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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.

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Competition experiments revealed that adenine and guanine were transported by a purine permease in both Candida glabrata 4 and a C. glabrata 4 cytosine permease negative mutant. The C. glabrata 4 cytosine permease negative mutant was isolated using 5-fluorocytosine selection. This mutant no longer transported cytosine, but transported adenine and guanine. A transport system for hypoxanthine was not detected. Hence, in addition to the cytosine permease, a purine permease exists in C. glabrata. This differs from the purine cytosine permeases in Saccharomyces cerevisiae and Candida albicans which transport adenine, cytosine, guanine and hypoxanthine.

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The purine-cytosine permease (PCP) of the yeast Saccharomyces cerevisiae was detected by immunological methods. Using antibodies directed against synthetic peptides, whose sequences were derived from the primary structure of the PCP, immunoprecipitation of [35S]methionine-labelled PCP was achieved either from cellular extracts or from in vitro translation mixtures. Non-labelled PCP was also detected on Western blots of membrane proteins. Similar migration rates were observed for PCP originating both from immunoprecipitated cellular extracts and from in vitro translation mixtures. Hence, post-translational processing, if any, only slightly affects the size of the protein. Also no evidence was found for N-linked core-glycosylation: identical migration rates were observed when immunoprecipitated PCP molecules were extracted from cells labelled for 10 min with [35S]methionine, pretreated or not with tunicamycin. On the other hand, the suppression of the two potential N-linked glycosylation sequences in the DNA did not lead to inactivation of the transport activity, confirming that N-linked glycosylation is not required for the permease activity.

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A Saccharomyces cerevisiae gene (1722 bp), encoding a protein (574 aa) highly homologous to the basic-amino-acid permeases LYP1 and CAN1, was sequenced. The gene, which was named APL1 (Amino-acid Permase Like), is located 881 bp upstream from LYP1 (lysine-specific permease), and in head-to-head orientation to it. These sequence data have been deposited in the EMBL/GenBank/DDBJ nucleotide sequence data libraries under Accession Number X74069.

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Using a gene bank of C. albicans, the lysine-permease deficiency in a strain of S. cerevisiae was complemented, and the restriction map of the corresponding C. albicans DNA fragment was constructed. Its expression in S. cerevisiae showed that the permease of C. albicans actively transports arginine (KT = 18 mumol/l, Jmax = 26 nmol/min per mg dry weight), lysine (KT = 12 mumol/l, Jmax = 18 nmol/min per mg dry weight), histidine (KT = 37 mumol/l, Jmax = 9.7 nmol/min per mg dry weight), as well as their toxic analogues canavanine and thialysine, with high affinity. The intracellular concentration of basic amino acids transported into S. cerevisiae by the C. albicans permease reaches more than a thousand-times-higher value compared to the external concentration in the medium. Accumulated amino acids do not leave the cells. The uptake is strongly reduced by the protonophores and inhibitors of plasma membrane H(+)-ATPase.

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The LYP1 gene of Saccharomyces cerevisiae was cloned by complementation in lysine-permease-deficient recipient yeast cells, and its nucleotide sequence was determined. An open reading frame of 1833 nucleotides was found encoding a polypeptide of 611 amino acids, with a calculated molecular weight of 68 118. Analysis of the deduced primary structure of the protein revealed ten membrane-spanning regions and three potential N-glycosylation sites. Analysis of the deduced sequence of protein LYP1 indicates homology with other yeast amino-acid permeases, in particular with CAN1, and also the lysine-specific permease of Escherichia coli. The strain transformed by a multi-copy plasmid harbouring the LYP1 gene, showed a 20-fold increase in the maximum velocity of lysine uptake over that in the wild type, with no changes in the affinity of the permease for its substrate.

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Three u.v.-induced mutants of the purine-cytosine permease gene of Saccharomyces cerevisiae, with altered apparent Michaelis constant of transport (Kmapp), were cloned and sequenced. One of the mutants had extensive nucleotide replacement, whereas the other two had a single mutation. To evaluate the contribution of the different amino acid replacements to the phenotype of the complex mutant, simpler mutants were created by site-directed mutagenesis. All the amino acid replacements found in the segment from amino acids 371 to 377 inclusive, contribute to the determination of the phenotype. According to the model postulated this segment lies on the cell surface. In particular, amino acids at position 374 and 377 modulate the affinity of the permease towards its substrates. In the wild-type, when asparagine is present at both of these positions, the lowest Kmapp values are found.

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Mutants defective in lysine transport were isolated and characterized. After UV-mutagenesis colonies resistant to thialysine, a toxic analogue of lysine, were isolated and L-lysine uptake into the mutant strains was analyzed. Among the thialysine-resistant strains a group of mutants was found, where the half-saturation constant, KT, of the high-affinity transport system for lysine was higher than in the wild-type, the high-affinity transport system for basic amino acids being specifically affected. This was confirmed by a complementation test in which all the thialysine-resistant strains with a higher KT for lysine uptake belonged to one complementation group. Kinetic and genetic analysis showed that our mutants were identical with can1-1 mutants, showing that a single high-affinity system for the transport of basic amino acids exists in S. pombe.

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The FCY2 gene of the purine-cytosine permease (PCP) of Saccharomyces cerevisiae and the allele fcy2-21 have been cloned on the yeast multicopy plasmid pJDB207. The corresponding plasmids were introduced into a S. cerevisiae strain carrying a chromosomal deletion at the FCY2 locus. The resulting strains were designated pAB4 and pAB25 respectively. The pAB25 strain, which carries the fcy2-21 allele, contains four amino acid changes in the open reading frame of the PCP (Weber et al., 1989). The influence of these mutations was studied on cells by determination of the uptake constants of purine bases and cytosine [apparent Michaelis constant of transport (Ktapp) and Vmax] and on plasma-membrane preparations, by measurements of binding parameters at equilibrium [(Kd and maximum amount of binding sites/Bmax)]. For strain pAB4, the Ktapp and Vmax of uptake were almost similar for all solutes considered [1.8-2.6 microM and 8.5-10.2 nmol.min-1.(10(7) cells)-1]. The main effect of the mutations in strain pAB25 was based on a large increase in Ktapp for all ligands except adenine. Plasma membranes of each strain displayed one class of specific binding sites. Variations in Kd of 0.4-1 microM were observed for pAB4. These slight variations had no effect on the Ktapp of uptake measured for the corresponding solutes. In contrast, using pAB25 membranes, Kd increased dramatically; 2.6 microM, 40 microM and 96 microM for adenine, cytosine and hypoxanthine, respectively. These increments were correlated to variations in Ktapp of the uptake for cytosine and hypoxanthine. Therefore, we conclude that modification in the Ktapp of uptake in the strain carrying fcy2-21 allele is merely due to a modification of the binding ability of the permease for its ligands.

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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.

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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.

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The comparison of the amino acid sequences of four yeast transport proteins indicates that there is a questionable relatedness between the uracil permease (FUR4) and the purine-cytosine permease (FCY2), whereas the arginine permease (CAN1) and the histidine permease (HIP1) clearly originated from a common molecular ancestor. The analysis of the primary structure of these transport proteins by two methods of secondary structure predictions suggests the presence of 9-12 membrane-spanning alpha-helices in each polypeptide chain. These results are concordant in that 90% of the alpha-helices were determined by both methods to be at the same positions. In the aligned sequences HIP1 and CAN1, the postulated membrane-spanning alpha-helices often start at corresponding sites, even though the overall sequence similarity of the two proteins is only 30%. In the aligned DNA coding sequences of CAN1 and HIP1, synonymous nucleotide substitutions occur with very similar frequencies in regions where the replacement substitution (changing the amino acids) frequencies are widely different. Moreover, our data suggest that the replacement substitutions can be considered as neutral in the N-terminal segment, whereas the other regions are subject to a conservative selective pressure because, if compared to a random drift, the replacement substitutions are underrepresented.

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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.

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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.

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8-Azido[2-3H]adenine was used as a photoaffinity label for the purine-cytosine transport system. After irradiation in the presence of the photoaffinity label, the cells were converted into protoplasts, their plasma membranes were purified, and the membrane proteins were extracted and separated by NaDodSO4/PAGE. The radioactivity was specifically incorporated into a protein with a molecular weight of 120,000. Photoaffinity labeling of this protein could be blocked by irradiation in the presence of natural substrates for the transport system. The molecular weight as determined by NaDod-SO4/PAGE was found to be twice the value calculated from mRNA analysis of the cloned gene. Incubation of exponentially growing cells with tunicamycin, an antibiotic that inhibits glycosylation of proteins, resulted in a 40% decrease in the overall initial uptake rate, which correlates with the reduction of the labeled Mr 120,000 protein. Treatment of the extracted labeled plasma membrane proteins with glycosidic enzymes resulted in disappearance of the Mr 120,000 peak and the appearance of new peaks at Mr 60,000 and Mr 73,000. These findings indicate that the purine-cytosine transport protein is a glycoprotein.

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The photoreactivation repair gene (PHR1) of the yeast Saccharomyces cerevisiae was cloned in a hybrid plasmid (pJDB207), which is able to replicate as a multicopy episome in S. cerevisiae and Escherichia coli cells. The size of the DNA fragment found to have the photoreactivation activity was 3.0 kb, determined by recloning of the isolated fragment. In wild type cells transformed by the plasmid containing the PHR1 gene, the number of DNA photolyase molecules was 15 times greater than in wild type cells with pJDB207 only. Using the same receptor strain the excision repair gen RAD1 was also isolated. The size of the insert of the DNA which complements excision repair deficiency in recipient yeast cells was 5.7 kb. The recipient cells after transformation with the plasmid containing RAD1 showed the same UV-sensitivty as wild type cells with pJDB207 only.

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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.

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A piece of DNA of the yeast Saccharomyces cerevisiae complementing the uracil permease gene was introduced into a plasmid able to replicate autonomously in Schizosaccharomyces pombe. A strain of S. pombe lacking uracil transport activity was transformed with this new plasmid carrying the gene of S. cerevisiae. The behaviour of the transformant shows not only an expression of the uracil permease gene in the heterologous membrane but also that the transport of uracil is active and coupled to the energy furnishing system of the heterologous host.

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