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A. Javelle et al.
as two other genes encoding transcriptional regulatory factors. Transcriptional
regulation is the main regulatory mechanism that controls the levels of
the enzymes involved in nitrate metabolism (Siverio 2002). The genetic and
molecular bases of repression and induction have been studied in detail in
Aspergillus nidulans and Neurospora crassa (Scazzocchio and Arst 1989; Caddick
et al. 1994; Marzluf 1997).
In both species, nitrate induction is mediated by a pathway-specific regulatory
gene (nirA and nit-4 in, respectively Aspergillus nidulans and Neurospora
crassa), whose product binds to the promoters of the nitrate pathway genes
when NO 3 – is present in the culture medium. Similarly, derepression is mediated
by a wide-domain regulatory gene (respectively areA and nit-2), which
encodes a GATA DNA-binding protein. Both areA and nit-2 are responsible, at
least in part, for the derepression, when ammonium is absent, of several other
genes involved in the use of other nitrogen sources, such as several amino
acids or proteins.
In Neurospora crassa and Aspergillus nidulans, glutamine appears to be the
critical metabolite which exerts nitrogen catabolite repression (Chang and
Marzluf 1979; Premakumar et al. 1979). Ammonia leads to strong nitrogen
repression in these fungi, but is not itself active, since it does not cause repression
in mutants lacking glutamine synthetase (Premakumar et al. 1979). Intracellular
glutamine, or possibly a metabolite derived from it, leads to repression,
but the cellular location of the glutamine pool responsible for this
control response, e.g., cytoplasmic or vacuolar, is unknown. An extremely
important, but still unknown feature is the identity of the element or signal
pathway system that senses the presence of repressing levels of glutamine. It is
conceivable that the AREA, NIT2, GLN3, and similar global regulators themselves
bind glutamine or that a complex such as a NIT2-NMR heterodimer
recognizes the amino acid. However, it is also possible that an as yet unidentified
factor detects glutamine and conveys the repression signal to the global
activating proteins. Thus, an important goal for future research is the creative
use of genetic and biochemical approaches to identify the signalling system
that recognizes and processes environmental nitrogen cues.
In the ectomycorrhizal fungus Hebeloma cylindrosporum, transcription of
nar1 coding for the NR protein, was repressed in the presence of ammonium,
suggesting that the organism might possess a gene homologous to nit-2 in
Neurospora crassa. According to Jargeat et al. (2000), inspection of the
sequences flanking the NR genes cloned from Hebeloma cylindrosporum
revealed that they contain several GATA elements to which regulatory GATA
proteins could bind.
In Neurospora crassa, expression of structural genes which encode the
nitrate assimilatory enzymes also has an absolute requirement for nitrate
induction mediated by a pathway-specific factor, NIT4 (or NIRA in
Aspergillus nidulans; Marzluf, 1997). The Neurospora crassa NIT4 protein is
composed of 1090 amino acids and contains at its amino terminus a Cys 6 /Zn 2