SF202-206 and SF209-211 were constructed as follows. First a BglII-NcoI fragment from the SF2 vector was replaced by a short DNA fragment obtained by hybridization of oligonucleotides (5 0 -GATCTTAATACGACT CACTATAGGGCTCAGGGAAACTGATCA-CGTGG C-3 0 ) and 5 0 -CATGGCCACGTGATCAGTTTCCCTGA GCCCTATAGTGAGTC-GTATTAA-3 0 ) creating a unique BclI restriction site just downstream of the slippery sequence. The resulting plasmid pSF201 was opened at the BclI and NcoI sites and sets of complementary oligonucleotides corresponding to mutants SF202-206 were inserted. In mutant SF207, which codes only for the À1 reading frame product, the BglII-NcoI fragment was replaced by two short complementary oligonucleotides: SF218/SF219: 5 0 GATCTGGCCACTAGTAC/5 0 CATGG TACTAGTGGCCA). Note that this in-frame product is 21 aa shorter than the frameshifted À1 product. A C B Figure 1 . Structure of the 'NMR' SRV-1 frameshift pseudoknot and the effect of substitutions in L2 on its frameshift-inducing capacity. This pseudoknot differs from the viral pseudoknot by having G2C18-to-CG, C10G32-to-UA, G20-to-C and deletion of GCU between C24 and A25 substitutions (26) . (A) The 3D model of the NMR pseudoknot (PDB 1E95); L2 bases, magenta; S1 and S2 base pairs in cyan, L1 base in red, 3 0 single-stranded sequence AC in yellow. (B) Substitutions in the NMR pseudoknot sequence. Dashed lines illustrate base triples. (C) Rabbit reticulocyte lysate translation products of mRNAs derived from BamHI-digested templates were separated on a 17.5% SDS polyacrylamide gel and detected by fluorography. The migration of the 19 kDa 0-frame product (NFS) and the 22 kDa 'frameshift' product (FS) are indicated. SF103, wild-type; see Table 1 for the explanation of other constructs.