Mutagenesis of stem 1 and stem 2 of the proposed pseudoknot structure in the BVDV SD-1 5' UTR. A. Schematic drawing of wild-type plasmids and pseudoknot mutants. Plasmid pSBCSSD1-9LUC (Fig. 4) was used as basic plasmid for construction of the pseudoknot mutants. Altered nucleotides are boxed. In mutants M1 and M2 stem 2 base-pairing is disturbed, in M4, M5, and M6 the stem 1 complementarity is impaired.