GlaR (YugA)—a novel RpiR‐family transcription activator of the Leloir pathway of galactose utilization in Lactococcus lactis IL1403

Bacteria can utilize diverse sugars as carbon and energy source, but the regulatory mechanisms directing the choice of the preferred substrate are often poorly understood. Here, we analyzed the role of the YugA protein (now designated GlaR—Galactose–lactose operon Regulatory protein) of the RpiR family as a transcriptional activator of galactose (gal genes) and lactose (lac genes) utilization genes in Lactococcus lactis IL1403. In this bacterium, gal genes forming the Leloir operon are combined with lac genes in a single so‐called gal–lac operon. The first gene of this operon is the lacS gene encoding galactose permease. The glaR gene encoding GlaR lies directly upstream of the gal–lac gene cluster and is transcribed in the same direction. This genetic layout and the presence of glaR homologues in the closest neighborhood to the Leloir or gal–lac operons are highly conserved only among Lactococcus species. Deletion of glaR disabled galactose utilization and abrogated or decreased expression of the gal–lac genes. The GlaR‐dependent regulation of the gal–lac operon depends on its specific binding to a DNA region upstream of the lacS gene activating lacS expression and increasing the expression of the operon genes localized downstream. Notably, expression of lacS‐downstream genes, namely galMKTE, thgA and lacZ, is partially independent of the GlaR‐driven activation likely due to the presence of additional promoters. The glaR transcription itself is not subject to catabolite control protein A (CcpA) carbon catabolite repression (CRR) and is induced by galactose. Up to date, no similar mechanism has been reported in other lactic acid bacteria species. These results reveal a novel regulatory protein and shed new light on the regulation of carbohydrate catabolism in L. lactis IL1403, and by similarity, probably also in other lactococci.