In addition to the six artificial pseudoknot-like structures, we also analysed two earlier investigated pseudoknots PK400 and PK401 (25), with over-all A B Figure 1 . Frameshift assay and pseudoknot structures. (A) All plasmid constructs contain an IPTG inducible promoter in front of T7 gene10 (light grey), a complete frameshift signal, and lacZ (dark grey). The frame shift stimulating pseudoknot-like structure is inserted downstream of gene10. Immediately, downstream from the pseudoknot lacZ is inserted in the À1 reading frame relative to gene10. In the 'Upstream Stop' construct the non-frameshifting ribosomes will translate gene10 and terminate at a UAA stop codon in the spacer sequence and produce a 28 kDa polypeptide. Ribosomes undergoing À1 frameshift at the slippery sequence translate lacZ thus producing $148 kDa polypeptide. In the 'Downstream Stop' construct the UAA stop codon is replaced by an AAA lysine codon thus resulting in $37 kDa polypeptide being produced by non-frameshifting ribosomes which terminate at an UGA stop codon downstream from the pseudoknot. (B) Sequence and structure of the inserted pseudoknots, the slippery sequence and the spacer. In pseudoknot, 10/6, 22/6a, 22/6b and 22/6c the first base in loop 2 has been removed in order to maintain the downstream reading frame (underlined). The boxed insert in panel B shows the structure and sequence of previously described constructs (25) .