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 suc k as caffeine are transported across the plasma membrane. Cytokinins, impor tant hormones structurally related to adenine, are produced mainly in root apices, from where they are translocated to shoots to control a multitude o f physiological processes. Complementation of a yeast mutant deficient in a denine uptake (fcy2) with an Arabidopsis cDNA expression library enabled th e identification of a gene, AtPUP1 (for Arabidopsis thaliana purine permeas e1), belonging to a large gene family (AtPUP1 to AtPUP15) encoding a new cl ass of small, integral membrane proteins. AtPUP1 transports adenine and cyt osine with high affinity. Uptake is energy dependent, occurs against a conc entration gradient, and is sensitive to protonophores, potentially indicati ng secondary active transport, Competition studies show that purine derivat ives (e.g., hypoxanthine), phytohormones (e.g., zeatin and kinetin), and al kaloids (e.g,, caffeine) are potent inhibitors of adenine and cytosine upta ke. Inhibition by cytokinins is competitive (competitive inhibition constan t K-i = 20 to 35 mu M), 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 derivativ es in shoots or that it exports nucleic acid base analogs from shoots by wa y of the phloem. The other family members may have different affinities for nucleic acid bases, perhaps functioning as transporters for nucleosides, n ucleotides, and their derivatives.