Genetic evidence for a defective xylan degradation pathway in Lactococcus lactis

Genetic and biochemical evidence for a defective xylan degradation pathway was found linked to the xylose operon in three lactococcal strains, Lactoco ccus lactis 210, L. lactis IO-1, and L, lactis NRRL B-4449. Immediately dow nstream of the xylulose kinase gene (xylB) (K, A. Erlandson, J.-H, Park, W, El Khal, H.-H. Kao, P, Basaran, S, Brydges, and C, A. Batt, Appl, Environ, Microbiol, 66:3974-3980, 1999) are two open reading frames encoding a muta rotase (xylM) and a xyloside transporter (xynT) and a partial open reading frame encoding a beta -xylosidase (xynB), These are functions previously un reported for lactococci or lactobacilli. The mutarotase activity of the put ative xylM gene product was confirmed by overexpression of the L. lactis en zyme in Escherichia coli and purification of recombinant XylM. We hypothesi ze that the mutarotase links xylan degradation to xylose metabolism due to the anomeric preference of xylose isomerase. In addition, Northern hybridiz ation experiments suggested that the xylM and xynTB genes are cotranscribed with the xylRAB genes, responsible for xylose metabolism. Although none of the three strains appeared to metabolize xylan or xylobiose, they exhibite d xylosidase activity, and L, lactis IO-1 and L. lactis NRRL, B-4449 had fu nctional mutarotases.