Identification of RNA determinants that modulate the competitive translational activity of cucumber mosaic virus

Abstract

Positive strand RNA viruses constitute the major proportion of plant viruses. RNA species of plant tripartite RNA viruses whose genome is composed of three species of RNA show distinct translational activities in vitro when the virion RNA concentration is high. However, it is not known what is responsible for the differential translation of the viral RNAs. In this study, I found that cucumber mosaic virus(CMV) RNAs are also distinctly translated when the virion RNA concentration is high. In the mRNA competitive environment, RNA 4, a subgenomic RNA that encodes coat protein, showed the strongest translational activity. The competitive translated determinant in the RNA 4 was demonstrated to exist in the 5′ untranslated region(UTR) using chimeric mRNA containing viral UTR and a luciferase-coding sequence. In CMV genomic RNAs 1, 2 and 3, translation efficiencies and competitive translational activities were also investigated using chimeric CMV RNAs that contained viral UTRs and a luciferase-coding sequence. The chimeric RNAs exhibited distinct translational efficiencies and competitive translational activities. For example, the translational activity of chimeric CMV RNA 4 was about 40-fold higher than that of chimeric CMV RNA 3 in a highly competitive environment. The distinct translational activities resulted mainly from differences in competitive translational activities did not require viral proteins or 3′ UTRs. Viral 5′ UTRs alone specified the differential competitive activities. The competitive translational activities of the 5′ UTRs were as follows: RNA 4(coat protein)>RNAs 2 and 1(2a and 1a protein, or replicase)>RNA3(3a protein). In order to identify RNA features that contribute to competitive translational activity, I constructed a series of mRNA transcripts that contained RNA 4-derived, mutated 5′ UTRs that differed in stability of an optimally predicted RNA secondary structure. Stabilization of the 5′ UTR RNA secondary structure inhibited competitive tr...