Although no other SHAPE-specific changes were observed, several other phosphodiester bonds 3 0 of specific 25S rRNA bases were reproducibly more, or less, intrinsically labile as compared to wild type ( Figure 5D ). In both mutants, G2531 and G2534 located in expansion segment 31 (ES31) were more stable than in wild-type ribosomes as evidenced by reduced intensity of strong reverse transcriptase stops 1-nt 5 0 of these bases. Additionally, bases A2779-A2780 (E. coli A2407-U2408) located in the terminal loop of Helix 88 were hyper-labile in 51-4A mutant ribosomes as compared to WT, as shown by the presence of strong stops with increased intensity 1-nt 5 0 of these bases. These are mapped onto the two-dimensional structure of yeast 25S rRNA ( Figure 5E ).