Nitrogen metabolite repression in Hansenula polymorpha : the nmr1-1 mutation

Abstract

In Hansenula polymorpha, the expression of the nitrate assimilation metabolism is subjected to re-pression-derepression mechanisms triggered by reduced nitrogen compounds such as ammonium. To further our knowledge on the genetics of these regulatory mechanisms, a screening strategy for the isolation of mutants exhibiting nitrate reductase activities in the presence of reduced nitrogen compounds was set up. This strategy makes use of a nitrate+ methylamine mutant to isolate suppressors of its characteristic phenotype--the inability to grow on a nitrate plus methylamine medium. A total of 21 regulatory mutants were isolated with this strategy and grouped into five complementation classes. One of these mutants harbours the recessive mutation nmr1-1, which determines the derepression of the nitrate assimilation metabolism in media containing nitrate plus a repressing nitrogen source (ammonium, methylamine, glutamate, urea or aspartate). Therefore, nitrate reductase activities are detected in the presence of reduced nitrogen sources, as long as nitrate is also in the medium. Our data indicate that the processes of repression-derepression and induction are controlled by elements which are distinct. Furthermore, they indicate that Nmrlp is involved in repressing circuits which control not only the nitrate-utilisation pathway, but also other pathways which are necessary for the utilisation of nitrogen sources alternative to ammonium. Of considerable interest is the fact that our nmr1-1 mutant is derepressed in glutamate but not in glutamine. Since the phenotype of this mutant seems to exclude a glutamine synthetase defect, we suggest that glutamate (or a derivative of this compound) might be involved in signalling nitrogen metabolite repression in H. polymorpha. Thus, in H. polymorpha, a glutamine-dependent circuit may co-exist with a glutamine-independent circuit.

AREA directly mediates nitrogen regulation of gibberellin biosynthesis in Gibberella fujikuroi, but its activity is not affected by NMR

“…In A. nidulans , glutamine, and to a less extent glutamate and ammonium, were shown to be the most sufficient signal components (Margelis et al ., 2001). In Hansenula polymorpha , a putative nmr mutant was obtained which is derepressed only by glutamate but not by glutamine (Serrani et al ., 2001). However, the nmr gene of this organism is not yet available to confirm these results at the molecular level.…”

Assimilation of nitrate by yeasts

“…N. crassa NMR1, the orthologue of Ure2 in S. cerevisiae , a central protein in the control of nitrogen metabolism in this organism [58], interacts with GLN3 and GAT1 products. H. polymorpha mutants defective in NMR1 (formally HpURE2 ) have been isolated [60].…”

and , two new involved in nitrogen metabolite repression

“…To access adequate nutrient supply, cells have evolved sophisticated signal transduction pathways that sense different environmental factors, such as nitrogen compounds. The targets of these transduction pathways are often transcriptional factors that mediate the tuning of gene expression, in order to maximise organisms’ fitness in a given environment [1].…”

“…Pichia angusta ) is able to use various compounds as nitrogen source, including ammonium, methylamine, glutamate and glutamine, nitrate and nitrite. H. polymorpha nitrate assimilation is subject to a dual control: nitrogen metabolite repression, triggered by reduced nitrogen compounds (e.g., ammonia, glutamine and glutamate) and specific induction mechanisms, elicited by nitrate itself [53,1]. In this yeast, nitrate assimilation is similar to that in other organisms.…”