Figure 3.

Base pair compensation in helices E23-8 to E23-12. Helices E23-8 to E23-12 comprise 15 base pairs in most species. Fourteen of these, in red in the P.palmata structure in the centre, show compensation in other species, as can be seen in the eight structures surrounding it. The first base pair of helix E23-11 (U·A, at right in the figure), though subject to compensation, is non-complementary in ∼1/4 of the sequences. In Trichonomas tenax this base pair cannot be formed if it is assumed that the pseudoknot loop between helices E23-10 and 11 must contain at least one nucleotide (G in this case). Compensation of the second base pair of helix E23-11 occurs between the structures of Ciliophrys infusionum (C·G) and Peranema trichophorum (U·A). The last base pair (A·U) is not compensated in presently known species but becomes G·U in two species. See also Table 2.