Brome Mosaic Virus (BMV) has a tripartite RNA genome; each RNA and the subgenomic RNA encoding the viral coat protein share a highly homologous region of about 200 nucleotides at the 3′ end, for which a tRNA-like structure has been proposed. Several sequences encoding functions, including replicase binding, initiation of (−) strand synthesis and tyrosine esterification are known to be nested within this region. Elongation factor EF-1α binds to aminoacylated viral RNAs, but not to the uncharged forms. An additional function of the tRNA-like structure is to serve as a substrate for nucleotidyl transferase, which adds the terminal adenosine residue to the (+) sense virion RNAs. A template-dependent and template-specific replicase preparation from BMV-infected barley leaves has been characterized and extensively used for replication studies in vitro that complement studies in vivo using protoplasts. The replicase has been shown to initiate de novo both (−) strand synthesis on supplied (+) strand RNAs, and (+) strand subgenomic RNA synthesis on supplied (−) sense RNA3 templates. RNA transcripts obtained by transcription in vitro of cDNA clones containing desired base substitutions and deletions, have been supplied as templates for replication, aminoacylation and other assays. Use of such mutant RNAs has allowed the promoters for both (−) strand synthesis and for synthesis of the subgenomic (+) strand RNA to be characterized and defined. The same approach has also been used to reveal regions of the tRNA-like structure involved in the tyrosylation of the BMV RNAs. These experiments showed that, although regions important in aminoacylation and replication functions overlap, they are not identical. Some of the mutations tested in vitro have also been tested for infectivity in vivo using both barley plants and protoplasts. Mutants retaining replicase and nucleotidyl transferase template activity, but having lost aminoacylation capability are of special interest in that they should reveal the role of aminoacylation in the infection process.