Assimilation of nitrate by yeasts

Abstract

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.

Down-regulation of Eukaryotic Nitrate Transporter by Nitrogen-dependent Ubiquitinylation

“…Further evidence for posttranscriptional regulation of Ynt1 was compiled using H. polymorpha ⌬ure2 mutant, where NCR of nitrate assimilation genes is abolished. 5 A similar S. cerevisiae mutant has been described in which NCR control of nitrogen assimilation genes is lost (6). The WT and ⌬ure2 strains were incubated in a medium containing nitrate or nitrate plus ammonium (Fig.…”

“…This pathway demands a high investment of energy per mole of nitrate assimilated because it must be reduced to ammonium to be utilized. As a result, most organisms restrict the use of nitrate to those environments not containing "preferred nitrogen sources" such as ammonium, asparagine, or glutamine (1)(2)(3)(4)(5). Furthermore, organisms have evolved molecular mechanisms, termed Nitrogen Catabolite Repression (NCR), 4 by which preferred nitrogen sources repress genes involved in the assimilation of non-preferred nitrogen sources (4 -6).…”

“…Isolation of genes involved in assimilation of nitrate and construction of the corresponding null mutants makes the yeast Hansenula polymorpha a valuable tool for studying different issues related to nitrate assimilation (5). This yeast presents a single high affinity bispecific nitrate/ nitrite transporter called Ynt1 that contains 12 putative transmembrane domains (7,8).…”

“…This yeast presents a single high affinity bispecific nitrate/ nitrite transporter called Ynt1 that contains 12 putative transmembrane domains (7,8). YNT1, YNR1 and YNI1, encoding, respectively, nitrate and nitrite reductases, are induced by nitrate and repressed by the preferred nitrogen sources ammonium or glutamine (5). Ynt1 has been included in the NNP (nitrate/nitrite porter) family that is involved in high affinity nitrate and/or nitrite transport (9) and is a member of the Major Facilitator Superfamily (10).…”

Characterization and expression analysis of a gene cluster for nitrate assimilation from the yeast Arxula adeninivorans

“…Since the classical model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe do not assimilate nitrate, nitrate assimilation in yeasts has not received much attention. However, with the biotechnological application of species such as Hansenula polymorpha and Candida utilis, and the complete sequencing of the H. polymorpha genome, this situation is now changing (Sengupta et al, 1996;Ramezani-Rad et al, 2003;Siverio, 2002).…”