Molecular and functional analyses of the metC gene of Lactococcus lactis, encoding cystathionine beta-lyase

The enzymatic degradation of amino acids in cheese is believed to generate aroma compounds and therefore to be essential for flavor development. Cysta thionine beta-lyase (CBL) can convert cystathionine to homocysteine but is also able to catalyze an alpha,gamma elimination. With methionine as a subs trate, it produces volatile sulfur compounds which are important for flavor formation in Gouda cheese. The metC gene, which encodes CBL, was cloned fr om the Lactococcus lactis model strain MG1363 and from strain B78, isolated from a cheese starter culture and known to have a high capacity to produce volatile compounds. The metC gene was found to be cotranscribed with a dow nstream cysK gene, which encodes a putative cysteine synthase. The MetC pro teins of both strains were overproduced in strain MG1363 with the NICE (nis in-controlled expression) system, resulting in a >25-fold increase in cysta thionine lyase activity. A disruption of the metC gene was achieved in stra in MG1363. Determination of enzymatic activities in the overproducing and k nockout strains revealed that MetC is essential for the degradation of cyst athionine but that at least one lyase other than CBL contributes to methion ine degradation via alpha,gamma elimination to form volatile aroma compound s.