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![[Figure 2]](rr5137fig2mag.jpg)
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Figure 2
Mutant and wild-type ISL structures in the U6 snRNP. (a) Overlay of the wild-type snRNP core with the U6-A62G snRNP core (gray with yellow A62G mutation; Montemayor et al., 2014  ). The U6 ISL was omitted from the selection used to construct the overlay. The pivot for the altered trajectories of the two ISLs is located at approximately the A59:U88 pairing at the base of the ISL. (b) Superposition of only the ISL in the wild-type and A62G complexes. Nucleotides 59–67 and 80–88 were used to construct the overlay, and the protein is omitted for clarity. Despite the altered sequences and helical trajectories, the pairing of nucleotides in the lower ISL is unchanged in the wild-type and A62G mutant structures. All nucleotides of the wild-type ISL are visible in the crystal form presented here. Nucleotide A79 is extrahelical, unlike in the A62G structure, which was crystallized in a different space group. (c, d) Base-pairing at U6 nucleotide 62. The interatomic distances are consistent with protonation of the N1 atom of adenine in the wild-type ISL presented here. (e) Structure of the U6 ISL in the C complex spliceosome (Galej et al., 2016). For clarity, only the proteins Prp8, Isy1, Cef1, Cwc15 and Clf1 are shown in gray. (f) Associated protein cofactors dramatically alter the conformation and pairing of the ISL in assembled spliceosomes, and the A62–C85 pairing is not preserved. Dashes and circles represent Watson–Crick and non-Watson–Crick base pairs, respectively, and `+' denotes protonation within the A62–C85 base pair. |
![[Figure 2]](rr5137fig2.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
