Applied and Environmental Microbiology, January 2000, p. 42-48, Vol. 66, No. 1
0099-2240/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
-Lyase

Microbial Ingredients Section, NIZO food research, Ede,1 and Wageningen Centre for Food Sciences, Wageningen,2 The Netherlands
Received 6 July 1999/Accepted 12 October 1999
The enzymatic degradation of amino acids in cheese is believed to
generate aroma compounds and therefore to be essential for flavor
development. Cystathionine
-lyase (CBL) can convert cystathionine to
homocysteine but is also able to catalyze an
,
elimination. With
methionine as a substrate, it produces volatile sulfur compounds which
are important for flavor formation in Gouda cheese. The metC gene, which encodes CBL, was cloned from 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 downstream cysK gene,
which encodes a putative cysteine synthase. The MetC proteins of both
strains were overproduced in strain MG1363 with the NICE
(nisin-controlled expression) system, resulting in a >25-fold increase
in cystathionine lyase activity. A disruption of the metC
gene was achieved in strain MG1363. Determination of enzymatic
activities in the overproducing and knockout strains revealed that MetC
is essential for the degradation of cystathionine but that at
least one lyase other than CBL contributes to methionine degradation via
,
elimination to form volatile aroma compounds.
Present address: Nestlé Research Center,
Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland.
Present address: Dept. of Genetics, University of Groningen,
Haren, The Netherlands.
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