Four mutant brome mosaic virus (BMV) RNA3 transcripts, bearing single or double base changes in the 3'-CCAOH terminus of the tRNA-like structure, previously characterized as being deficient in vitro with respect to aminoacylation and replication activities, have now been assayed in vivo for their ability to replicate (in the presence of transcripts of wild-type RNA1 and -2) in barley protoplasts and plants. In tests conducted with protoplasts, irrespective of the time post-infection, all four mutants were fully viable, and the relative levels of both plus and minus strand replication for each mutant were similar to that of the wild type. Inoculation of barley plants with these mutants resulted in phenotypic symptoms and viral yields that were similar to those from wild-type infections. Analysis of each mutant progeny RNA3 indicated that the altered sequence at the 3' terminus was restored to that of wild type. These observations indicate that there is a rapid turnover and correction of the 3' termini of BMV RNAs in vivo. Such correction is commensurate with the action of tRNA nucleotidyltransferase, but it differs from recombination processes that appear to be relatively infrequent for BMV RNA3. These results support the conclusion that the 3'-CCAOH termini of viral tRNA-like structures function analogously to telomeres of chromosomal DNA.