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Systematic reverse transcription-polymerase chain reaction (RT-PCR) isolations of cDNA fragments using specific primers for HAK mRNAs have revealed that plant HAK K+ transporters are extensively expressed in shoots and roots. At least 13 genes encoding this type of transporter have been identified in Arabidopsis. Apparently, most plant HAK transporters do not show functional expression in trk1 trk2 yeast mutants. In one of them, however, a point mutation increased the Vmax of transport approximately 10-fold without affecting the Km or cation selectivity, suggesting that regulatory problems or targeting to the plasma membrane are the cause of failure for functional expression of this clone in yeast. The K+:Rb+:Cs+ selectivity of bacterial and eukaryotic Kup-HAK transporters are coincident with the selectivity data given in the literature about alkali cation transport in different plant tissues, indicating that HAK transporters may be the most representative plant K+ transporters. Phylogenetic analysis of the 19 plant translated sequences that belong to this type of transporter shows that there are 4 different groups. In group I and II there are members in which high-affinity K+ or Rb+ transport activity has been demonstrated. In other groups this has not been proved. However, present information suggests that all HAK transporters may be K+ transporters.