A single-stranded loop in the 5′ untranslated region of cucumber mosaic virus RNA 4 contributes to competitive translational activity

The 5′ untranslated region (UTR) of cucumber mosaic virus (CMV) RNA 4 confers a highly competitive translational advantage on a heterologous luciferase open reading frame. Here we investigated whether secondary structure in the 5′ UTR contributes to this translational advantage. Stabilization of the 5′ UTR RNA secondary structure inhibited competitive translational activity. Alteration of a potential single-stranded loop to a stem by substitution mutations greatly inhibited the competitive translational activity. Tobacco plants infected with wild type virus showed a 2.5-fold higher accumulation of maximal coat protein than did plants infected with a loop-mutant virus. Amplification of viral RNA in these plants could not explain the difference in accumulation of coat protein. Phylogenetic comparison showed that potential single-stranded loops of 12–23 nucleotides in length exist widely in subgroups of CMV.