Supplementary Figure 2: Conformational dynamics of ES27 on the 80S ribosome.

(a,d) Two distinct conformations of ES27 in the cryo-EM structures of the empty 80S ribosome. The two maps (filtered to 12 Å) are from the dataset of RAC1 (Supplementary Table 1). ES27, as an inherent dynamic component of the 25S rRNA, has two predominant states, with the helix stem toward the PTE (peptide tunnel exit) (a) or L1 stalk (d). (b,e) Two representative cryo-EM structures of the 80S-RAC complex, with ES27 in different conformations. The two maps (filtered to 10 Å) are from datasets RAC2 (b and c) and RAC11 (e and f). (c,f) Same as (b) and (e), with view directions centered at RAC. The 60S, 40S, RAC and ES27 are colored cyan, yellow, orange and green, respectively. ES27 is a dynamic rRNA helix, which can exist in sharply different conformations⁵, and has been proposed to have a possible role in regulating the binding of various co-translational factors at the PTE^5-8. Our structural analysis shows that the RAC binding is independent of the conformational state of ES27, which suggests that ES27 might regulate the binding of Ssb (Supplementary Fig. 1), given the possible spatial relationship of ES27 and Ssb at the PTE.