In vitro transcripts of a full-length cDNA of a naturally deleted RNA2 of barley mild mosaic virus (BaMMV) replicate in BaMMV-infected plants.

The RNA2 of a German isolate of the bipartite barley mild mosaic bymovirus (BaMMV-ASL1) is 3524 nucleotides long excluding the 3'-terminal poly(A) tail. The isolate was propagated by mechanical inoculation for several years. Electrophoretic comparison of viral nucleic acids during this period revealed that a spontaneous reduction in the length of the RNA2 occurred, resulting in the isolate BaMMV-ASL1a. The deleted RNA2 of BaMMV lacked a fragment that was 630 nucleotides long. The deletion occurred in the 3' half of the single open reading frame (ORF) found in RNA2; this ORF encodes a polyprotein that has a molecular mass of 98 kDa, which is assumed to be processed autocatalytically into proteins of 25 kDa and 73 kDa. A full-length cDNA of the deleted RNA2 was synthesized and cloned under the control of the phage T7 promoter. In vitro transcripts of the BaMMV-ASL1a clone replicated in barley plants after co-inoculation with a wild-type-like isolate of BaMMV. The deletions in RNA2 of BaMMV-ASL1a and those of a number of other isolates that were examined were found to affect a domain of the putative 73 kDa protein that is obviously not essential for replication but may be important for the transmission of BaMMV by its natural vector Polymyxa graminis.