Plant viruses as model systems for the study of non-canonical translation mechanisms in higher plants

During protein synthesis the translational apparatus of the cell performs three basic steps, (i) initiation by the recognition of translational start codons, (ii) elongation of the nascent polypeptide chain by the sequential decoding of triplets within an open reading frame (ORF) and (iii) termination at appropriate stop codons. This canonical sequence of events is controlled firstly by the primary structure of mRNAs and the interactions of coding triplets with the anticodons of tRNAs. In addition, for prokaryotes ample evidence has accumulated to demonstrate that ribosomes, both by themselves and through interaction with the elongation factor EF-Tu, play an important role in the initial selection and proofreading of the appropriate aminoacyl-tRNA (Maden, 1993; Powers & Noller, 1994). Lastly, translational efficiency and specificity is modulated by 5′ or 3′ untranslated mRNA regions, in that primary (enhancer sequences), secondary (stable stem-loops) and tertiary (pseudoknots) structures or small ORFs in the untranslated region influence the expression of the major ORF(s) (for a recent review on post-transcriptional regulation of plant gene expression see Gallie, 1993).