Cryo-EM structures of isolated pre-50S particles lacking or containing the SRL. (A) Cryo-EM maps representing the ΔSRL particles isolated from cells. The five classes of particles (classes 1–5) are arranged on a pathway of increasing complexity (bottom panel). In parentheses are the percentages of each class within the total population. The rRNA is shown in gray, and the r-proteins are shown in dark red. The diagrams of the 50S subunit shown between classes in the pathway indicate the structural motifs adopting their mature conformation in each transition. These structural motifs are color-coded according to the diagrams in the top panel. The left-most diagram in the top panel indicates the landmarks of the mature 50S subunit. (B) Cryo-EM maps representing the three classes of control particles isolated from cells, annotated as in A.
