Differential roles of the 5 ' untranslated regions of cucumber mosaic virus RNAs 1, 2, 3 and 4 in translational competition

RNA species of plant tripartite RNA viruses show distinct translational act ivities in vitro when the Viral RNA concentration is high. However, it is n ot known what causes the differential translation of virion RNAs. Using an in vitro wheat germ translation system, we investigated the translation eff iciencies and competitive activities of chimeric cucumber mosaic virus (CMV ) RNAs that contained viral untranslated regions (UTRs) and a luciferase-co ding sequence. The chimeric RNAs exhibited distinct translation efficiencie s and competitive activities. For example, the translation of chimeric CMV RNA 4 was about 40-fold higher than that of chimeric CMV RNA 3 in a competi tive environment. The distinct translation resulted mainly from differences in competitive activities rather than translation efficiencies of the chim eric RNAs. The differential competitive activities were specified by viral 5' UTRs, but not by 3' UTRs or viral proteins. The competitive translationa l activities of the 5' UTRs were as follows: RNA 4 (coat protein) > RNAs 2 and 1 (2a and 1a protein, or replicase) > RNA 3 (3a protein). (C) 2000 Else vier Science B.V. All. rights reserved.