The 3' untranslated region of alfalfa mosaic virus RNA3 contains a core promoter for minus-strand RNA synthesis and an enhancer element

The 3' untranslated regions (UTRs) of the three genomic RNAs of alfalfa mosaic virus consist of a 3' homologous sequence of 145 nt and upstream unique sequences 18-34 nt in length. Mutations were made in the 3' UTR of a cDNA clone of RNA3. Point mutations in five AUGC motifs which interfere with specific binding of coat protein to the 3' UTR had no effect on template activity of RNA3 for minus-strand RNA synthesis in vitro by purified viral RNA-dependent RNA polymerase (RdRp). Deletion analysis showed that the 3' homologous sequence of 145 nt was sufficient for a low level of template activity in the in vitro RdRp assay and a similarly low level of RNA3 accumulation in plants. The presence of an additional sequence of nucleotides 145-165 from the 3' end of RNA3 enhanced template recognition by RdRp in vitro and accumulation of RNA3 in vivo to wild-type levels.