NONTRANSLATABILITY AND DISSIMILAR BEHAVIOR IN PLANTS AND PROTOPLASTS OF VIRAL-RNA AND MOVEMENT PROTEIN COMPLEXES FORMED IN-VITRO

It was found that the fusion (His)(6)-movement proteins (MPs) of two t obamoviruses (TMV UI and a crucifer-infecting tobamovirus, crTMV) were efficient nonspecific translational repressors. The in vitro translat ion of viral RNAs was blocked by incomplete 30K MP-RNA complexes forme d at the MP:RNA molar ratios of 100-150:1. Similar results were obtain ed with the barley stripe mosaic hordeivirus (BSMV)-encoded 58K MP; ho wever, the translation inhibiting activity of the 58K MP was manifeste d only in the presence of magnesium. By contrast, the 25K MP of potato virus X (PVX) was incapable of forming MP-RNA complexes under experim ental conditions used and did not inhibit in vitro translation. The tr anslation repressing ability correlated with the level of MP affinity to RNA. The complexes of the 30K MP and 58K MP with TMV RNA were not i nfectious in isolated protoplasts; however, they were infectious in in dicator plants. Reduction of MP affinity to RNA resulted in translatab ility of MP-TMV RNA complexes that apparently was due to their destabi lization. Thus, the deletion mutant DEL4 MP formed MP-TMV RNA complexe s that were translatable in vitro, infectious to protoplasts and plant s. In contrast to this, the complexes of TMV RNA with the mammalian RN A-binding protein p50 were nontranslatable and noninfectious to either protoplasts or intact plants. These results implied that nontranslata ble MP-RNA complexes which could not replicate in the primary infected cells were converted into a translatable and replicatable form in the course of passage through plasmodesmata in planta. (C) 1997 Academic Press.