Control of start codon choice on a plant viral RNA encoding overlapping genes.

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Abstract
The signals that control initiation of translation in plants are not well understood. To dissect some of these signals, we used a plant viral mRNA on which protein synthesis initiates at two out-of-frame start codons. On the large subgenomic RNA (sgRNA1) of barley yellow dwarf virus-PAV serotype, the coat protein (CP) and overlapping 17K open reading frames (ORFs) are translated beginning at the first and second AUG codons, respectively. The roles of bases at positions -3 and +4 relative to the AUG codons in efficiency of translation initiation were investigated by translation of sgRNA1 mutants in a cell-free extract and by expression of a reporter gene from mutant sgRNA1 leaders in protoplasts. The effects of mutations that disrupted and restored secondary structure encompassing the CP AUG independently of, and in combination with, changes to bases -3 and +4 were also examined. Partial digestion of the 5' end of the sgRNA1 leader with structure-sensitive nucleases gave products that were consistent with the predicted secondary structure. Secondary structure had an overall inhibitory effect on translation of both ORFs. In general, the "Kozak rules" of start codon preference predominate in determining start codon choice. Unexpectedly, for a given CP AUG sequence context, changes that decreased initiation at the downstream 17K AUG also reduced initiation at the CP AUG. To explain this observation, we propose a new model in which pausing of the ribosome at the second AUG allows increased initiation at the first AUG. This detailed analysis of the roles of primary and secondary structure in controlling translation initiation should be of value for understanding expression of any plant gene and in the design of artificial constructs.