Nitrogen regulation in Saccharomyces cerevisiae.

Ammonium is a nitrogen source supporting growth of yeast cells at an optimal rate. We recently reported the first characterization of an NH4+ transport protein (Mep1p) in Saccharomyces cerevisiae. Here we describe the characterization of two additional NH4+ transporters, Mep2p and Mep3p, both of which are highly similar to Mep1p. The Mep2 protein displays the highest affinity for NH4+ (Km, 1 to 2 microM), followed closely by Mep1p (Km, 5 to 10 microM) and finally by Mep3p, whose affinity is much lower (Km, approximately 1.4 to 2.1 mM). A strain lacking all three MEP genes cannot grow on media containing less than 5 mM NH4+ as the sole nitrogen source, while the presence of individual NH4+ transporters enables growth on these media. Yet, the three Mep proteins are not essential for growth on NH4+ at high concentrations (>20 mM). Feeding experiments further indicate that the Mep transporters are also required to retain NH4+ inside cells during growth on at least some nitrogen sources other than NH4+. The MEP genes are subject to nitrogen control. In the presence of a good nitrogen source, all three MEP genes are repressed. On a poor nitrogen source, MEP2 expression is much higher than MEP1 and MEP3 expression. High-level MEP2 transcription requires at least one of the two GATA family factors Gln3p and Nil1p, which are involved in transcriptional activation of many other nitrogen-regulated genes. In contrast, expression of either MEP1 or MEP3 requires only Gln3p and is unexpectedly down-regulated in a Nil1p-dependent manner. Analysis of databases suggests that families of NH4+ transporters exist in other organisms as well.

A family of ammonium transporters in Saccharomyces cerevisiae.

Fungi contribute extensively to a wide range of ecosystem processes, including decomposition of organic carbon, deposition of recalcitrant carbon, and transformations of nitrogen and phosphorus. In this review, we discuss the current knowledge about physiological and morphological traits of fungi that directly influence these processes, and we describe the functional genes that encode these traits. In addition, we synthesize information from 157 whole fungal genomes in order to determine relationships among selected functional genes within fungal taxa. Ecosystem-related traits varied most at relatively coarse taxonomic levels. For example, we found that the maximum amount of variance for traits associated with carbon mineralization, nitrogen and phosphorus cycling, and stress tolerance could be explained at the levels of order to phylum. Moreover, suites of traits tended to co-occur within taxa. Specifically, the genetic capacities for traits that improve stress tolerance-β-glucan synthesis, trehalose production, and cold-induced RNA helicases-were positively related to one another, and they were more evident in yeasts. Traits that regulate the decomposition of complex organic matter-lignin peroxidases, cellobiohydrolases, and crystalline cellulases-were also positively related, but they were more strongly associated with free-living filamentous fungi. Altogether, these relationships provide evidence for two functional groups: stress tolerators, which may contribute to soil carbon accumulation via the production of recalcitrant compounds; and decomposers, which may reduce soil carbon stocks. It is possible that ecosystem functions, such as soil carbon storage, may be mediated by shifts in the fungal community between stress tolerators and decomposers in response to environmental changes, such as drought and warming.

Fungal Traits That Drive Ecosystem Dynamics on Land

https://pure.rug.nl/ws/files/13640729/1995MicrobiolRevvdRest.pdf

The plasma membrane of Saccharomyces cerevisiae: structure, function, and biogenesis.

Publisher Summary This chapter provides a brief description of nitrogen catabolite repression in Saccharomyces cerevisiae. The ancestral use of yeasts in industry has led to the selection of many wild-type strains of Saccharomyces cerevisiae. Saccharomyces spp. are very different from bacteria, although in vegetative growth they can be handled in a similar way. This is probably the origin of the choice of these organisms when prokaryotic cellular physiologists became interested in eukaryotic organisms. Anabolism, catabolism, and regulation of arginine metabolism have also been studied in parallel in Saccharomyces cerevisiae. The chapter also discusses the nitrogen metabolite repression in filamentous fungi. A number of filamentous fungi are able to utilize a wide range of nitrogen sources. This metabolic versatility implies a strong selective pressure for a mechanism to ensure preferential utilization of the favorable nitrogen sources. This mechanism, nitrogen metabolite repression of the syntheses of many enzymes and permeases involved in nitrogen nutrition, has been extensively studied in two filamentous fungi, the ascomycetes Aspergillus nidulans and Neurospora crassa.

Nitrogen catabolite repression in yeasts and filamentous fungi.

Kinetic and genetic evidences are presented to show that, in addition to specific amino acid permeases, Saccharomyces cerevisiae has a general amino acid permease which catalyzes the transport of basic and neutral amino acids, but most probably not that of proline. The general amino acid permease appears to be constitutive, and its activity is inhibited when ammonium ions are added to the culture medium. A mutant which has lost the general amino acid permease activity was isolated. Its mutation, named gap (general amino acid permease), is not allelic to the aap (amino acid permease) mutation of Surdin et al., which has a quite different phenotype and cannot be considered as having selectively lost the general amino acid permease activity.

Multiplicity of the Amino Acid Permeases in Saccharomyces cerevisiae IV. Evidence for a General Amino Acid Permease

Mutations at the apf locus selectively depress the activity of a number of distinct amino acid permeases in Saccharomyces cerevisiae. The activity of the general amino acid permease and specific amino acid permeases is decreased, but the uptake of pyrimidines and adenine is unaffected. Mutations at the apf locus are allelic to the aap mutation isolated by Surdin et al. Amino acid uptake is normal in a heterozygous diploid (apf/+) and in a tetraploid strain with only one functional allele at the apf locus.

M. Grenson

Papers

Multiplicity of the Amino Acid Permeases in Saccharomyces cerevisiae IV. Evidence for a General Amino Acid Permease

Mutation Affecting Activity of Several Distinct Amino Acid Transport Systems in Saccharomyces cerevisiae