Fig. 1.

Pseudoknots and frameshift sites. (a) Sequences of frameshift sites encoded in plasmids pTH400, pTH401, and pTH421. The slippery sequence is underlined, and the stop codon UAA is marked by a black “stop sign.” pTH400 and pTH401 encode the pseudoknots PK400 and PK401. The nucleotides that can form double-stranded stems are underlined by arrows. Stem1 is indicated by black arrows, and stem2 is indicated by gray arrows. The three nucleotides present only in PK401 are in bold. (b) Schematic drawing of an mRNA where the ribosome is positioned with the slippery sequence in its A and P sites. The secondary structure of PK401 is indicated with coaxially stacked stems and single-stranded loops. Differences from PK400 are indicated by arrows. (c) A schematic drawing of a pseudoknot.