FIG. 3.

Alteration of the putative stem-loop structure downstream from the slippery site by site-directed mutagenesis and assay of the effect from the alteration on the −1 ribosomal frameshift. The five transcripts used for transfecting Giardia trophozoites are as follows: 1, wild type; 2, the AUUA/uagu mutant at nt 2868 to 2871 that shortens the stem from its top by 4 bp and enlarges the loop from 11 to 19 nt without altering its distance from the slippery heptamer; 3, the CGUG/gcgc mutant at nt 2849 to 2852 that shortens the stem from its bottom by 5 bp and increases its distance from the slippery heptamer from 5 to 10 nt without altering the loop; 4, the CGUG/gcgc mutant at nt 2872 to 2875 that shortens the stem from its bottom by 5 bp and increases its distance from the slippery heptamer to 10 nt without altering its loop structure; and 5, the double mutant of CGUC/gcgc (nt 2849 to 2852) and CGCG/gugc (nt 2872 to 2875) that restored the wild-type stem structure with altered nucleotide sequence. The upper panel presents relative frequencies of the −1 ribosomal frameshift derived from luciferase expression in mutant transcript-transfected Giardia trophozoites in triplicate experiments (see Table 1 for actual data).