Mol. Cell. Biol., 04 1995, 2321-2330, Vol 15, No. 4
S Xu, DA Falvey and MC Brandriss
The yeast Saccharomyces cerevisiae can use alternative nitrogen sources
such as arginine, urea, allantoin, gamma-aminobutyrate, or proline when
preferred nitrogen sources like glutamine, asparagine, or ammonium ions are
unavailable in the environment. Utilization of alternative nitrogen sources
requires the relief of nitrogen repression and induction of specific
permeases and enzymes. The products of the GLN3 and URE2 genes are required
for the appropriate transcription of many genes in alternative nitrogen
assimilatory pathways. GLN3 appears to activate their transcription when
good nitrogen sources are unavailable, and URE2 appears to repress their
transcription when alternative nitrogen sources are not needed. The
participation of nitrogen repression and the regulators GLN3 and URE2 in
the proline utilization pathway was evaluated in this study. Comparison of
PUT gene expression in cells grown in repressing or derepressing nitrogen
sources, in the absence of the inducer proline, indicated that both PUT1
and PUT2 are regulated by nitrogen repression, although the effect on PUT2
is comparatively small. Recessive mutations in URE2 elevated expression of
the PUT1 and PUT2 genes 5- to 10-fold when cells were grown on a
nitrogen-repressing medium. Although PUT3, the proline utilization pathway
transcriptional activator, is absolutely required for growth on proline as
the sole nitrogen source, a put3 ure2 strain had somewhat elevated PUT gene
expression, suggesting an effect of the ure2 mutation in the absence of the
PUT3 product. PUT1 and PUT2 gene expression did not require the GLN3
activator protein for expression under either repressing or derepressing
conditions. Therefore, regulation of the PUT genes by URE2 does not require
a functional GLN3 protein. The effect of the ure2 mutation on the PUT genes
is not due to increased internal proline levels. URE2 repression appears to
be limited to nitrogen assimilatory systems and does not affect genes
involved in carbon, inositol, or phosphate metabolism or in mating-type
control and sporulation.
Copyright © 1995, American Society for Microbiology
Roles of URE2 and GLN3 in the proline utilization pathway in Saccharomyces cerevisiae
Department of Microbiology and Molecular Genetics, UMD-New Jersey Medical School, Newark 07103.
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