Affiliation:
1. Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Abstract
ABSTRACT
In most bacteria, inorganic sulfur is assimilated into cysteine, which provides sulfur for methionine biosynthesis via transsulfurylation. Here, cysteine is transferred to the terminal carbon of homoserine via its sulfhydryl group to form cystathionine, which is cleaved to yield homocysteine. In the enteric bacteria
Escherichia coli
and
Salmonella enterica
, these reactions are catalyzed by irreversible cystathionine-γ-synthase and cystathionine-β-lyase enzymes. Alternatively, yeast and some bacteria assimilate sulfur into homocysteine, which serves as a sulfhydryl group donor in the synthesis of cysteine by reverse transsulfurylation with a cystathionine-β-synthase and cystathionine-γ-lyase. Herein we report that the related enteric bacterium
Klebsiella pneumoniae
encodes genes for both transsulfurylation pathways; genetic and biochemical analyses show that they are coordinately regulated to prevent futile cycling.
Klebsiella
uses reverse transsulfurylation to recycle methionine to cysteine during periods of sulfate starvation. This
m
ethionine-
t
o-
c
ysteine (
mtc
) transsulfurylation pathway is activated by cysteine starvation via the CysB protein, by adenosyl-phosphosulfate starvation via the Cbl protein, and by methionine excess via the MetJ protein. While
mtc
mutants cannot use methionine as a sulfur source on solid medium, they will utilize methionine in liquid medium via a sulfide intermediate, suggesting that an additional nontranssulfurylation methionine-to-cysteine recycling pathway(s) operates under these conditions.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
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