Uracil transport in Saccharomyces cerevisiae

FUN26 (function unknown now 26) protein from saccharomyces cerevisiae is a broad selectivity, high affinity, nucleoside and nucleobase transporter

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that transport nucleosides and, to a lesser extent, nucleobases across cell membranes. ENTs modulate efficacy for a range of human therapeutics and function in a diffusion-controlled bidirectional manner. A detailed understanding of ENT function at the molecular level has remained elusive. FUN26 (function unknown now 26) is a putative ENT homolog from S. cerevisiae that is expressed in vacuole membranes. In the present system, proteoliposome studies of purified FUN26 demonstrate robust nucleoside and nucleobase uptake into the luminal volume for a broad range of substrates. This transport activity is sensitive to nucleoside modifications in the C(2')- and C(5')-positions on the ribose sugar and is not stimulated by a membrane pH differential. [(3)H]Adenine nucleobase transport efficiency is increased ∼4-fold relative to nucleosides tested with no observed [(3)H]adenosine or [(3)H]UTP transport. FUN26 mutational studies identified residues that disrupt (G463A or G216A) or modulate (F249I or L390A) transporter function. These results demonstrate that FUN26 has a unique substrate transport profile relative to known ENT family members and that a purified ENT can be reconstituted in proteoliposomes for functional characterization in a defined system.

Global expression analysis of the yeast Lachancea (Saccharomyces) kluyveri reveals new URC genes involved in pyrimidine catabolism

Pyrimidines are important nucleic acid precursors which are constantly synthesized, degraded, and rebuilt in the cell. Four degradation pathways, two of which are found in eukaryotes, have been described. One of them, the URC pathway, has been initially discovered in our laboratory in the yeast Lachancea kluyveri. Here, we present the global changes in gene expression in L. kluyveri in response to different nitrogen sources, including uracil, uridine, dihydrouracil, and ammonia. The expression pattern of the known URC genes, URC1-6, helped to identify nine putative novel URC genes with a similar expression pattern. The microarray analysis provided evidence that both the URC and PYD genes are under nitrogen catabolite repression in L. kluyveri and are induced by uracil or dihydrouracil, respectively. We determined the function of URC8, which was found to catalyze the reduction of malonate semialdehyde to 3-hydroxypropionate, the final degradation product of the pathway. The other eight genes studied were all putative permeases. Our analysis of double deletion strains showed that the L. kluyveri Fui1p protein transported uridine, just like its homolog in Saccharomyces cerevisiae, but we demonstrated that is was not the only uridine transporter in L. kluyveri. We also showed that the L. kluyveri homologs of DUR3 and FUR4 do not have the same function that they have in S. cerevisiae, where they transport urea and uracil, respectively. In L. kluyveri, both of these deletion strains grew normally on uracil and urea.

Overexpression of SNG1 causes 6-azauracil resistance in Saccharomyces cerevisiae

The mechanism of action of 6AU, a growth inhibitor for many microorganisms causing depletion of intracellular nucleotide pools of GTP and UTP, is not well understood. To gain insight into the mechanisms leading to 6AU resistance, and in an attempt to uncover novel genes required for this resistance, we undertook a high-copy-number suppressor screening to identify genes whose overexpression could repair the 6AU(S) growth defect caused by rpb1 mutations in Saccharomyces cerevisiae. We have identified SNG1 as a multicopy suppressor of the 6AU(S) growth defect caused by the S. cerevisiae rpb1 mutant. The mechanism by which Sng1 causes 6AU resistance is independent of the transcriptional elongation and of the nucleotide-pool regulation through Imd2 and Ura2, as well as of the Ssm1-mediated 6AU detoxification. This resistance to 6AU is not extended to other uracil analogues, such as 5-fluorouracil, 5FU. In addition, our results suggest that 6AU enters S. cerevisiae cells through the uracil permease Fur4. Our results demonstrate that Sng1 is localised in the plasma membrane and evidence SNG1 and FUR4 genes as determinants of resistance and susceptibility to this inhibitory compound, respectively. Taken together, these results show new mechanisms involved in the resistance and susceptibility to 6AU.

Yeast pre-rRNA processing and modification occur cotranscriptionally

To better understand yeast ribosome synthesis, we developed techniques for the rapid harvesting and analysis of metabolically labeled cultures. Modeling of the resulting kinetic data allowed predicted lifetimes and processing patterns to be compared with the experimental data. This supported a transcription time for the 35S primary transcripts of approximately 170 s at 30 degrees C (approximately 40 nt s(-1)), with a high fraction (approximately 70%) of nascent transcripts cleaved at the early processing sites that generate the 20S precursor to the 18S rRNA. This level of nascent transcript cleavage apparently conflicted with previous reports that modification of yeast pre-rRNA exclusively occurred on released transcripts. A second round of high-resolution kinetic labeling showed that 20S pre-rRNA predominately undergoes methylation as nascent transcripts, whereas the 27S precursor to the 25S/5.8S rRNAs was partially methylated on the nascent transcript. The results demonstrate that quantitative analyses of pre-rRNA processing can yield important biological insights.

Regulation of expression and kinetic modeling of substrate interactions of a uracil transporter inAspergillus nidulans

Early genetic evidence suggested that A. nidulans possesses at least one uracil transporter. A gene, named furD, was recently identified by reverse genetics and in silico approaches and we confirm here that it encodes a high-affinity, high-capacity, uracil transporter. In this work, we study the regulation of expression of FurD and develop a kinetic model describing transporter-substrate interactions. The furD gene is not expressed in resting conidiospores, is transcriptionally activated and reaches a peak during the isotropic growth phase of conidiospore germination, and stays at a basic low level in mycelium. Transcriptional expression is correlated to uracil transport activity. Expression in a strain blocked in uracil biosynthesis (pyrG-) is moderately increased and extended to later stages of germination. The presence of excess uracil in the medium leads to down-regulation of furD expression and FurD activity. A detailed kinetic analysis using a number of pyrimidine and purine analogues showed that FurD is able to recognize with high-affinity uracil (Km 0.45 microM), thymine (Ki 3.3 microM) and several 5-substituted analogues of uracil, and with moderate affinity uric acid and xanthine (Ki 94-99 microM). Kinetic evidence supports a model in which the positions N1-H, =O2, N3-H, =O4, as well as planarity play a central role for the substrate binding. This model, which rationalizes the unique specificity of FurD for uracil, is compared to and found to be very similar to analogous models for protozoan uracil transporters.

Transcriptional regulation of the Saccharomyces cerevisiae DAL5 gene family and identification of the high affinity nicotinic acid permease TNA1 (YGR260w)

We have studied the transcript levels of YGR260w and YLR004c, two genes encoding members of the yeast Dal5p subfamily of the major facilitator family, and we show that they increase when extracellular nicotinic acid and thiamine, respectively, are absent. The deletion of YGR260w in a bna1 auxotrophic mutant for nicotinic acid prevents growth at low nicotinic acid concentration. This suggests that YGR260w is necessary for nicotinic acid import into the cell. The direct measurement of nicotinic acid uptake on whole cells demonstrates that YGR260w encodes the yeast high affinity nicotinic acid permease. Its apparent K(m) of 1.7 microM is low enough to allow the uptake of the low concentrations of nicotinic acid normally secreted by wild type cells.

Nucleobase transporters (review)

Purines and pyrimidines play a key role in nucleic acid and nucleotide metabolism of all cells. In addition, they can be used as nitrogen sources in plants and many microorganisms. Transport of nucleobases across biological membranes is mediated by specific transmembrane transport proteins. Nucleobase transporters have been identified genetically and/or physiologically in bacteria, fungi, protozoa, algae, plants and mammals. A limited number of bacterial and fungal transporter genes have been cloned and analysed in great detail at the molecular level. Very recently, nucleobase transporters have been identified in plants. In other systems, with less accessible genetics, such as vertebrates and protozoa, no nucleobase transporter genes have been identified, and the transporters have been characterized and classified by physiological and biochemical approaches instead. In this review, it is shown that nucleobase transporters and similar sequences of unknown function present in databases constitute three basic families, which will be designated NAT, PRT and PUP. The first includes members from archea, eubacteria, fungi, plants and metazoa, the second is restricted to prokaryotes and fungi, and the last one is only found in plants. Interestingly, mammalian ascorbate transporters are homologous to NAT sequences. The function of different nucleobase transporters is also described, as is how their expression is regulated and what is currently known about their structure-function relationships. Common features emerging from these studies are expected to prove critical in understanding what governs nucleobase transporter specificity and in selecting proper model microbial systems for cloning and studying plant, protozoan and mammalian nucleobase transporters of agricultural, pharmacological and medical importance.

Uracil-induced down-regulation of the yeast uracil permease

In Saccharomyces cerevisiae the FUR4-encoded uracil permease catalyzes the first step of the pyrimidine salvage pathway. The availability of uracil has a negative regulatory effect upon its own transport. Uracil causes a decrease in the level of uracil permease, partly by decreasing the FUR4 mRNA level in a promoter-independent fashion, probably by increasing its instability. Uracil entry also triggers more rapid degradation of the existing permease by promoting high efficiency of ubiquitination of the permease that signals its internalization. A direct binding of intracellular uracil to the permease is possibly involved in this feedback regulation, as the behavior of the permease is similar in mutant cells unable to convert intracellular uracil into UMP. We used cells impaired in the ubiquitination step to show that the addition of uracil produces rapid inhibition of uracil transport. This may be the first response prior to the removal of the permease from the plasma membrane. Similar down-regulation of uracil uptake, involving several processes, was observed under adverse conditions mainly corresponding to a decrease in the cellular content of ribosomes. These results suggest that uracil of exogenous or catabolic origin down-regulates the cognate permease to prevent buildup of excess intracellular uracil-derived nucleotides.

Proton stoichiometry of the overexpressed uracil symport of the yeast Saccharomyces cerevisiae

Information about the molecular biology of the uracil carrier of Saccharomyces cerevisiae is now available but its properties as a symport are unexplored. We have now studied its proton stoichiometry at pH 6.5, 4.8 and 4.2, in a yeast strain overexpressing the symport and unable to metabolize uracil. After the depletion of cellular ATP, uracil uptake followed an approximately exponential time course to reach a plateau. Proton uptake was then indistinguishable from the basal rate shown in controls without added uracil. It was concluded that more than 87% of the uracil flux through the system was coupled to symported protons. Because the basal rate was a significant fraction of the total rate of proton uptake at pH 4.2 or 4.8, the ratio of the proton and uracil flows could not be defined uniquely during the initial near-linear phase of uptake. However, the average rate of proton uptake during increasing time intervals up to 120 s was shown to be a linear function of the corresponding average rate of uracil uptake. This relationship, defining approx. 90% of the eventual uracil uptake, was used to deduce the mean proton stoichiometry as approx. 3 at pH 4.2, falling to near 2 at pH 6.5. The rate of uracil uptake increased 4-fold when the negative proton gradient (Delta mu(H)) acting across the plasma membrane increased from 60 to 200 mV. The rate fell markedly after depolarization by KCl. The maximum uracil gradient (Delta mu(S)) was less affected by depolarization. The ratio Delta mu(S)/Delta mu(H) fell from approx. 2 to near 1 as Delta mu(H) increased in the above range. In contrast with the starved yeast, uracil accumulation during energy metabolism followed a pump-leak model.

The uracil permease of Schizosaccharomyces pombe: a representative of a family of 10 transmembrane helix transporter proteins of yeasts

The uracil permease gene of Schizosaccharomyces pombe was cloned and sequenced. The deduced protein sequence shares strong similarities with five open reading frames from Saccharomyces cerevisiae, namely the uracil permease encoded by the FUR4 gene, the allantoin permease encoded by DAL4, a putative uridine permease (YBL042C) and two unknown ORFs YOR071c and YLR237w. A topological model retaining ten transmembrane helices, based on predictions and on experimental data established for the uracil permease of S. cerevisiae by Galan and coworkers (1996), is discussed for the four closest proteins of this family of transporters. The sequence of the uracil permease gene of S. pombe has been deposited in the EMBL data bank under Accession Number X98696.

Expulsion of uracil and thymine from the yeast Saccharomyces cerevisiae: contrasting responses to changes in the proton electrochemical gradient

The outflow of uracil from the yeast Saccharomyces cerevisiae is known to be relatively fast in certain circumstances, to be retarded by proton conductors and to occur in strains lacking a uracil proton symport. In the present work, it was shown that uracil exit from washed yeast cells is an active process, creating a uracil gradient of the order of -80 mV relative to the surrounding medium. Glucose accelerated uracil exit, while retarding its entry. DNP or sodium azide each lowered the gradient to about -30 mV, simultaneously increasing the rate of uracil entry. They also lowered cellular ATP content. Manipulation of the external ionic conditions governing delta mu H+ at the plasma membrane had no detectable effect on uracil transport in yeast preparations thoroughly depleted of ATP. It was concluded that uracil exit is probably not driven by the proton gradient but may utilize ATP directly. It is known that thymine is not normally absorbed by yeast. However, thymine expulsion was here observed during deamination of the substrate 5-methylcytosine in the presence of glucose. In the absence of glucose, or following ATP depletion, thymine uptake from the medium only occurred when delta mu H+ was dissipated, either by DNP or azide, or by manipulation of the external ionic environment. The yeast expelled absorbed thymine when delta mu H+ was restored to the physiological range. The properties of the system corresponded to those of an H+/thymine antiport that is distinct from the mechanism expelling uracil.

Replacement of Lys by Glu in a transmembrane segment strongly impairs the function of the uracil permease from Saccharomyces cerevisiae

The co-transport of uracil and protons through the plasma membrane of the yeast Saccharomyces cerevisiae is mediated by a specific permease encoded by the FUR4 gene. The uracil permease is a multi-spanning membrane protein that follows the secretory pathway to the plasma membrane. Recent experimental data led to the proposal of a two-dimensional model of its topology. A spontaneous mutant corresponding to the substitution of Lys-272 by glutamic acid was obtained. The influence of this mutation was studied by comparing the wild-type and mutant permeases produced in a strain carrying a chromosomal deletion of the FUR4 gene. The mutant permease is correctly targeted to the plasma membrane and its stability is similar to that of the wild-type permease. The uptake parameters for the mutant permease were impaired and showed an approximately 65-fold increase of apparent K(m) and a decrease in apparent Vmax. Equilibrium binding measurements with enriched plasma membrane preparations showed an approximately 70-fold increase in apparent Kd in the mutant, whereas its Bmax. was similar to that of the wild type. Lys-272 is fully conserved in the uracil permease family and is predicted to lie in the fourth transmembrane segment of the protein. It seems to be essential for both efficient uracil binding and translocation.

Uracil uptake in Escherichia coli K-12: isolation of uraA mutants and cloning of the gene

Mutants defective in utilization of uracil at low concentrations have been isolated and characterized. The mutations in question (uraA) map close to the upp gene encoding uracil phosphoribosyltransferase. By complementation analysis, a plasmid that complements the uraA mutation has been isolated. The uraA gene was shown to be the second gene in a bicistronic operon with upp as the promoter proximal gene. The nucleotide sequence of the gene was determined, and the gene encodes a hydrophobic membrane protein with a calculated Mr of 45,030. The UraA protein has been identified in sodium dodecyl sulfate-polyacrylamide gels in the membrane fraction of minicells harboring the uraA plasmids.

Fenpropimorph affects uptake of uracil and cytosine in Saccharomyces cerevisiae

Fenpropimorph was shown to inhibit the accumulation of the pyrimidine bases uracil and cytosine from the growth media in Saccharomyces cerevisiae. Uracil prototrophs of S. cerevisiae were more resistant to the growth-inhibitory effects of fenpropimorph than were uracil auxotrophs. High concentrations of uracil rescued fenpropimorph-treated uracil auxotrophs, and cytosine, which is accumulated by a separate mechanism, could also support growth of treated uracil auxotrophs. Fenpropimorph caused a significant decrease in the uptake of radiolabeled uracil, which was not due to accumulation of ergosta-8,14-dienol (ignosterol) in the treated cultures. Radiolabeled cytosine uptake was unaffected by drug treatment in a wild-type strain but was inhibited in a sterol mutant, in which ergosterol was absent from the cell. The role of fenpropimorph in causing membrane dysfunction through a mechanism other than altered sterol metabolism is discussed.

Regulation of the pyrimidine salvage pathway by the FUR1 gene product of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the protein encoded by the FUR1 gene is absolutely required for the expression of uracil phosphoribosyl transferase activity. The occurrence of semi-dominant mutations for 5-fluorouracil-(5FU)-resistance at this locus led us to clone and sequence the semi-dominant fur1-5 allele. A single point mutation, resulting in the substitution of arginine 134 for serine, is responsible for this mutant phenotype. The fur1-5 allele is transcribed and expressed at the same level as the wild-type allele. But, in contrast with the wild-type, the UPRTase activity of the fur1-5 mutant strain is stimulated in vitro by UTP and does not, therefore, correspond to a loss of feedback of UPRTase activity. We found that uracil, as a free base, induces a significative increase in transcription and UPRTase activity in a wild-type strain as well as in uracil-overproducing mutants which principally explains the high efficiency of the pyrimidine salvage pathway in S. cerevisiae.

Membrane insertion of uracil permease, a polytopic yeast plasma membrane protein

Uracil permease is a multispanning protein of the Saccharomyces cerevisiae plasma membrane which is encoded by the FUR4 gene and produced in limited amounts. It has a long N-terminal hydrophilic segment, which is followed by 10 to 12 putative transmembrane segments, and a hydrophilic C terminus. The protein carries seven potential N-linked glycosylation sites, three of which are in its N-terminal segment. Overexpression of this permease and specific antibodies were used to show that uracil permease undergoes neither N-linked glycosylation nor proteolytic processing. Uracil permease N-terminal segments of increasing lengths were fused to a reporter glycoprotein, acid phosphatase. The in vitro and in vivo fates of the resulting hybrid proteins were analyzed to identify the first signal anchor sequence of the permease and demonstrate the cytosolic orientation of its N-terminal hydrophilic sequence. In vivo insertion of the hybrid protein bearing the first signal anchor sequence of uracil permease into the endoplasmic reticulum membrane was severely blocked in sec61 and sec62 translocation mutants.

Membrane insertion of uracil permease, a polytopic yeast plasma membrane protein

Uracil permease is a multispanning protein of the Saccharomyces cerevisiae plasma membrane which is encoded by the FUR4 gene and produced in limited amounts. It has a long N-terminal hydrophilic segment, which is followed by 10 to 12 putative transmembrane segments, and a hydrophilic C terminus. The protein carries seven potential N-linked glycosylation sites, three of which are in its N-terminal segment. Overexpression of this permease and specific antibodies were used to show that uracil permease undergoes neither N-linked glycosylation nor proteolytic processing. Uracil permease N-terminal segments of increasing lengths were fused to a reporter glycoprotein, acid phosphatase. The in vitro and in vivo fates of the resulting hybrid proteins were analyzed to identify the first signal anchor sequence of the permease and demonstrate the cytosolic orientation of its N-terminal hydrophilic sequence. In vivo insertion of the hybrid protein bearing the first signal anchor sequence of uracil permease into the endoplasmic reticulum membrane was severely blocked in sec61 and sec62 translocation mutants.

19F nuclear magnetic resonance study of fluoropyrimidine metabolism in strains of Candida glabrata with specific defects in pyrimidine metabolism

Flucytosine (5-FC)-resistant strains were isolated from the haploid opportunistic pathogen Candida glabrata by UV-induced mutation and fluoropyrimidine selection. These strains were characterized biochemically, and the metabolism of fluorinated pyrimidines was studied by 19F nuclear magnetic resonance spectroscopy. No evidence was obtained from these studies for degradative metabolism of the fluorinated derivatives. In the parental susceptible strain of C. glabrata, 5-fluorouracil but not 5-FC was detected within the cells. 5-Fluorouracil was also present in the culture supernatant after incubation of the cells with 5-FC. The distribution of fluorinated derivatives within the 5-FC-resistant strains was consistent with their genotype. Two strains of C. glabrata which had only a partial loss of cytosine deaminase and UMP pyrophosphorylase activity had high levels of resistance to 5-FC. Both C. glabrata and Candida albicans were susceptible to 5-fluorouridine. This compound but not the anticancer drug 5-fluoro-2-deoxyuridine was shown to be transported into susceptible cells by a specific uridine permease.

The purine-cytosine permease gene of Saccharomyces cerevisiae: primary structure and deduced protein sequence of the FCY2 gene product

A 2.1 kb DNA segment carrying the purine-cytosine permease gene (FCY2) of Saccharomyces cerevisiae was sequenced, the primary structure of the protein (533 amino acids) deduced and a folding pattern in the membrane is proposed for the permease protein. Expression of the FCY2 gene product requires a functional secretory pathway and is reduced in mnn9, a mutant strain deficient in outer chain glycosylation. The FCY2 gene was mapped on the right arm of chromosome V close to the HIS1 gene.

Pathogenicity of 5-fluorocytosine resistant strains of Candida albicans

Mutants of Candida albicans blocked in pyrimidine transport and salvage metabolism were produced by a two step mutagenic procedure and selected by their resistance to 5-fluorocytosine (flucytosine). The growth rates and growth yields of these mutants did not differ significantly from the parental strain of C. albicans. Examination of their pathogenicity to mice demonstrated that a defect in the uridine transport function decreased the pathogenicity of C. albicans.

Primary structure of the uracil transport protein of Saccharomyces cerevisiae

We present in this paper the nucleotidic sequence of the FUR4 gene encoding the uracil permease in the yeast Saccharomyces cerevisiae. The deduced amino acid sequence of the permease has 633 residues; it consists of many hydrophobic stretches, only the N-terminal and C-terminal ends of the protein (about 100 and 50 amino acids respectively) being mostly hydrophilic. No N-terminal hydrophobic signal peptide is present, although it is shown in this work that the biosynthesis of the uracil permease goes through the secretion/glycosylation pathway. Using the results of three different methods, allowing the prediction of transmembrane alpha helices in proteic sequences, we drew a model of folding of the permease in the membrane.

Chromosomal mapping of the uracil permease gene of Saccharomyces cerevisiae

The gene FUR4, coding for the uracil permease in Saccharomyces cerevisiae, was mapped on chromosome II, at a distance of 7.8 cM from the centromere on the right arm of the chromosome. In a first step, we used the chromosome loss mapping method developed by Falco and Botstein (1983) to determine on which chromosome the gene mapped. After the observation that FUR4 was closely linked to GAL10, one of the three genes forming the gal cluster (Bassel and Mortimer 1971), we could determine precisely the position of the gene on chromosome II.

Accumulation of dinucleoside polyphosphates in Saccharomyces cerevisiae under stress conditions. High levels are associated with cell death

Adenosine tetraphosphonucleosides (Ap4X) were measured in Saccharomyces cerevisiae by a coupled phosphodiesterase-luciferase assay. After exposure of the cells to cadmium or to hyperthermic treatment (46 degrees C) a marked increase of the cellular pool from 0.08 microM (base level) to 4 microM or higher was observed. The accumulation of Ap4X to high levels is associated with irreversible processes leading to cell death.

Metabolism of pyrimidine bases and nucleosides in the coryneform bacteria Brevibacterium ammoniagenes and Micrococcus luteus

The metabolism of exogenous pyrimidine bases and nucleosides was investigated in Brevibacterium ammoniagenes and Micrococcus luteus with fluorinated analogs and radioactive precursors. Salvage of thymine and thymidine was found in M. luteus, but not in B. ammoniagenes. Exogenous uracil or uracil nucleosides, but not cytosine or cytosine nucleosides, were nucleic acid precursors for both bacteria. By examining the possible nucleoside-metabolizing enzymes, it can be suggested that the pyrimidine salvage pathways in the coryneform bacteria are different from those of members of the family Enterobacteriaceae.

Cloning and transcriptional control of a eucaryotic permease gene

The uracil permease gene of the yeast Saccharomyces cerevisiae was cloned on a hybrid plasmid which replicates autonomously in both yeast and Escherichia coli. Cloning was carried out by complementation in yeast. The smallest DNA fragment found to complement the uracil permease deficiency in recipient yeast cells measured approximately 2.3 kilobases. In strains transformed by the plasmid with the uracil permease gene inserted, initial rates of uracil uptake increased up to 25 times more than the rates found in the wild type. Using DNA probes carrying several regions of the cloned gene, I showed that a strain carrying the dhul-I mutation, which is not linked to the permease structural gene and is responsible for enhanced uptake velocity of uracil, had enhanced transcription of the permease gene. By using DNA probes recloned in phage M13 mp7, the direction of transcription of the permease gene relative to the restriction map was deduced. A half-life of 2 min was found for the permease mRNA in labeling kinetics experiments.

Cloning and transcriptional control of a eucaryotic permease gene

The uracil permease gene of the yeast Saccharomyces cerevisiae was cloned on a hybrid plasmid which replicates autonomously in both yeast and Escherichia coli. Cloning was carried out by complementation in yeast. The smallest DNA fragment found to complement the uracil permease deficiency in recipient yeast cells measured approximately 2.3 kilobases. In strains transformed by the plasmid with the uracil permease gene inserted, initial rates of uracil uptake increased up to 25 times more than the rates found in the wild type. Using DNA probes carrying several regions of the cloned gene, I showed that a strain carrying the dhul-I mutation, which is not linked to the permease structural gene and is responsible for enhanced uptake velocity of uracil, had enhanced transcription of the permease gene. By using DNA probes recloned in phage M13 mp7, the direction of transcription of the permease gene relative to the restriction map was deduced. A half-life of 2 min was found for the permease mRNA in labeling kinetics experiments.

Properties of three distinct pyrimide transport systems in yeast. Evidence for distinct energy coupling

In Saccharomyces cerevisiae the uptake of cytosine, uracil and uridine is mediated by three permeases. Using mutants blocked in the metabolic utilization of these three compounds we were able to study their specific uptake. Cytosine and uridine show simple saturation kinetics, whereas uracil uptake is a biphasic process. A comparison of the effects of several inhibitors of energy metabolism on these uptake systems was made. Striking differences were found. 2,4-Dinitrophenol (10(-3) M) and NaN3 (10(-2) M) inhibit the entry of the three compounds to similar extent, but chlorhexidine (10(-5) M) and Dio 9 (50 microgram/ml) which are ATPase inhibitors in vitro strongly impaired cytosine and uridine entry and remained without effect on uracil uptake. We provisionally conclude that these systems may be energized by different mechanisms. In the case of cytosine and uridine permease, a membrane ATPase is possibly involved in the process of energetic coupling whereas this does not seem to be so for uracil.