Regulation of cation-coupled high-affinity phosphate uptake in the yeast Saccharomyces cerevisiae

Studies of the high-affinity phosphate transporters in the yeast Saccharomyces cerevisiae using mutant strains lacking either the Pho84 or the Pho89 permease revealed that the transporters are differentially regulated. Although both genes are induced by phosphate starvation, activation of the Pho89 transporter precedes that of the Pho84 transporter early in the growth phase in a way which may possibly reflect a fine tuning of the phosphate uptake process relative to the availability of external phosphate.     

Isolation and characterization of two distinct myo-inositol transporter genes of Saccharomyces cerevisiae

By the complementation of a yeast mutant defective in myo-inositol transport (Nikawa, J., Nagumo, T., and Yamashita, S. (1982) J. Bacteriol. 150, 441-446), we isolated two myo-inositol transporter genes, ITR1 and ITR2, from a yeast gene library. The ITR1 and ITR2 genes contained long open reading frames capable of encoding 584 and 612 amino acids with calculated relative molecular masses of 63,605 and 67,041, respectively. The sequence similarity between the ITR1 and ITR2 products was extremely high, suggesting that the two genes arose from a common ancestor. Both gene products show significant sequence homology with a superfamily of sugar transporters, including human HepG2 hepatoma/erythrocyte glucose transporter and Escherichia coli xylose transporter. Hydropathy analysis indicated that the ITR1 and ITR2 products are both hydrophobic and contain 12 putative membrane-spanning regions. Thus, yeast myo-inositol transporters could be classified into the sugar transporter superfamily. Gene disruption and tetrad analysis showed that yeast cells contain two separate myoinositol transporters. The ITR1 product was the major transporter and the ITR2 product the minor one in cells grown in minimum medium containing glucose. Northern blot analysis showed that ITR1 mRNA was much more abundant than ITR2 mRNA. The previously isolated myo-inositol transport mutant was determined to be defective in ITR1.     

Isolation and characterization of rice cesium transporter genes from a rice‐transporter‐enriched yeast expression library

A considerable portion of agricultural land in central‐east Japan has been contaminated by radioactive material, particularly radioactive Cs, due to the industrial accident at the Fukushima Daiichi nuclear power plant. Understanding the mechanism of absorption, translocation and accumulation of Cs⁺ in plants will greatly assist in developing approaches to help reduce the radioactive contamination of agricultural products. At present, however, little is known regarding the Cs⁺ transporters in rice. A transporter‐enriched yeast expression library was constructed and the library was screened for Cs⁺ transporter genes. The 1452 full length cDNAs encoding transporter genes were obtained from the Rice Genome Resource Center and 1358 clones of these transporter genes were successively subcloned into yeast expression vectors; which were then transferred into yeast. Using this library, both positive and negative selection screens can be performed, which have not been previously possible. The constructed library is an excellent tool for the isolation of novel transporter genes. This library was screened for clones that were sensitive to Cs⁺ using a SD‐Gal medium containing either 30 or 70 mM CsCl; resulting in the isolation of 13 Cs⁺ sensitive clones. ¹³⁷Cs absorption experiments were conducted and confirmed that all of the identified clones were able to absorb ¹³⁷Cs. A total of 3 potassium transporters, 2 ABC transporters and 1 NRAMP transporter were among the 13 identified clones.     

The conservation of polyol transporter proteins and their involvement in lichenized Ascomycota

In lichen symbiosis, polyol transfer from green algae is important for acquiring the fungal carbon source. However, the existence of polyol transporter genes and their correlation with lichenization remain unclear. Here, we report candidate polyol transporter genes selected from the genome of the lichen-forming fungus (LFF) Ramalina conduplicans. A phylogenetic analysis using characterized polyol and monosaccharide transporter proteins and hypothetical polyol transporter proteins of R. conduplicans and various ascomycetous fungi suggested that the characterized yeast’ polyol transporters form multiple clades with the polyol transporter-like proteins selected from the diverse ascomycetous taxa. Thus, polyol transporter genes are widely conserved among Ascomycota, regardless of lichen-forming status. In addition, the phylogenetic clusters suggested that LFFs belonging to Lecanoromycetes have duplicated proteins in each cluster. Consequently, the number of sequences similar to characterized yeast’ polyol transporters were evaluated using the genomes of 472 species or strains of Ascomycota. Among these, LFFs belonging to Lecanoromycetes had greater numbers of deduced polyol transporter proteins. Thus, various polyol transporters are conserved in Ascomycota and polyol transporter genes appear to have expanded during the evolution of Lecanoromycetes.     

Zeroing in on zinc uptake in yeast and plants.

Zinc is an essential micronutrient. Genes responsible for zinc uptake have now been identified from yeast and plants. These genes belong to an extended family of cation transporters called the ZIP gene family. Zinc efflux genes that belong to another transporter family, the CDF family, have also been identified in yeast and Arabidopsis. It is clear that studies in yeast can greatly aid our understanding of zinc metabolism in plants.     

The Saccharomyces cerevisiae high affinity phosphate transporter encoded by PHO84 also functions in manganese homeostasis

In the bakers' yeast Saccharomyces cerevisiae, high affinity manganese uptake and intracellular distribution involve two members of the Nramp family of genes, SMF1 and SMF2. In a search for other genes involved in manganese homeostasis, PHO84 was identified. The PHO84 gene encodes a high affinity inorganic phosphate transporter, and we find that its disruption results in a manganese-resistant phenotype. Resistance to zinc, cobalt, and copper ions was also demonstrated for pho84Delta yeast. When challenged with high concentrations of metals, pho84Delta yeast have reduced metal ion accumulation, suggesting that resistance is due to reduced uptake of metal ions. Pho84p accounted for virtually all the manganese accumulated under metal surplus conditions, demonstrating that this transporter is the major source of excess manganese accumulation. The manganese taken in via Pho84p is indeed biologically active and can not only cause toxicity but can also be incorporated into manganese-requiring enzymes. Pho84p is essential for activating manganese enzymes in smf2Delta mutants that rely on low affinity manganese transport systems. A role for Pho84p in manganese accumulation was also identified in a standard laboratory growth medium when high affinity manganese uptake is active. Under these conditions, cells lacking both Pho84p and the high affinity Smf1p transporter accumulated low levels of manganese, although there was no major effect on activity of manganese-requiring enzymes. We conclude that Pho84p plays a role in manganese homeostasis predominantly under manganese surplus conditions and appears to be functioning as a low affinity metal transporter.     

Monosaccharide transporters in plants: structure, function and physiology

Monosaccharide transport across the plant plasma membrane plays an important role both in lower and higher plants. Algae can switch between phototrophic and heterotrophic growth and utilize organic compounds, such as monosaccharides as additional or sole carbon sources. Higher plants represent complex mosaics of phototrophic and heterotrophic cells and tissues and depend on the activity of numerous transporters for the correct partitioning of assimilated carbon between their different organs. The cloning of monosaccharide transporter genes and cDNAs identified closely related integral membrane proteins with 12 transmembrane helices exhibiting significant homology to monosaccharide transporters from yeast, bacteria and mammals. Structural analyses performed with several members of this transporter superfamily identified protein domains or even specific amino acid residues putatively involved in substrate binding and specificity. Expression of plant monosaccharide transporter cDNAs in yeast cells and frog oocytes allowed the characterization of substrate specificities and kinetic parameters. Immunohistochemical studies, in situ hybridization analyses and studies performed with transgenic plants expressing reporter genes under the control of promoters from specific monosaccharide transporter genes allowed the localization of the transport proteins or revealed the sites of gene expression. Higher plants possess large families of monosaccharide transporter genes and each of the encoded proteins seems to have a specific function often confined to a limited number of cells and regulated both developmentally and by environmental stimuli.     

The hexose transporter family of Saccharomyces cerevisiae

Saccharomyces cerevisiae accomplishes high rates of hexose transport. The kinetics of hexose transport are complex. The capacity and kinetic complexity of hexose transport in yeast are reflected in the large number of sugar transporter genes in the genome. Twenty hexose transporter genes exist in S. cerevisiae. Some of these have been found by genetic means; many have been discovered by the comprehensive sequencing of the yeast genome. This review codifies the nomenclature of the hexose transporter genes and describes the sequence homology and structural similarity of the proteins they encode. Information about the expression and function of the transporters is presented. Access to the sequences of the genes and proteins at three sequence databases is provided via the World Wide Web. Received: 24 June 1996 / Accepted: 29 July 1996     

NTR1 encodes a high affinity oligopeptide transporter in Arabidopsis

Heterologous complementation of yeast mutants has enabled the isolation of genes encoding several families of amino acid transporters. Among them, NTR1 codes for a membrane protein with weak histidine transport activity. However at the sequence level, NTR1 is related to rather non-specific oligopeptide transporters from a variety of species including Arabidopsis and to the Arabidopsis nitrate transporter CHL1. A yeast mutant deficient in oligopeptide transport was constructed allowing to show that NTR1 functions as a high affinity, low specificity peptide transporter. In siliques NTR1-expression is restricted to the embryo, implicating a role in the nourishment of the developing seed.     

Hexose and pentose transport in ascomycetous yeasts: an overview

The biochemical characterization of sugar uptake in yeasts started five decades ago and led to the early production of abundant kinetic and mechanistic data. However, the first accurate overview of the underlying sugar transporter genes was obtained relatively late, due mainly to the genetic complexity of hexose uptake in the model yeast Saccharomyces cerevisiae . The genomic era generated in turn a massive amount of information, allowing the identification of a multitude of putative sugar transporter and sensor-encoding genes in yeast genomes, many of which are phylogenetically related. This review aims to briefly summarize our current knowledge on the biochemical and molecular features of the transporters of hexoses and pentoses in yeasts, when possible establishing links between previous kinetic studies and genomic data currently available. Emphasis is given to recent developments concerning the identification of d -xylose and l -arabinose transporter genes, which are thought to be key players in the optimization of S. cerevisiae strains for bioethanol production from lignocellulose hydrolysates.