structures more similar to naturally occurring pseudoknots (Figure 1 insert), inspired from structures in the infectious bronchitis virus (22, 28, 30) . The pseudoknot structures in this type of virus are selected for their effects on vertebrate ribosomes, but the stem1 length variations were found to yield approximately the same relative stimulatory effect in E. coli (25) and suggest that stem1 strength is equally important for stimulating bacterial ribosomes to frameshift.