It was found that the fusion (His)6-movement proteins (MPs) of two tobamoviruses (TMV UI and a crucifer-infecting tobamovirus, crTMV) were efficient nonspecific translational repressors. The in vitro translation of viral RNAs was blocked by incomplete 30K MP-RNA complexes formed at the MP:RNA molar ratios of 100-150:1. Similar results were obtained with the barley stripe mosaic hordeivirus (BSMV)-encoded 58K MP; however, the translation inhibiting activity of the 58K MP was manifested only in the presence of magnesium. By contrast, the 25K MP of potato virus X (PVX) was incapable of forming MP-RNA complexes under experimental conditions used and did not inhibit in vitro translation. The translation 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 infectious in isolated protoplasts; however, they were infectious in indicator plants. Reduction of MP affinity to RNA resulted in translatability of MP-TMV RNA complexes that apparently was due to their destabilization. Thus, the deletion mutant DEL4 MP formed MP-TMV RNA complexes that were translatable in vitro, infectious to protoplasts and plants. In contrast to this, the complexes of TMV RNA with the mammalian RNA-binding protein p50 were nontranslatable and noninfectious to either protoplasts or intact plants. These results implied that nontranslatable 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.