A minimal RNA promoter for minus-strand RNA synthesis by the brome mosaic virus polymerase complex.

The approximately 150 nt tRNA-like structure present at the 3' end of each of the brome mosaic virus (BMV) genomic RNAs is sufficient to direct minus-strand RNA synthesis. RNAs containing mutations in the tRNA-like structure that decrease minus-strand synthesis were tested for their ability to interact with RdRp (RNA-dependent RNA polymerase) using a template competition assay. Mutations that are predicted to disrupt the pseudoknot and stem B1 do not affect the ability of the tRNA-like structure to interact with RdRp. Similarly, the +1 and +2 nucleotides are not required for stable template-RdRp interaction. Mutations in the bulge and hairpin loops of stem C decreased the ability of the tRNA-like structure to interact with RdRp. Furthermore, in the absence of the rest of the BMV tRNA, stem C is able to interact with RdRp. The addition of an accessible initiation sequence containing ACCA3' to stem C created an RNA capable of directing RNA synthesis. Synthesis from this minimal minus-strand template is dependent on sequences in the hairpin and bulged loops.