Role of an upstream open reading frame in the translation of polycistronic mRNAs in plant cells.

The influence of an upstream small open reading frame (URF) on the translation of two consecutive coding regions on an eukaryotic mRNA was studied. The cis effects of leader length, URF length, the sequences of the URF and neighboring regions, and the trans effects of the Cauliflower mosaic virus transactivator (TAV) were analyzed. Translation efficiency of the immediate downstream open reading frame (ORF) decreased with increasing URF length. Short URFs did not drastically inhibit translation of immediate downstream ORFs but supported far downstream translation in the presence of TAV. In the latter case, the optimal URF length was 30 codons.