Replication of tobacco mosaic virus RNA

The replication of tobacco mosaic virus (TMV) RNA involves synthesis of a n egative-strand RNA using the genomic positive-strand RNA as a, template, fo llowed by the synthesis of positive-strand RNA on the negative-strand RNA t emplates. Intermediates of replication isolated from infected cells include completely double-stranded RNA (replicative form) and partly double-strand ed and partly single-stranded RNA (replicative intermediate), but it is not known whether these structures are double-stranded or largely single-stran ded in vivo. The synthesis of negative strands ceases before that of positi ve strands, and positive and negative strands may be synthesized by two dif ferent polymerases. The genomic-length negative strand also serves as a tem plate for the synthesis of subgenomic mRNAs for the virus movement and coat proteins. Both the virus-encoded 126-kDa protein, which has amino-acid seq uence motifs typical of methyltransferases and helicases, and the 183-kDa p rotein, which has additional motifs characteristic of RNA-dependent RNA pol ymerases, are required for efficient TMV RNA replication. Purified TMV RNA polymerase also contains a host protein serologically related to the RNA-bi nding subunit of the yeast translational initiation factor, eIF3. Study of Arabidopsis mutants defective in RNA replication indicates that at least tw o host proteins are needed for TMV RNA. replication. The tomato resistance gene Im-l may also encode a mutant form of a host protein component of the TMV replicase. TMV replicase complexes are located on the endoplasmic retic ulum in close association with the cytoskeleton in cytoplasmic bodies calle d viroplasms, which mature to produce 'X bodies'. Viroplasms are sites of b oth RNA replication and protein synthesis, and may provide compartments in which the various stages of the virus multiplication cycle (protein synthes is, RNA replication, virus movement, encapsidation) are localized and coord inated. Membranes may also be important for the configuration of the replic ase with respect to initiation of RNA synthesis, and synthesis and release of progeny single-stranded RNA.