Inhibitory effect of myb7 uORF on downstream gene expression in homologous (rice) and heterologous (tobacco) systems.

The rice myb7 mRNA contains in its long leader an upstream open reading frame (uORF) putatively coding for a 40 amino acid peptide. uORFs have been found in the leader of mRNAs encoding transcriptional factors or other proteins involved in cellular growth and development. They are thought to translationally regulate the expression of downstream ORFs. Here, we showed the ability of the myb7 uORF to inhibit the expression of downstream reporter genes both in homologous (rice) and heterologous (tobacco) systems. This effect seems partially related to its translation, as indicated by the comparison with the mutagenized uORF. In both systems most of the inhibitory effect was due to the presence of the intercistronic region, in disagreement with the Kozak model. Moreover, replacing the uORF or the intercistronic region with a different one, we demonstrated that the inhibitory effect strictly depends on their co-presence. Finally, in vitro assays showed that the myb7 uORF is translated and inhibits the downstream ORF translation.