TRANSLATION OF THE MESSENGER-RNA OF THE MAIZE TRANSCRIPTIONAL ACTIVATOR OPAQUE-2 IS INHIBITED BY UPSTREAM OPEN READING FRAMES PRESENT IN THE LEADER SEQUENCE

The protein encoded by the Opaque-2 (02) gene is a transcription factor, translated from an mRNA that possesses an unusually long 5' leader sequence containing three upstream open reading frames (uORFs). The efficiency of translation of O2 mRNA has been tested in vivo by a transient assay in which the level of activation of the b32 promoter, a natural target of 02 protein, is measured. We show that uORF-less O2 alleles possess a higher transactivation value than the wild-type allele and that the reduction in transactivation due to the uORFs is a cis-dominant effect. The data presented indicate that both uORF1 and uORF2 are involved in the reducing effect and suggest that both are likely to be translated.