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Applied and Environmental Microbiology, July 2003, p. 4049-4056, Vol. 69, No. 7
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.7.4049-4056.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Characterization of Genes Involved in the Metabolism of -Galactosides by Lactococcus raffinolactis

Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie, Groupe de Recherche en Écologie Buccale (GREB), Faculté de Médecine Dentaire, Université Laval, Québec, Canada G1K 7P4

Received 9 December 2002/ Accepted 31 March 2003

Lactococcus raffinolactis, unlike most lactococci, is able to ferment -galactosides, such as melibiose and raffinose. More than 12 kb of chromosomal DNA from L. raffinolactis ATCC 43920 was sequenced, including the -galactosidase gene and genes involved in the Leloir pathway of galactose metabolism. These genes are organized into an operon containing aga (-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, -galactosidase activity was higher in lactose-, melibiose-, and raffinose-grown cells than in galactose-grown cells. No -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.

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