Two iron-regulated cation transporters from tomato complement metal uptake-deficient yeast mutants

Although iron deficiency poses severe nutritional problems to crop plants, to date iron transporters have only been characterized from the model plant Arabidopsis thaliana. To extend our molecular knowledge of Fe transport in crop plants, we have isolated two cDNAs (LeIRT1 and LeIRT2) from a library constructed from roots of iron-deficient tomato (Lycopersicon esculentum) plants, using the Arabidopsis iron transporter cDNA, IRT1, as a probe. Thei r deduced polypeptides display 64% and 62% identical amino acid residues to the IRT1 protein, respectively. Transcript level analyses revealed that bo th genes were predominantly expressed in roots. Transcription of LeIRT2 was unaffected by the iron status of the plant, while expression of LeIRT1 was strongly enhanced by iron limitation. The growth defect of an iron uptake- deficient yeast (Saccharomyces cerevisiae) mutant was complemented by LeIRT 1 and LeIRT2 when ligated to a yeast expression plasmid. Transport assays r evealed that iron uptake was restored in the transformed yeast cells. This uptake was temperature-dependent and saturable, and Fe2+ rather than Fe3+ w as the preferred substrate. A number of divalent metal ions inhibited Fe2uptake when supplied at 100-fold or 10-fold excess. Manganese, zinc and cop per uptake-deficient yeast mutants were also rescued by the two tomato cDNA s, suggesting that their gene products have a broad substrate range. The ge ne structure was determined by polymerase chain reaction experiments and, s urprisingly, both genes are arranged in tandem with a tail-to-tail orientat ion.