SSL135 Chromosomal DNA.

A cloned chromosomal fragment of Lactococcus lactis subsp. lactis SSL135 on plasmid p VS8 in an L. lactis subsp. lactis MG1614 background enabled proteinase-negative strain MG1614 to grow in autoclaved milk. The strain (VS230) did not, however, degrade milk proteins and did not grow in pasteurized milk. In contrast, a strain (VS150) carrying p VS9, the proteinase plasmid of SSL135, in an MG1614 background degraded beta-casein but did not grow in milk. VS230 was shown to utilize peptides produced by VS150 in growth experiments in pasteurized milk preincubated with the latter strain. The peptide utilization phenotype linked with p VS8 was further confirmed by growth of VS230 on tryptic peptide fractions, on which the plasmid-free but otherwise isogenic strain MG1614 failed to grow. Plasmid p VS8 produced 69-, 42-, 38-, and 36-kilodalton proteins, as determined by in vitro transcription-translation. At least three of these proteins affected the peptide utilization phenotype. We suggest that there could be a coupled peptidase-peptide transport system encoded by the chromosomal fragment.