Characterization of genes involved in the metabolism of α-galactosides by Lactococcus raffinolactis

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 I-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 GaIR 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 GaIR and induced by a metabolite derived from the metabolism of galactosides.