Enhanced translation of the downstream ORF attributed to a long 5^|^#8242; untranslated region in the OsMac1 gene family members, OsMac2 and OsMac3

The 5′-untranslated region (5′UTR) of mRNAs often affects the translational efficiency of the downstream open reading frames (ORFs), and some of its regulatory elements are involved in the initiation of translation. We found that the 5′UTR of the rice OsMac1 mRNA, which consisted of more than 500 nucleotides, yielded a significant enhancement of the translational efficiency of the downstream ORF. In the rice genome, OsMac1 represents a conserved gene family with two homologues, OsMac2 and OsMac3, which contain DUF300 (domain of unknown function 300) domains with predicted transmembrane regions. Similarly to the OsMac1 mRNA, the OsMac2 and OsMac3 mRNAs possess long 5′UTRs consisting of 312 and 318 nucleotides, respectively, that precede the main ORFs, which allow the elevation of the translational efficiency of the downstream ORF. The estimation of the translational efficiency of the GUS gene, which is located after the 5′UTRs, in suspension cultures of rice protoplasts showed that it was significantly greater than that of the control. These results suggest that 5′UTRs of OsMac2 and OsMac3 enhance the translation of the downstream ORF. Our results indicate that these 5′UTRs play a role of novel translational enhancer elements that enable the efficient translation of the downstream ORF.