BOTH CODON CONTEXT AND LEADER LENGTH CONTRIBUTE TO EFFICIENT EXPRESSION OF 2 OVERLAPPING OPEN READING FRAMES OF A CUCUMBER NECROSIS VIRUS BIFUNCTIONAL SUBGENOMIC MESSENGER-RNA

The importance of codon context and leader length in the translational regulation of p20 and p21 from the bifunctional 0.9-kb subgenomic mRN A of cucumber necrosis virus was investigated. Nucleotide substitution s introduced into the -3 and +4 positions of the p21 AUG codon (where the A of the AUG is +1) verified that purines in these positions are f avored and demonstrated the similar contribution of the -3 and +4 posi tions to the efficiency of initiation codon selection in plants. The e ffect of nucleotide substitutions in the +5 position, most clearly dem onstrated when pyrimidines occupy the -3 and +4 positions, also provid ed direct insight into the influence of the +5 position in plants. The codon context of the upstream p21 initiation codon affected expressio n from the downstream p20 AUG codon, In addition, an increase in the l ength of the subgenomic mRNA leader decreased expression from the down stream p20 initiation site. These latter observations are in accordanc e with the ''Kozak rules'' for accession of internal AUG codons by lea ky ribosomal scanning and provide the first example of an effect of le ader length on the efficiency of translation initiation in a plant (vi ral) mRNA. (C) 1996 Academic Press, Inc.