A novel autophosphorylation mediated regulation of nitrite reductase in Candida utilis

Nitrite metabolism in Debaryomyces hansenii TOB-Y7, a yeast strain involved in tobacco fermentation

The Italian cigar manufacturing process includes a fermentation step that leads to accumulation of nitrite and tobacco-specific nitrosamines (TSNA), undesirable by-products due to their negative impact on health. In this study, growth and biochemical properties of Debaryomyces hansenii TOB-Y7, a yeast strain that predominates during the early phase of fermentation, have been investigated. With respect to other D. hansenii collection strains (Y7426, J26, and CBS 1796), TOB-Y7 was characterized by the ability to tolerate very high nitrite levels and to utilize nitrite, but not nitrate, as a sole nitrogen source in a chemically defined medium, a property that was enhanced in microaerophilic environment. The ability to assimilate nitrite was associated to the presence of YNI1, the gene encoding the assimilatory NAD(P)H:nitrite reductase (NiR), absent in Y7426, J26, and CBS 1796 by Southern blot data. YNI1 from TOB-Y7 was entirely sequenced, and its expression was analyzed in different media by Northern blot and reverse transcriptase polymerase chain reaction. The evidence that, in D. hansenii TOB-Y7, YNI1 was transcriptional active also in the presence of high ammonia concentration typical of tobacco fermentation, stimulated the development of an improved process that, on a laboratory scale, was proved to be effective in minimizing nitrite and TSNA accumulation.

Down-regulation of eukaryotic nitrate transporter by nitrogen-dependent ubiquitinylation

In the yeast Hansenula polymorpha, the YNT1 gene encodes the high affinity nitrate transporter, which is repressed by reduced nitrogen sources such as ammonium or glutamine. Ynt1 protein is degraded in response to glutamine in the growth medium. Ynt1 disappears independently of YNT1 glutamine repression as shown in strains where YNT1 repression is abolished. Ynt1-green fluorescent protein chimera and a mutant defective in vacuolar proteinase A (deltapep4) showed that Ynt1 is degraded in the vacuole in response to glutamine. The central hydrophilic domain of Ynt1 contains PEST-like sequences whose deletion blocked Ynt1 down-regulation. Site-directed mutagenesis showed that Lys-253 and Lys-270, located in this sequence, were involved in internalization and subsequent vacuolar degradation of Ynt1. Ynt1-ubiquitin conjugates were induced by glutamine and not nitrate. We conclude that glutamine triggers Ynt1 down-regulation via ubiquitinylation of lysines in the central hydrophilic domain, and proteolysis in the vacuole.

Assimilation of nitrate by yeasts

Nitrate assimilation has received much attention in filamentous fungi and plants but not so much in yeasts. Recently the availability of classical genetic and molecular biology tools for the yeast Hansenula polymorpha has allowed the advance of the study of this metabolic pathway in yeasts. The genes YNT1, YNR1 and YNI1, encoding respectively nitrate transport, nitrate reductase and nitrite reductase, have been cloned, as well as two other genes encoding transcriptional regulatory factors. All these genes lie closely together in a cluster. Transcriptional regulation is the main regulatory mechanism that controls the levels of the enzymes involved in nitrate metabolism although other mechanisms may also be operative. The process involved in the sensing and signalling of the presence of nitrate in the medium is not well understood. In this article the current state of the studies of nitrate assimilation in yeasts as well as possible venues for future research are reviewed.

Cloning, sequencing, and expression of H.a.YNR1 and H.a.YNI1, encoding nitrate and nitrite reductases in the yeast Hansenula anomala

A single Hansenula anomala genomic DNA fragment containing the genes H.a.YNR1 (yeast nitrate reductase) and H.a.YNI1 (yeast nitrite reductase) encoding nitrate and nitrite reductase, respectively, was isolated from a lambda EMBL3 genomic DNA library. As probe, a 3.2 kb DNA fragment isolated from a lambda gt11 H. anomala genomic DNA library screened with antiserum anti-NR from H. anomala was used. H. a.YNR1 and H.a.YNI1 genes are separated by 473 bp and encode putative proteins of 870 and 1077 amino acids, respectively, with great similarity to nitrate and nitrite reductases from other organisms. Northern blot analysis revealed that both genes are highly expressed in nitrate, very low in nitrate plus ammonium, and no expression was detected in ammonium or nitrogen-free media. Levels of nitrate reductase and nitrite reductase were very low or undetectable by Western blot analysis in nitrogen-free and ammonium media, whereas both proteins were present in nitrate and ammonium plus nitrate media. The nucleotide sequence Accession No. is AF123281.