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J. Bacteriol., 09 1995, 5035-5039, Vol 177, No. 17
E Zdych, R Peist, J Reidl and W Boos
The Escherichia coli maltose system consists of a number of genes whose
products are involved in the uptake and metabolism of maltose and
maltodextrins. MalT is the central positive gene activator of the regulon
and is, together with the cyclic AMP-catabolite gene activator protein
system, necessary for the expression of the maltose genes. Expression of
malY, a MalT-independent gene, leads to the repression of all
MalT-dependent genes. We have purified MalY to homogeneity and found it to
be a pyridoxal-5-phosphate-containing enzyme with the enzymatic activity of
a beta C-S lyase (cystathionase). MalY is a monomeric protein of 42,000 to
44,000 Da. Strains expressing MalY constitutively abolish the methionine
requirement of metC mutants. The enzymatic activity of MetC, the cleavage
of cystathionine to homocysteine, ammonia, and pyruvate, can be catalyzed
by MalY. However, the cystathionase activity is not required for the
function of MalY in repressing the maltose system. By site-directed
mutagenesis, we changed the conserved lysine residue at the pyridoxal
phosphate binding site (position 233) of MalY to isoleucine. This abolished
beta C-S lyase activity but not the ability of the protein to repress the
maltose system. Also, the overexpression of plasmid-encoded metC did not
affect mal gene expression, nor did the deduced amino acid sequence of MetC
show homology to that of MalY.
Copyright © 1995, American Society for Microbiology
MalY of Escherichia coli is an enzyme with the activity of a beta C-S lyase (cystathionase)
Department of Biology, University of Konstanz, Germany.
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