Regulatory Nascent Peptides in the Ribosomal Tunnel Tanel Tenson, Måns Ehrenberg  Cell  Volume 108, Issue 5, Pages 591-594 (March 2002) DOI: 10.1016/S0092-8674(02)00669-4

Figure 1 Exit of the Nascent Peptide from the Ribosome as Determined by Cryo-EM (A) Cross-section of the E. coli large ribosomal subunit (from Gabashvili et al., 2001). PT, the peptidyl transferase center. The erythromycin binding site is red, and the nascent peptide tunnel is yellow. The tunnel branches at the end. e1–e4, different potential exit sites; e1 is the widest exit where the membrane pore complex binds (B). (B) Yeast ribosome bound to the membrane pore complex formed by Sec61 (red). The small ribosomal subunit is yellow, the large subunit is blue, and tRNA inside the ribosome is green (from Beckmann et al., 2001). Cell 2002 108, 591-594DOI: (10.1016/S0092-8674(02)00669-4)

Figure 2 Regulation of SecA Synthesis by SecM (A) Structure of the mRNA encoding SecM and SecA proteins. The Shine-Dalgarno region (SD) of the open reading frame coding for SecA is trapped in a RNA helix. (B) If there is enough SecA protein in the cell, SecM (represented by a chain of white, red, and gray balls) is rapidly translocated through the membrane pore (shown in green). (C) If there is not enough SecA in the cell, the signal sequence in the N terminus of SecM (gray balls) cannot start the membrane translocation process. The peptide sequence in the C terminus of SecM (red balls) then causes ribosome stalling. This event causes rearrangement of the mRNA secondary structure, exposing the SD sequence in the mRNA, thereby promoting efficient translation of SecA. Cell 2002 108, 591-594DOI: (10.1016/S0092-8674(02)00669-4)