Role of mRNA Secondary Structure in Translational Repression of the Maize Transcriptional ActivatorLc ,

Lc, a member of the maize (Zea mays) R/B gene family, encodes a basic helix-loop-helix transcriptional activator of the anthocyanin biosynthetic pathway. It was previously shown that translation of the Lc mRNA is repressed by a 38-codon upstream open reading frame (uORF) in the 5' leader. In this study, we report that a potential hairpin structure near the 5'end of the Lc mRNA also represses downstream translation in the rabbit reticulocyte in vitro translation system and in transient transformation assays. Base pairing of the hairpin is important for repression because its destabilization increases translation of the uORF and the downstream ORF. However, translation of the uORF is not required for the hairpin-mediated repression. Instead, the uORF and the 5'-proximal hairpin mediate two independent levels of repression. Although the uORF represses downstream translation due to inefficient reinitiation of ribosomes that translate uORF, the hairpin inhibits ribosome loading at the 5' end of the mRNA.