A long 5a2 UTR of the rice OsMac1 mRNA enabling the sufficient translation of the downstream ORF

Untranslated regions (UTRs) of mRNA are involved in many posttranscriptional regulatory pathways. The rice OsMac1 mRNA had a 5a2 UTR of more than 500 nucleotides (nt), containing a CU-rich region and three upstream open reading frames (uORFs) preceding the downstream ORF. The expected GFP-fusion protein was detected at the cell membrane, indicating the occurrence of translation of the downstream ORF. The 5a2 UTR contained three splicing variants that were generated by alternative splicing. A reporter analysis using I2-glucuronidase indicated that only the longest one, UTRc, showed a significant ability for the efficient translation of the downstream ORF, whereas other splicing variants showed low level of translational efficiency of the corresponding ORF. These results suggested that the additional 38-nt sequence unique to UTRc localized between the CU-rich region and the uORFs may be involved in the increased translational efficiency of the downstream ORF located after the long 5a2 UTR.