Dissolution of xylose metabolism in Lactococcus lactis

Xylose metabolism, a variable phenotype in strains of Lactococcus lactis, w as studied and evidence was obtained for the accumulation of mutations that inactivate the xyl operon, The xylose metabolism operon (xylRAB) was seque nced from three strains of lactococci, Fragments of 4.2, 4.2, and 5.4 kb th at included the xyl lotus were sequenced from L. lactis subsp, lactis B-444 9 (formerly Lactobacillus xylosus), L, lactis subsp, lactis IO-1, and L. la ctis subsp. lactis 210, respectively. The two environmental isolates, L. la ctis B-4449 and L. lactis IO-1, produce active xylose isomerases and xylulo kinases and can metabolize xylose. L. lactis 210, a dairy starter culture s train, has neither xylose isomerase nor xylulokinase activity and is Xyl(-) . Xylose isomerase and xylulokinase activities are induced by xylose and re pressed by glucose in the two Xyl(+) strains. Sequence comparisons revealed a number of point mutations in the xylA, xylB, and xglR genes in L. lactis 210, IO-1, and B-4449. None of these mutations, with the exception of a pr emature stop codon in xylB, are obviously lethal, since they lie outside of regions recognized as critical for activity. Nevertheless, either cumulati vely or because of indirect affects on the structures of catalytic sites, t hese mutations render some strains of L. lactis unable to metabolize xylose .