Potassium Transport by Amino Acid Permeases in Saccharomyces cerevisiae

Deletion of the potassium transporter genesTRK1 and TRK2 impairs potassium uptake inSaccharomyces cerevisiae, resulting in a greatly increased requirement for the ion and the inability to grow on low pH medium. Selection for mutations that restored growth of trk1Δ trk2Δ cells on low pH (3.0) medium led to the isolation of a dominant suppressor that also partially suppressed the increased K+ requirement of these cells. Molecular analysis revealed the suppressor to be an allele of BAP2 that encodes a permease for branched chain amino acids. The suppressor mutation (BAP2-1) converts a phenylalanine codon, highly conserved among the amino acid permease genes, to a serine codon in a region predicted to lie within the sixth membrane-spanning domain. Generation of the analogous mutation in the histidine permease produced an allele,HIP1-293, that similarly suppressed the low pH sensitivity of trk1Δ trk2Δ cells. Suppression of trk1Δ trk2Δ phenotypes by BAP2-1 or HIP1-293was correlated with increased Rb+ uptake. The presence of the substrate amino acids enhanced but was not essential for suppression of trk1Δ trk2Δ phenotypes and increased Rb+ uptake. The conserved site altered by the suppressor mutations appears to be important; his4 HIP1-293 cells show an increased requirement for histidine compared with his4 HIP1 cells. Deletion of the potassium transporter genesTRK1 and TRK2 impairs potassium uptake inSaccharomyces cerevisiae, resulting in a greatly increased requirement for the ion and the inability to grow on low pH medium. Selection for mutations that restored growth of trk1Δ trk2Δ cells on low pH (3.0) medium led to the isolation of a dominant suppressor that also partially suppressed the increased K+ requirement of these cells. Molecular analysis revealed the suppressor to be an allele of BAP2 that encodes a permease for branched chain amino acids. The suppressor mutation (BAP2-1) converts a phenylalanine codon, highly conserved among the amino acid permease genes, to a serine codon in a region predicted to lie within the sixth membrane-spanning domain. Generation of the analogous mutation in the histidine permease produced an allele,HIP1-293, that similarly suppressed the low pH sensitivity of trk1Δ trk2Δ cells. Suppression of trk1Δ trk2Δ phenotypes by BAP2-1 or HIP1-293was correlated with increased Rb+ uptake. The presence of the substrate amino acids enhanced but was not essential for suppression of trk1Δ trk2Δ phenotypes and increased Rb+ uptake. The conserved site altered by the suppressor mutations appears to be important; his4 HIP1-293 cells show an increased requirement for histidine compared with his4 HIP1 cells. Potassium transport in Saccharomyces cerevisiaeis primarily dependent on the product of the TRK1 gene. Mutations in TRK1 lead to a significant increase in the requirement for potassium due to decreased uptake of the ion (1Ramos J. Contreras P. Rodriguez-Navarro A. Arch. Microbiol. 1985; 143: 88-93Google Scholar, 2Gaber R.F. Styles C.A. Fink G.R. Mol. Cell. Biol. 1988; 8: 2848-2859Google Scholar). Cells in which TRK1 is deleted lose the ability to grow on medium containing micromolar concentrations of potassium but readily grow on medium containing millimolar concentrations of the ion due to the presence of a second, highly related transporter encoded byTRK2 (3Ko C.H. Gaber R.F. Mol. Cell. Biol. 1991; 11: 4266-4273Google Scholar, 4Ko C.H. Buckley A.M. Gaber R.F. Genetics. 1990; 125: 305-312Google Scholar). Under conditions where transcription ofTRK2 is genetically derepressed, trk1Δ TRK2cells regain the ability to grow on micromolar potassium (5Vidal M. Buckley A.M. Yohn C. Hoeppner D.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2370-2374Google Scholar, 6Vidal M. Buckley A.M. Hilger F. Gaber R.F. Genetics. 1990; 125: 313-320Google Scholar). Consistent with the ability of Trk2 under these conditions to suppress the trk1Δ phenotype, kinetic analysis showed that cells expressing either TRK1 or TRK2 exhibit high affinity K+ uptake compared with trk1Δ trk2Δcells (7Ramos J. Alijo R. Haro R. Rodriguez-Navarro A. J. Bacteriol. 1994; 176: 249-252Google Scholar). Thus, the dual affinity potassium uptake activities inS. cerevisiae identified by Rodriguez-Navarro and Ramos (8Rodriguez-Navarro A. Ramos J. J. Bacteriol. 1984; 159: 940-945Google Scholar) appear to require Trk1 and Trk2 for high affinity transport and other, unidentified pathways for low affinity transport. Proteins highly related to Trk1 have been identified inSaccharomyces uvarum (9Anderson J.A. Best L.A. Gaber R.F. Gene ( Amst. ). 1991; 99: 39-46Google Scholar), Schizosaccharomyces pombe (10Soldatenkov V.A. Velasco J.A. Avila M.A. Dritschilo A. Notario V. Gene ( Amst. ). 1995; 161: 97-101Google Scholar), andNeurospora, 1A. Rodriguez-Navarro, personal communication. but few details have been discerned regarding their mechanism of transport. The Trk proteins do not contain sequences that are conserved in ion-transporting ATPases, nor do they contain the signature sequences of K+ channels. Recent electrophysiological analysis has addressed the participation of Trk proteins in K+ transport more directly. Whole cell patch-clamp experiments with wild-typeS. cerevisiae cells detected Trk-dependent inward K+ currents in response to hyperpolarizing membrane potentials (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar). These results strongly suggest that K+transport via the Trk system is a passive process driven by the membrane potential. Trk-dependent K+ currents were not detected until the membrane potential was increased beyond −100 mV (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar), which is consistent with previous estimates of the membrane potential in S. cerevisiae (12Calahorra M. Ramı́rez J. Clemente M. Peña A. Biochim. Biophys. Acta. 1987; 899: 229-238Google Scholar, 13Borst-Pauwels G. Biochim. Biophys. Acta. 1981; 650: 88-127Google Scholar). Disruption of TRK1 and TRK2 significantly impairs the ability of the cell to take up K+, resulting in a 1,000-fold increase in the K+ requirement for growth (3Ko C.H. Gaber R.F. Mol. Cell. Biol. 1991; 11: 4266-4273Google Scholar). The hyperpolarization-dependent inward K+currents observed with wild-type cells by whole cell patch clamp analysis were essentially abolished in trk1Δ trk2Δ cells (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar). However, the membrane potential that appears to be the driving force for K+ uptake is intact. This is supported by the functional expression of Arabidopsis K+ channels in trk1Δ trk2Δ cells. Expression of Kat1 restores the ability of trk1Δ trk2Δ cells to grow on very low concentrations of potassium (14Anderson J.A. Huprikar S.S. Kochian L.V. Lucas W.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 3736-3740Google Scholar). Because the inward Kat1-dependent K+ channels are also activated by hyperpolarization beyond −100 mV (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar, 15Schachtman D.P. Schroeder J.I. Lucas W.J. Anderson J.A. Gaber R.F. Science. 1992; 258: 1654-1658Google Scholar), the membrane potential in trk1Δ trk2Δ cells is probably at least of this magnitude and thus can provide sufficient driving force for the uptake of K+ ions provided a portal of entry is available. Such routes of monovalent cation transport have been previously described for bacterial systems. Dosch et al. (16Dosch D.C. Salvacion F.F. Epstein W. J. Bacteriol. 1984; 160: 1188-1190Google Scholar) showed that expression of the wild-type tetracycline/H+ antiporter resulted in partial suppression of a K+ transport-defectiveEscherichia coli mutant by increasing K+ uptake. More recently, it was shown that expression of the wild-type Ca2+/H+ antiporter gene chaA fromE. coli and the wild-type tetracycline/H+antiporter gene tetA(L) from Bacillus subtilisrestore Na+ transport activity in Na+/H+ antiport-defective mutants of E. coli (17Guffanti A.A. Krulwich T.A. J. Bacteriol. 1995; 177: 4557-4561Google Scholar, 18Ivey D.M. Guffanti A.A. Zemsky J. Pinner E. Karpel R. Padan E. Schuldiner S. Krulwich T.A. J. Biol. Chem. 1993; 268: 11296-11303Google Scholar). By selecting for suppressors in trk1Δ trk2Δ cells that restore the ability to grow on low potassium medium, we have obtained dominant mutations in genes encoding glucose transporters (19Ko C.H. Liang H. Gaber R.F. Mol. Cell. Biol. 1993; 13: 638-648Google Scholar), the galactose transporter, 2T. Herman and R. F. Gaber, unpublished results. and one of the inositol transporters 3R. Alijo, S. Grove, C. Ko, A. Dalcanto, and R. F. Gaber, manuscript in preparation. among such revertants. A second phenotype of trk1Δ trk2Δ cells is their hypersensivity to low pH. Whereas wild-type cells can grow quite well on medium adjusted below pH 3.0, growth of trk1Δ trk2Δcells is severely inhibited below pH 4.5. This phenotype appears to be the consequence of the impaired potassium uptake because high concentrations of K+, but not sodium or sorbitol, restore growth of trk1Δ trk2Δ at low pH (4Ko C.H. Buckley A.M. Gaber R.F. Genetics. 1990; 125: 305-312Google Scholar). Further support of this interpretation is provided by the observation that K+uptake by the K+-selective channel Kat1 fully suppresses the low pH sensitivity of trk1Δ trk2Δ cells (14Anderson J.A. Huprikar S.S. Kochian L.V. Lucas W.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 3736-3740Google Scholar). In this report, we describe the isolation and analysis of mutations at a highly conserved site in the amino acid permease genes BAP2and HIP1 that suppress the low pH hypersensitivity and, to a lesser degree, the potassium requirement of trk1Δ trk2Δcells by conferring increased K+ (Rb+) uptake. We present evidence that suggests wild-type amino acid permeases are also somewhat permeable to K+ ions. S. cerevisiae and E. coli strains used in this study are listed in Table I. YPD, YNB, sporulation medium, and routine genetic manipulations are described by Sherman et al. (20Sherman F. Fink G.R. Hicks J. Methods in Yeast Genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY1986Google Scholar). LS medium was made as described previously (1Ramos J. Contreras P. Rodriguez-Navarro A. Arch. Microbiol. 1985; 143: 88-93Google Scholar, 2Gaber R.F. Styles C.A. Fink G.R. Mol. Cell. Biol. 1988; 8: 2848-2859Google Scholar). YNB media containing amino acids as nitrogen sources were made by omitting ammonium sulfate from the recipe and adding the appropriate amount of filter-sterilized amino acid stock solution to medium after sterilization. Yeast and bacterial transformations were performed by electroporation (21Becker D.M. Guarente L. Methods 1991; cerevisiae and E. coli strains were in this trk1Δ strains were in this in a Yeast and were or performed as described by et al. J. Molecular A Cold Spring Harbor Laboratory, Cold Spring Harbor, Scholar). analysis was performed to the of the were by of by of by Scholar). were obtained from and was obtained from A S. cerevisiae was by previously described R.F. Fink G.R. Mol. Cell. Biol. 1990; Scholar) from a trk1Δ trk2Δ (BAP2-1) mutant used partially were the P. Genetics. Scholar), and the was a trk1Δ Table were on medium and to potential (BAP2-1) by their ability to suppress the growth phenotype trk2Δ cells on low pH medium A was obtained that suppression of the low pH sensitivity of trk1Δ trk2Δ cells. was by in E. coli and was to that trk1Δ trk2Δ cells by led to suppression of the low pH A from was P. Genetics. Scholar) which was of the trk1Δ trk2Δ The of of this was and the S. The wild-type allele of the suppressor gene was by an and with was by a that the of from the P. Genetics. Scholar). of the with the region of was with and used to to from one was with with and used to E. coli to The of the region of the in the resulting was the of used for (BAP2-1) and were performed with the and the of of the J. P. 1984; Scholar). A of of the BAP2 region was made by the P. Genetics. Scholar). A of BAP2 was by the described and a to the region to region the P. Genetics. Scholar). was by and used to and and strains to of the trk1Δ trk2Δ BAP2-1 the suppressor phenotype, that they a of The due to and the at the resulting in of the with The presence of the in was by analysis not strains and are of strains and suppression of the trk1Δ trk2Δ phenotypes (BAP2-1) amino acids in the growth medium trk1Δ trk2Δ strains were was with and were strains and which require M. Gaber R.F. 1994; Scholar) was performed to by of and with P. Genetics. Scholar). were to that by at the This strains and that are to grow on medium after and were with BAP2 and BAP2-1 by and and and The of containing the functional HIP1 gene provided by P. and G. was and was performed to the The which is to the HIP1 was used to a to in the second of amino acid codon The presence of the mutation was by the in this the allele and the mutant HIP1-293 allele were as from and the and P. Genetics. Scholar) and M. P. Gene ( 1992; Scholar). Cells were in either YNB and or YNB and with amino acids with of cells were by and in uptake adjusted to pH with containing and either or with a of amino acids of in uptake was and the of uptake was the of cells were by with and in The of was by Mutations that suppress the low pH hypersensitivity of trk1Δ trk2Δ cells were by selecting for to grow on medium with but adjusted to pH analysis of these mutants the presence of be and dominant The dominant suppressors also partially suppressed the increased potassium requirement by the trk1Δ trk2Δ mutations and were thus for their potassium the dominant suppressors revealed a of the described analysis of from a the trk1Δ trk2Δ mutant and a trk2Δ revealed that the suppressor mutation as a the a of made from the was for that suppress the low pH sensitivity and potassium requirement trk2Δ cells. A suppressor was and a that suppression of the trk1Δ trk2Δ phenotypes was for The wild-type on a by an and and was and and and trk2Δ to suppress the trk1Δ trk2Δ that the the wild-type allele of the suppressor analysis of the that the functional suppressor revealed a of amino acid which a of analysis J. R.F. J. Mol. Biol. Scholar) of the revealed membrane-spanning A of the S. cerevisiae revealed that the encoding the allele was essentially to the H. M. G. C. A. A.M. E. et J. 1994; 13: Scholar) and identified as an amino acid permease of branched chain amino acids M. Biochim. Biophys. Acta. 1995; Scholar). A with related sequences revealed that to as with the amino acid permease M. Gene ( Amst. ). 1994; 143: Scholar) and least with the M. M. Gene ( Amst. ). Scholar). The ability of trk1Δ trk2Δ cells to grow on medium suggests that membrane proteins Trk1 or Trk2 be of K+ uptake. a in potassium a allele containing a of the was in trk1Δ trk2Δ BAP2 and trk1Δ trk2Δ BAP2-1 cells and and for of the BAP2 sequences in trk2Δ BAP2-1 abolished suppression of trk2Δ The trk1Δ trk2Δ cells growth compared with the cells that BAP2 is M. Biochim. Biophys. Acta. 1995; Scholar). In the potassium requirement trk2Δ by for growth on medium containing concentrations of the was not increased compared with the trk1Δ trk2Δ not Thus, wild-type not provide a of K+ uptake in trk1Δ trk2Δ cells and is not for their ability to grow on The product of gene has been shown to be within the and is to be for the of not amino acid permeases in S. cerevisiae Styles C.A. Fink G.R. Cell. 1992; Scholar). amino acid permeases are in the in membrane the are Thus, the of appears to be for the of amino acid or not of is dependent on allele was in a trk1Δ trk2Δ BAP2-1 to for of suppressor The mutation abolished the ability of trk1Δ trk2Δ BAP2-1 cells to grow on low pH and on low potassium medium but increased the K+ requirement of trk1Δ trk2Δ cells expressing the wild-type BAP2 allele not In the of TRK1 trk2Δ cells containing mutation was from that of TRK1 trk2Δ cells the gene not that the activity of is not for of In S. acid permeases as or not the have altered the of an site the amino acid transport mechanism the ability of BAP2-1 to suppress trk1Δ was in the of amino acids in the growth medium. A trk1Δ trk2Δ containing the dominant BAP2-1 suppressor on a growth on low potassium or low pH medium in the presence and the of amino acids that suppression of the trk1Δ trk2Δ phenotypes is not dependent on the uptake of amino acids. Thus, the suppressor mutation in BAP2-1 the of the permease to potassium this is not to amino acid The of the of containing the wild-type BAP2 gene was and compared with the BAP2-1 A the sequences was a to (BAP2-1) of the second of codon resulting in a phenylalanine to serine in the amino acid This site is within the sixth of the transporter and at a site highly conserved among S. cerevisiae amino acid but one of the of the amino acid permease gene in S. cerevisiae contain phenylalanine at this site In the of the a to is present at this The of this site suggests that it an in the of amino acid the conserved phenylalanine that is to serine in BAP2-1 an in an mutation in amino acid permease also suppress the trk1Δ trk2Δ this the analogous mutation was made in the histidine permease was from a phenylalanine to a serine codon by The resulting allele was and and The were to suppress the sensitivity to low pH and the potassium requirement of a trk1Δ trk2Δ suppression of the trk1Δ trk2Δ low potassium and low pH phenotypes be detected on medium amino that suppression of histidine transport. Cells the HIP1-293 allele on the suppression of the trk1Δ trk2Δ potassium requirement compared with cells that the suppressor from the that transport of K+ by was for growth as a gene. expression of the wild-type HIP1 allele from a not suppress either of the trk1Δ trk2Δ expression from a suppressed the low pH phenotype that wild-type amino acid permeases transport of In support of this in trk1Δ trk2Δ cells resulted in growth on medium containing compared with trk1Δ trk2Δ This was not due to a on growth because in TRK1 cells on growth under and R. F. Gaber, unpublished results. BAP2-1 increase potassium Rb+ uptake were performed the trk1Δ trk2Δ that was with either the or expressing the wild-type or suppressor of the amino acid Because amino acids and the presence of these in the growth medium results in greatly increased BAP2 transcription M. C. J. Bacteriol. Scholar), Rb+ uptake were performed cells either in the presence or the of amino acids in the growth medium and in the uptake The of Rb+ uptake in trk1Δ trk2Δ cells expressing the BAP2-1 allele was from trk1Δ trk2Δ cells the wild-type allele the cells were in medium with their In the of Rb+ uptake increased in trk1Δ trk2Δ BAP2-1 cells under conditions Table Rb+ uptake was not observed with trk1Δ trk2Δ cells expressing the wild-type BAP2 gene under these The presence of amino acids in the increased Rb+ uptake by trk1Δ trk2Δ BAP2-1 uptake were in YNB with or with with amino acids and for The was similarly as and with and histidine of Rb+ uptake is cells not Cells were in YNB with or with with amino acids and for The was similarly as and with and histidine of Rb+ uptake is cells not in a The expression of HIP1-293 in trk1Δ trk2Δcells increased Rb+ uptake compared with cells expressing the wild-type HIP1 allele The ability to this increase was dependent on the cells in the presence of amino acids and enhanced by the of amino acids to the the The was obtained histidine was in the of amino acids that amino acid transport increased K+ uptake by the mutant The requirement of amino acids in the growth medium to or Rb+ uptake is in with the ability of BAP2-1 or to suppress the trk1Δ trk2Δ phenotypes on medium. the for increased growth of trk1Δ trk2Δ cells on or low pH medium is that to an increase in Rb+ uptake by the used in The requirement for an amino acid at the conserved site altered by the BAP2-1 and was addressed by the ability of and HIP1-293 to suppress the histidine of a his4 mutant containing mutation Table HIP1 his4 cells require of histidine to support growth in medium with amino acids Fink G.R. Gene ( Amst. ). 1985; Scholar). Suppression of the histidine requirement from a was very that histidine transport is significantly impaired but not abolished by mutation at Consistent with this from a in the resulted in growth from cells expressing the wild-type HIP1 The of high affinity K+ by the of the TRK1 and TRK2 genes, phenotypes a increase in potassium to support growth and the inability to grow on medium below pH 4.5. The phenotypes or trk1Δ trk2Δ cells have genetic to be K+ transport. These the and functional analysis of proteins that restore K+ uptake and the analysis of suppressor The has led to the of K+ transporters from (9Anderson J.A. Best L.A. Gaber R.F. Gene ( Amst. ). 1991; 99: 39-46Google M.A. A. J. 1995; Scholar) and D.P. Schroeder J.I. 1994; Scholar) and K+ channels (14Anderson J.A. Huprikar S.S. Kochian L.V. Lucas W.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 3736-3740Google Scholar, H. M. F. F. C. Science. 1992; Scholar), the second is that a of S. cerevisiae membrane transporters (19Ko C.H. Liang H. Gaber R.F. Mol. Cell. Biol. 1993; 13: 638-648Google Scholar), and H. unpublished results. the inositol and amino acid transporters can be genetically altered to increased potassium transport. we describe a mutation in the amino acid permease gene as a suppressor of the low phenotype of trk1Δ trk2Δ cells. Consistent with previous that either high concentrations of potassium in the growth medium (3Ko C.H. Gaber R.F. Mol. Cell. Biol. 1991; 11: 4266-4273Google or the expression of a K+ transporter (14Anderson J.A. Huprikar S.S. Kochian L.V. Lucas W.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 3736-3740Google Scholar) can suppress this phenotype, we show that a increase in the of K+ uptake by trk1Δ trk2Δ cells. Thus, suppression of the low pH sensitivity by appears to the of an K+ transport in these cells. A mutation analogous to BAP2-1 was in the histidine permease and it similarly trk2Δ phenotypes by conferring a increase in the of K+ that the ability of a amino acid to amino acid permeases proteins of K+ ions be a of this of The BAP2-1 and HIP1-293 mutations serine for a highly conserved phenylalanine predicted to within the sixth membrane-spanning domain. We have shown that this site is for amino acid transport in the of the histidine The suppressor mutations a amino acid for a The increased to K+ by this mutation be a consequence of the of or a in the at this The ability of to suppression of trk1Δ trk2Δ phenotypes and HIP1-293 unpublished suggest that the site of this entry for is not for The uptake of amino acids in S. cerevisiae can be with the uptake of via a This was observed as increased uptake by amino acids in cells A. C. A.A. J. Scholar). of acid from to on the A.A. J.A. J. Scholar, A. J.A. Slayman Slayman C.L. Biophys. Acta. 1984; Scholar, A. G. A.A. J. Scholar). Thus, amino acid permeases inS. cerevisiae have ion-transporting but the of conserved or in or has not been The presence of a at the of the sixth predicted is a conserved of transporters to uptake by is not present in transporters to Biol. 1992; Scholar). and contain a amino acids beyond the of their predicted sixth domain. These are consistent with and by a acid We that the amino acid altered by the BAP2-1 mutations of a or site to a The mutation K+ for as a for amino acid or it increase K+ ion and amino acid transport. suggest that K+ transport by the mutants is to amino acid uptake. increased K+ transport appears to of amino acid uptake because BAP2-1 cells suppress the trk1Δ phenotypes in the of their substrate amino acids. the presence of amino acids growth was to the increased Rb+ the presence of substrate amino acids in the increased Rb+ transport by either or However, the observation that the HIP1-293 mutant impaired histidine uptake that the mutation the transport a K+ channel D.P. Schroeder J.I. Lucas W.J. Anderson J.A. Gaber R.F. Science. 1992; 258: 1654-1658Google Scholar), suppresses the low pH hypersensitivity and potassium requirement of trk1Δ trk2Δ cells (14Anderson J.A. Huprikar S.S. Kochian L.V. Lucas W.J. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 3736-3740Google Scholar) by K+ uptake (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar). Because Kat1 is activated membrane hyperpolarization in of (11Bertl A. Anderson J.A. Slayman C.L. Gaber R.F. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2701-2705Google Scholar, 15Schachtman D.P. Schroeder J.I. Lucas W.J. Anderson J.A. Gaber R.F. Science. 1992; 258: 1654-1658Google Scholar) the membrane potential of trk1Δ trk2Δ cells is to cation and is to provide the driving force for K+ uptake via and of the wild-type histidine permease was observed to suppress the low pH sensitivity trk2Δ cells. In of in trk1Δ trk2Δ cells increased their J. M. J. M. G. and R. F. Gaber, unpublished results. Thus, it is that one or more amino acid permeases in S. cerevisiae K+ transport is that acid not have an requirement for as a and be of K+ ions to amino acid uptake. the analysis of mutations that increased K+uptake by amino acid permeases or for the We P. and G. Fink for and