Expression of AtMHX, an Arabidopsis vacuolar metal transporter, is repressed by the 5′ untranslated region of its gene

AtMHX is an Arabidopsis tonoplast transporter that can exchange protons with Mg²⁺ and Zn²⁺ ions. This transporter, which may play a role in ion homeostasis of plants, is encoded by a single gene in Arabidopsis. The molecular mechanisms that regulate the expression of this transporter are practically unknown. While AtMHX transcript can be easily visualized, expression of the corresponding protein is apparently low. To understand whether AtMHX expression is repressed at the translational level, the 5′ untranslated region (5′ UTR) of this gene was fused to reporter genes. In vitro analyses showed that the 5′ UTR of AtMHX can repress the translation of downstream coding sequences. The major cause of the repression was efficient initiation at an upstream open-reading-frame (uORF) included in the 5′ UTR. Although the sequence context of the upstream AUG (uAUG) codon was highly unfavourable, it was recognized by over 90% of the scanning ribosomes both in vitro and in vivo. The inhibitory effect of the uORF was mediated by imposing the need for reinitiation and not by ribosome stalling, as the inhibition was not dependent on the amino-acid sequence of the uORF peptide. The efficiency of reinitiation was low. The in vivo studies, carried out with transiently transformed tobacco plants, indicated that alternations in the Mg²⁺ or Zn²⁺ levels did not affect the rate of translation. These data suggest that AtMHX expression is repressed by the 5′ UTR of its gene.