The role of the 3'-untranslated region of non-polyadenylated plant viral mRNAs in regulating translational efficiency.

Tobacco mosaic virus (TMV) is a positive-sense RNA virus in which the single genomic RNA functions as a messenger RNA. It is a member of a class of plant viral RNAs that are the only known non-polyadenylated mRNAs in plants. The 3'-untranslated region (UTR) of TMV genomic RNA is the functional equivalent of a poly(A) tail in that it increases mRNA stability and regulates translational efficiency. To determine whether the 3'-UTR of other non-polyadenylated plant viral mRNAs regulate translation, those from turnip yellow mosaic (TYMV), brome mosaic (BMV), and alfalfa mosaic (AlMV) viruses were investigated. Chimeric gene constructs were made in which the viral 3'-UTRs were introduced immediately downstream from the reporter genes encoding beta-glucuronidase (GUS) and luciferase (LUC), and were translated in plant protoplasts following delivery of the mRNA using electroporation. The 3'-UTR from BMV RNA3 regulated reporter gene expression in vivo to an extent comparable to that observed for the TMV 3'-UTR. The BMV 3'-UTR increased both message stability and translational efficiency. As regulators of translation, the BMV and TMV 3'-UTR were dependent on the presence of a cap at the 5' terminus for function. The 3' UTR of TYMV or AlMV RNA4 had little impact on translation or transcript stability. These data suggest that although the TMV 3'-UTR is not unique in regulating translation, the 3'-UTR of plant viral mRNAs do vary in their regulatory ability.