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Microbiol. Mol. Biol. Rev., 12 1997, 503-532, Vol 61, No. 4
D Thomas and Y Surdin-Kerjan
Sulfur amino acid biosynthesis in Saccharomyces cerevisiae involves a large
number of enzymes required for the de novo biosynthesis of methionine and
cysteine and the recycling of organic sulfur metabolites. This review
summarizes the details of these processes and analyzes the molecular data
which have been acquired in this metabolic area. Sulfur biochemistry
appears not to be unique through terrestrial life, and S. cerevisiae is one
of the species of sulfate-assimilatory organisms possessing a larger set of
enzymes for sulfur metabolism. The review also deals with several enzyme
deficiencies that lead to a nutritional requirement for organic sulfur,
although they do not correspond to defects within the biosynthetic pathway.
In S. cerevisiae, the sulfur amino acid biosynthetic pathway is tightly
controlled: in response to an increase in the amount of intracellular S-
adenosylmethionine (AdoMet), transcription of the coregulated genes is
turned off. The second part of the review is devoted to the molecular
mechanisms underlying this regulation. The coordinated response to AdoMet
requires two cis-acting promoter elements. One centers on the sequence
TCACGTG, which also constitutes a component of all S. cerevisiae
centromeres. Situated upstream of the sulfur genes, this element is the
binding site of a transcription activation complex consisting of a basic
helix-loop-helix factor, Cbf1p, and two basic leucine zipper factors, Met4p
and Met28p. Molecular studies have unraveled the specific functions for
each subunit of the Cbf1p-Met4p- Met28p complex as well as the modalities
of its assembly on the DNA. The Cbf1p-Met4p-Met28p complex contains only
one transcription activation module, the Met4p subunit. Detailed mutational
analysis of Met4p has elucidated its functional organization. In addition
to its activation and bZIP domains, Met4p contains two regulatory domains,
called the inhibitory region and the auxiliary domain. When the level of
intracellular AdoMet increases, the transcription activation function of
Met4 is prevented by Met30p, which binds to the Met4 inhibitory region. In
addition to the Cbf1p-Met4p-Met28p complex, transcriptional regulation
involves two zinc finger-containing proteins, Met31p and Met32p. The
AdoMet-mediated control of the sulfur amino acid pathway illustrates the
molecular strategies used by eucaryotic cells to couple gene expression to
metabolic changes.
Copyright © 1997, American Society for Microbiology
Metabolism of sulfur amino acids in Saccharomyces cerevisiae [In Process Citation]
Centre de Genetique Moleculaire, CNRS, Gif sur Yvette, France.
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