Crystal structures of two solute receptors for L-cystine and L-cysteine, respectively, of the human pathogen Neisseria gonorrhoeae

ATP-binding cassette (ABC) transporters are integral membrane proteins that carry a variety of substrates across biological membranes at the expense of ATP. The here considered prokaryotic canonical importers consist of three entities: an extracellular solute receptor, two membrane-intrinsic proteins forming a translocation pathway, and two cytoplasmic ATP-binding subunits. The ngo0372-74 and ngo2011-14 gene clusters from the human pathogen Neisseria gonorrhoeae were predicted by sequence homology as ABC transporters for the uptake of cystine and cysteine, respectively, and chosen for structural characterization. The structure of the receptor component Ngo0372 was obtained in a ligand-free "open" conformation and in a "closed" conformation when co-crystallized with L-cystine. Our data provide the first structural information of an L-cystine ABC transporter. Dissociation constants of 21 and 33 nM for L-cystine and L-selenocystine, respectively, were determined by isothermal titration calorimetry. In contrast, L-cystathionine and L-djenkolic acid are weak binders, while no binding was detectable for S-methyl-L-cysteine. Mutational analysis of two residues from the binding pocket, Trp97 and Tyr59, revealed that the latter is crucial for L-cystine binding. The structure of the Ngo2014 receptor was obtained in closed conformation in complex with co-purified L-cysteine. The protein binds L-cysteine with a K(d) of 26 nM. Comparison of the structures of both receptors and analysis of the ligand binding sites shed light on the mode of ligand recognition and provides insight into the tight binding of both substrates. Moreover, since L-cystine limitation leads to reduction in virulence of N. gonorrhoeae, Ngo0372 might be suited as target for an antimicrobial vaccine.     

Regulation of potassium uptake in the sodium-resistant (NaCl(r)) and thalium-resistant (TlCl(r)) mutant strain of diazotrophic cyanobacterium Anabaena variabilis

A thalium chloride-resistant (TlCl(r)) mutant strain and a sodium chloride-resistant (NaCl(r)) mutant strain of the diazotrophic cyanobacterium Anabaena variabilis have been isolated by spontaneous and chemical mutagenesis by using TlCl, a potassium (K(+)) analog, and nitrosoguanidine (NTG), respectively. The TlCl(r) mutant strain was found to be defective in K(+) transport and showed resistance against 10 microM TlCl. However, it also showed sensitivity against NaCl (LD(50), 50 m M). In contrast, neither wild-type A. variabilis nor its NaCl(r) mutant strain could survive in the presence of 10 microM TlCl and died even at 1 microM TlCl. The TlCl(r) mutant strain exhibited almost negligible K(+) uptake, indicating the lack of a K(+) uptake system. High K(+) uptake was, however, observed in the NaCl(r) mutant strain, reflecting the presence of an active K(+) uptake system in this strain.DCMU, an inhibitor of PS II, inhibited the K(+) uptake in wild-type A. variabilis and its TlCl(r) and NaCl(r) mutant strains, suggesting that K(+) uptake in these strains is an energy-dependent process and that energy is derived from photophosphorylation. This contention is further supported by the inhibition of K(+) uptake under dark conditions. Furthermore, the inhibition of K(+) uptake by KCN, DNP, and NaN(3) also suggests the involvement of oxidative phosphorylation in the regulation of an active K(+) uptake system.The whole-cell protein profile of wild-type A. variabilis and its TlCl(r) and NaCl(r) mutant strains growing in the presence of 50 m M KCl was made in the presence and absence of NaCl. Lack of transporter proteins in TlCl(r) mutant strain suggests that these proteins are essentially required for the active transport and accumulation of K(+) and make this strain NaCl sensitive. In contrast, strong expression of the transporter proteins in NaCl(r) mutant strain and its weak expression in wild-type A. variabilis is responsible for their resistance and sensitivity to NaCl, respectively. Therefore, it appears that the increased salt tolerance of the NaCl(r) mutant strain was owing to increased K(+) uptake and accumulation, whereas the salt sensitivity of the TlCl(r) mutant strain was owing to the lack of K(+) uptake and accumulation.     

Glutamate-induced glioma cell proliferation is prevented by functional expression of the glutamate transporter GLT-1

A tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of l-cystine. The addition of glutamate (100 microM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. This suggests that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation.     

Vitamin C inhibits diethylmaleate-induced L-cystine transport in human vascular smooth muscle cells

Adaptive increases in intracellular glutathione (GSH) in response to oxidative stress are mediated by induction of L-cystine uptake via the anionic amino acid transport system x(c)(-). The recently cloned transporter xCT forms a heteromultimeric complex with the heavy chain of 4F2 cell surface antigen (4F2hc/CD98). Depletion of GSH by the electrophile diethylmaleate (DEM) induces the activity and expression of xCT in peritoneal macrophages. We here examine the effects of vitamin C on induction of xCT by DEM in human umbilical artery smooth muscle cells. DEM caused time- (3-24 h) and concentration- (25-100 microM) dependent increases in L-cystine transport, with GSH depleted by 50% after 6 h and restored to basal values after 24 h. xCT mRNA levels increased after 4 h DEM treatment with negligible changes detected for 4F2hc mRNA. DEM caused a rapid (5-30 min) phosphorylation of p38(MAPK). Inhibition of p38(MAPK) by SB203580 (10 microM) enhanced DEM-induced increases in L-cystine transport and GSH, whereas inhibition of p42/p44(MAPK) (PD98059, 10 microM) had no effect. Pretreatment of cells with vitamin C (100 microM, 24 h) attenuated DEM-induced adaptive increases in L-cystine transport and GSH levels. Inhibition of p38(MAPK), but not p42/p44(MAPK), reduced the cytoprotective action of vitamin C. Our findings suggest that DEM induces activation of xCT via intracellular signaling pathways involving p38(MAPK), and that vitamin C, in addition to its antioxidant properties, may modulate this signaling pathway to protect smooth muscle cells from injury.     

AtKuP1: a dual-affinity K+ transporter from Arabidopsis.

Plant roots contain both high- and low-affinity transport systems for uptake of K+ from the soil. In this study, we characterize a K+ transporter that functions in both high- and low-affinity uptake. Using yeast complementation analysis, we isolated a cDNA for a functional K+ transporter from Arabidopsis (referred to as AtKUP1 for Arabidopsis thaliana K+ uptake). When expressed in a yeast mutant, AtKUP1 dramatically increased K+ uptake capacity at both a low and high [K+] range. Kinetic analyses showed that AtKUP1-mediated K+ uptake displays a "biphasic" pattern similar to that observed in plant roots. The transition from the high-affinity phase (K(m) of 44 microM) to the low-affinity phase (K(m) of 11 mM) occurred at 100 to 200 microM external K+. Both low- and high-affinity K+ uptake via AtKUP1 were inhibited by 5 mM or higher concentrations of NaCl. In addition, AtKUP1-mediated K+ uptake was inhibited by K+ channel blockers, including tetraethylammonium, Cs+, and Ba2+. Consistent with a possible function in K+ uptake from the soil, the AtKUP1 gene is primarily expressed in roots. We conclude that the AtKUP1 gene product may function as a K+ transporter in Arabidopsis roots over a broad range of [K+] in the soil.     

Absence of light-induced proton extrusion in a cotA-less mutant of Synechocystis sp. strain PCC6803.

cotA of Synechocystis sp. strain PCC6803 was isolated as a gene that complemented a mutant defective in CO2 transport and is homologous to cemA that encodes a chloroplast envelope membrane protein (A. Katoh, K.S. Lee, H. Fukuzawa, K. Ohyama, and T. Ogawa, Proc. Natl. Acad. Sci. USA 93:4006-4010, 1996). A mutant (M29) constructed by replacing cotA in the wild-type (WT) Synechocystis strain with the omega fragment was unable to grow in BG11 medium (approximately 17 mM Na+) at pH 6.4 or at any pH in a low-sodium medium (100 microM Na+) under aeration with 3% (vol/vol) CO2 in air. The WT cells grew well in the pH range between 6.4 and 8.5 in BG11 medium but only at alkaline pH in the low-sodium medium. Illumination of the WT cells resulted in an extrusion followed by an uptake of protons. In contrast, only proton uptake was observed for the M29 mutant in the light without proton extrusion. There was no difference in sodium uptake activity between the WT and mutant. The mutant still possessed 51% of the WT CO2 transport activity in the presence of 15 mM NaCl. On the basis of these results we concluded that cotA has a role in light-induced proton extrusion and that the inhibition of CO2 transport in the M29 mutant is a secondary effect of the inhibition of proton extrusion.     

Cysteine metabolism in Legionella pneumophila: characterization of an L-cystine-utilizing mutant

Growth of Legionella pneumophila on buffered charcoal-yeast extract (BCYE) medium is dependent on L-cysteine (but not L-cystine), which is added in excess over what is required for nutrition. We investigated the biochemical and genetic bases for this unusual requirement and determined that much of the L-cysteine in BCYE medium is rapidly oxidized to L-cystine and is unavailable to the bacteria. Analysis of cysteine consumption during bacterial growth indicated that of the 11% consumed, 3.85% (approximately 0.1 mM) was incorporated into biomass. The activities of two key cysteine biosynthetic enzymes (serine acetyltransferase and cysteine synthase) were not detected in cell extracts of L. pneumophila, and the respective genes were not present in the genome sequences, confirming cysteine auxotrophy. Kinetic studies identified two energy-dependent cysteine transporters, one with high affinity (apparent Km, 3.29 microM) and the other with low affinity (apparent Km, 93 microM), each of which was inhibited by the uncoupling agent carbonyl cyanide m-chlorophenylhydrazone. Cystine was not transported by L. pneumophila; however, a mutant strain capable of growth on L-cystine (CYS1 mutant) transported L-cystine with similar kinetics (Km, 4.4 microM and 90 microM). Based on the bipartite kinetics, requirement for proton motive force, and inhibitor studies, we suggest that a high-affinity periplasmic binding protein and a major facilitator/symporter (low affinity) mediate uptake. The latter most likely is functional at high cysteine concentrations and most likely displays altered substrate specificity in the CYS-1 mutant. Our studies provide biochemical evidence to support a general view that L. pneumophila is restricted to an intracellular lifestyle in natural environments by an inability to utilize cystine, which most likely ensures that the dormant cyst-like transmissible forms do not germinate outside suitable protozoan hosts.     

Genes from Debaryomyces hansenii increase salt tolerance in Saccharomyces cerevisiae W303

The yeast Debaryomyces hansenii has been chosen as a model for molecular studies of tolerance to NaCl. A gene library was built and transformants of Saccharomyces cerevisiae W303 containing genes from D. hansenii were selected for their ability to grow in the presence of high concentrations of NaCl and/or low concentrations of KCl. In three of these transformants 500 mM NaCl improved growth at pH 7.6 like in D. hansenii but not in S. cerevisiae. One of the plasmids restored growth at 50 microM KCl and K(+) uptake in a mutant of S. cerevisiae lacking genes that encode K(+) transporters.     

Conservative mutations of glutamine-125 in herpes simplex virus type 1 thymidine kinase result in a ganciclovir kinase with minimal deoxypyrimidine kinase activities

The herpes simplex virus type 1 thymidine kinase (HSV-1 TK) is the major anti-herpes virus pharmacological target, and it is being utilized in combination with the prodrug ganciclovir as a toxin gene therapeutic for cancer. One active-site amino acid, glutamine-125 (Gln-125), has been shown to form hydrogen bonds with bound thymidine, thymidylate, and ganciclovir in multiple X-ray crystal structures. To examine the role of Gln-125 in HSV-1 TK activity, three site-specific mutations of this residue to an aspartic acid, an asparagine, or a glutamic acid were introduced. These three mutants and wild-type HSV-1 TK were expressed in E. coli and partially purified and their enzymatic properties compared. In comparison to the Gln-125 HSV-1 TK, thymidylate kinase activity of all three mutants was decreased by over 90%. For thymidine kinase activity relative to Gln-125 enzyme, the K(m) of thymidine increased from 0.9 microM for the parent Gln-125 enzyme to 3 microM for the Glu-125 mutant, to 6000 microM for the Asp-125 mutant, and to 20 microM for the Asn-125 mutant. In contrast, the K(m) of ganciclovir decreased from 69 microM for the parent Gln-125 enzyme to 50 microM for the Asn-125 mutant and increased to 473 microM for the Glu-125 mutant. The Asp-125 enzyme was able to poorly phosphorylate ganciclovir, but with nonlinear kinetics. Molecular simulations of the wild-type and mutant HSV-1 TK active sites predict that the observed activities are due to loss of hydrogen bonding between thymidine and the mutant amino acids, while the potential for hydrogen bonding remains intact for ganciclovir binding. When expressed in two mammalian cell lines, the Glu-125 mutant led to GCV-mediated killing of one cell line, while the Asn-125 mutant was equally as effective as wild-type HSV-1 TK in metabolizing GCV and causing cell death in both cell lines.     

Non-identity of cystine lyase with β-cystathionase in turnip roots

An active preparation of cystine lyase (EC 4.4.1.-) was prepared from turnip roots and its substrate specificity examined. Only L-cysteine, cysteine-S-SO₃, and the sulphoxides of L-djenkolic acid, S-methyl-and S-ethyl-L-cysteine were substrates. L-Cystathione, L-djenkolic acid, S-methyl-and S-ethyl-cysteines were not cleaved by this enzyme. The K_m for L-cystine was 1.3 mM and L-cystathionine acted as an effective competitive inhibitor with a K_i of 0.7 mM. After dialysis against 10 mM potassium phosphate buffer pH 7.5, added pyridoxal phosphate was absolutely necessary for activity. In addition a marked stimulation was observed in the presence of ammonium sulphate. The products of the reaction were cysteine persulphide, pyruvate and presumably ammonia. The persulphide was easily demonstrated by cleavage with CN^? to yield SCN^? under conditions in which elemental sulphur was unreactive.     

Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na(+)-dependent and highly selective. The K(m) for tryptophan was approximately 145 nm, and tryptophan transport was unchanged in the presence of 100 microM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K(I) values of 200 and 440 microM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.     

Production of a recombinant hybrid hemoflavoprotein: engineering a functional NADH:cytochrome c reductase.

A gene has been constructed coding for a unique fusion protein, NADH:cytochrome c reductase, that comprises the soluble heme-containing domain of rat hepatic cytochrome b(5) as the amino-terminal portion of the protein and the soluble flavin-containing domain of rat hepatic cytochrome b(5) reductase as the carboxyl terminus. The gene has been expressed in Escherichia coli resulting in the highly efficient production of a functional hybrid hemoflavoprotein which has been purified to homogeneity by a combination of ammonium sulfate precipitation, affinity chromatography on 5'-ADP agarose, and size-exclusion chromatography. The purified protein exhibited a molecular mass of approximately 46 kDa by polyacrylamide gel electrophoresis and 40,875 Da, for the apoprotein, using mass spectrometry which also confirmed the presence of both heme and FAD prosthetic groups. The fusion protein showed immunological cross-reactivity with both anti-rat cytochrome b(5) and anti-rat cytochrome b(5) reductase antibodies indicating the conservation of antigenic determinants from both native domains. Spectroscopic analysis indicated the fusion protein contained both a b-type cytochrome and flavin chromophors with properties identical to those of the native proteins. Amino-terminal and internal amino acid sequencing confirmed the identity of peptides derived from both the heme- and flavin-binding domains with sequences identical to the deduced amino acid sequence. The isolated fusion protein retained NADH:ferricyanide reductase activity (k(cat) = 8.00 x 10(2) s(-1), K(NADH)(m) = 4 microM, K(FeCN(6))(m) = 11 microM) comparable to that of that of native NADH:cytochrome b(5) reductase and also exhibited both NADH:cytochrome c reductase activity (k(cat) = 2.17 x 10(2) s(-1), K(NADH)(m) = 2 microM, K(FeCN(6))(m) = 11 microM, K(Cyt.c)(m) = 1 microM) and NADH:methemoglobin reductase activity (k(cat) = 4.40 x 10(-1) s(-1), K(NADH)(m) = 3 microM, K(mHb)(m) = 47 microM), the latter two activities indicating efficient electron transfer from FAD to heme and retention of physiological function. This work represents the first successful bacterial expression of a soluble, catalytically competent, rat hepatic cytochrome b(5)-cytochrome b(5) reductase fusion protein that retains the functional properties characteristic of the individual heme and flavin domain.     

Functional expression and characterization of the wild-type mammalian renal cortex sodium/phosphate cotransporter and an 215R mutant in Saccharomyces cerevisiae.

The wild-type and an R215E mutant of the rat renal cortex sodium/phosphate cotransporter type 2 (NaPi-2) were functionally expressed in the yeast Saccharomyces cerevisiae strain MB192, a cell line lacking the high-affinity endogenous H+/P(i) cotransporter. The expression of the mRNA molecules and corresponding proteins was confirmed by Northern and Western blot analysis, respectively. As detected by indirect immunofluorescence and antibody capture assay, both wild-type and mutant NaPi-2 proteins are expressed in the yeast plasma membrane in comparable amounts. In the presence of 5 microM phosphate, Na+ promotes phosphate uptake into yeast cells expressing the wild-type NaPi-2 with a K(0.5) of 5.6 +/- 1.1 mM. The maximum uptake of phosphate (649 +/- 30 pmol/10 min) is approximately 8-fold higher than the uptake obtained with nontransformed cells (76.8 +/- 8 pmol/10 min). Yeast cells expressing the R215E mutant of NaPi-2 accumulate 213 +/- 9 pmol of phosphate/10 min under the same conditions. The K(0.5) for the stimulation of phosphate uptake by Na+ is 4.2 +/- 0.8 mM for the R215E mutant and thus not significantly different from the value obtained with cells expressing the wild-type cotransporter. The reduced level of accumulation of phosphate in yeast cells expressing the R215E mutant is probably due to a reduction of the first-order rate constant k for phosphate uptake: while cells expressing wild-type NaPi-2 accumulate phosphate with a k of 0.06 min(-1), the rate for phosphate uptake into cells expressing the R215E mutant (k) is 0.016 min(-1) and therefore about 4-fold lower. In comparison, the rate for phosphate uptake into nontransformed cells (k) is 0.0075 min(-1). Phosphate uptake into yeast cells that express the wild-type NaPi-2 in the presence of 150 mM NaCl is promoted by extracellular phosphate with a K(0.5) of 45 +/- 4 microM. A phosphate-dependent phosphate accumulation is also observed with cells expressing the R215E mutant, but the K(0.5) is twice as high (86 +/- 5 microM) as that obtained with the wild-type cotransporter. We conclude that the yeast expression system is a useful tool for the investigation of structure-function relationships of the renal sodium/phosphate cotransporter and that (215)R, although not involved in Na+ recognition, is a part of the structure involved in phosphate recognition and considerably influences the rate of phosphate uptake by the NaPi-2 cotransporter.     

Genetic and biochemical characterization of 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase and its role in the protocatechuate 4,5-cleavage pathway in Sphingomonas paucimobilis SYK-6

Protocatechuate (PCA) is the key intermediate metabolite in the lignin degradation pathway of Sphingomonas paucimobilis SYK-6 and is metabolized to pyruvate and oxaloacetate via the PCA 4,5-cleavage pathway. We characterized the 4-carboxy-2-hydroxymuconate-6-semialdehyde (CHMS) dehydrogenase gene (ligC). CHMS is the 4,5-cleavage product of PCA and is converted into 2-pyrone-4,6-dicarboxylate (PDC) by LigC. We found that ligC was located 295 bp downstream of ligB, which encodes the large subunit of the PCA 4,5-dioxygenase. The ligC gene consists of a 945-bp open reading frame encoding a polypeptide with a molecular mass of 34,590 Da. The deduced amino acid sequence of ligC showed 19 to 20% identity with 3-chlorobenzoate cis-dihydrodiol dehydrogenase of Alcaligenes sp. strain BR60 and phthalate cis-dihydrodiol dehydrogenases of Pseudomonas putida NMH102-2 and Burkholderia cepacia DBO1, which are unrelated to group I, II, and III microbial alcohol dehydrogenases (M. F. Reid and C. A. Fewson, Crit. Rev. Microbiol. 20:13-56, 1994). The ligC gene was expressed in Escherichia coli and LigC was purified to near homogeneity. Production of PDC from CHMS catalyzed by LigC was confirmed in the presence of NADP(+) by electrospray ionization-mass spectrometry and gas chromatography-mass spectrometry. LigC is a homodimer. The isoelectric point, optimum pH, and optimum temperature were estimated to be 5.3, 8.0, and 25 degrees C, respectively. The K(m) for NADP(+) was estimated to be 24.6 +/- 1.5 microM, which was approximately 10 times lower than that for NAD(+) (252 +/- 3.9 microM). The K(m)s for CHMS in the presence of NADP(+) and NAD(+) are 26.0 +/- 0.5 and 20.6 +/- 1.0 microM, respectively. Disruption of ligC in S. paucimobilis SYK-6 prevented growth with vanillate. Only PCA was accumulated during the incubation of vanillate with the whole cells of the ligC insertion mutant (DLC), indicating a lack of PCA 4,5-dioxygenase activity in DLC. However, the introduction of ligC into DLC restored its ability to grow on vanillate. PDC was suggested to be an inducer for ligAB gene expression.     

The dual-specific active site of 7,8-diaminopelargonic acid synthase and the effect of the R391A mutation

7,8-diaminopelargonic acid (DAPA) synthase (EC 2.6.1.62) is a pyridoxal phosphate (PLP)-dependent transaminase that catalyzes the transfer of the alpha-amino group from S-adenosyl-L-methionine (SAM) to 7-keto-8-aminopelargonic acid (KAPA) to form DAPA in the antepenultimate step in the biosynthesis of biotin. The wild-type enzyme has a steady-state kcat value of 0.013 s(-1), and the K(m) values for SAM and KAPA are 150 and <2 microM, respectively. The k(max) and apparent K(m) values for the half-reaction of the PLP form of the enzyme with SAM are 0.016 s(-1) and 300 microM, respectively, while those for the reaction with DAPA are 0.79 s(-1) and 1 microM. The R391A mutant enzyme exhibits near wild-type kinetic parameters in the reaction with SAM, while the apparent K(m) for DAPA is increased 180-fold. The 2.1 A crystal structure of the R391A mutant enzyme shows that the mutation does not significantly alter the structure. These results indicate that the conserved arginine residue is not required for binding the alpha-amino acid SAM, but it is important for recognition of DAPA.     

A cloned prokaryotic Cd2+ P-type ATPase increases yeast sensitivity to Cd2+

CadA, the P1-type ATPase involved in Listeria monocytogenes resistance to Cd(2+), was expressed in Saccharomyces cerevisiae and did just the opposite to what was expected, as it strikingly decreased the Cd(2+) tolerance of these cells. Yeast cells expressing the non-functional mutant Asp(398)Ala could grow on selective medium containing up to 100 microM Cd(2+), whereas those expressing the functional protein could not grow in the presence of 1 microM Cd(2+). The CadA-GFP fusion protein was localized in the endoplasmic reticulum membrane, suggesting that yeast hyper-sensitivity was due to Cd(2+) accumulation in the reticulum lumen. CadA is also known to transport Zn(2+), but Zn(2+) did not protect the cells against Cd(2+) poisoning. In the presence of 10 microM Cd(2+), transformed yeasts survived by rapid loss of their expression vector.     

A new family of high-affinity transporters for adenine, cytosine, and purine derivatives in Arabidopsis

In many organisms, including plants, nucleic acid bases and derivatives such as caffeine are transported across the plasma membrane. Cytokinins, important hormones structurally related to adenine, are produced mainly in root apices, from where they are translocated to shoots to control a multitude of physiological processes. Complementation of a yeast mutant deficient in adenine uptake (fcy2) with an Arabidopsis cDNA expression library enabled the identification of a gene, AtPUP1 (for Arabidopsis thaliana purine permease1), belonging to a large gene family (AtPUP1 to AtPUP15) encoding a new class of small, integral membrane proteins. AtPUP1 transports adenine and cytosine with high affinity. Uptake is energy dependent, occurs against a concentration gradient, and is sensitive to protonophores, potentially indicating secondary active transport. Competition studies show that purine derivatives (e.g., hypoxanthine), phytohormones (e.g., zeatin and kinetin), and alkaloids (e.g., caffeine) are potent inhibitors of adenine and cytosine uptake. Inhibition by cytokinins is competitive (competitive inhibition constant K(i) = 20 to 35 microM), indicating that cytokinins are transported by this system. AtPUP1 is expressed in all organs except roots, indicating that the gene encodes an uptake system for root-derived nucleic acid base derivatives in shoots or that it exports nucleic acid base analogs from shoots by way of the phloem. The other family members may have different affinities for nucleic acid bases, perhaps functioning as transporters for nucleosides, nucleotides, and their derivatives.