title: Characterization of genes involved in the metabolism of alpha-galactosides by Lactococcus raffinolactis.
abstract: Lactococcus raffinolactis, unlike most lactococci, is able to ferment alpha-galactosides, such as melibiose and raffinose. More than 12 kb of chromosomal DNA from L. raffinolactis ATCC 43920 was sequenced, including the alpha-galactosidase gene and genes involved in the Leloir pathway of galactose metabolism. These genes are organized into an operon containing aga (alpha-galactosidase), galK (galactokinase), and galT (galactose 1-phosphate uridylyltransferase). Northern blotting experiments revealed that this operon was induced by galactosides, such as lactose, melibiose, raffinose, and, to a lesser extent, galactose. Similarly, alpha-galactosidase activity was higher in lactose-, melibiose-, and raffinose-grown cells than in galactose-grown cells. No alpha-galactosidase activity was detected in glucose-grown cells. The expression of the aga-galKT operon was modulated by a regulator encoded by the upstream gene galR. The product of galR belongs to the LacI/GalR family of transcriptional regulators. In L. lactis, L. raffinolactis GalR acted as a repressor of aga and lowered the enzyme activity by more than 20-fold. We suggest that the expression of the aga operon in lactococci is negatively controlled by GalR and induced by a metabolite derived from the metabolism of galactosides.
authors.affiliation: Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie, Groupe de Recherche en Ecologie Buccale (GREB), Faculté de Médecine Dentaire, Université Laval, Québec, Canada G1K 7P4.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Boucher
authors.forename: Christian
authors.initials: C
authors.lastname: Vadeboncoeur
authors.forename: Sylvain
authors.initials: S
authors.lastname: Moineau
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D005697:Galactosides / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D017045:Lactococcus / Q000201:enzymology* / Q000235:genetics; D008969:Molecular Sequence Data; D009876:Operon; D017422:Sequence Analysis, DNA; D000519:alpha-Galactosidase / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D005697:Galactosides; D000519:alpha-Galactosidase