Extended Data Fig. 11: Model for MutS2 function in sensing ribosome collisions and eliciting downstream responses.

The model depicts, from top to bottom: ribosomes translating an mRNA with a stalling site within the open reading frame (“Translation”); the leading ribosome becoming stalled (“Internal stalling”); a trailing ribosome colliding with the stalled ribosome (“Ribosomal collision”); MutS2 sensing the collision and promoting both separation of the ribosomal subunits (“Ribosome splitting”) and endonucleolytic cleavage (“mRNA cleavage”). Left side: Ribosomal splitting generates a 50S subunit still obstructed with a nascent chain-tRNA conjugate, which is sensed by RqcH and RqcP, resulting in elongation of the nascent chain with a C-terminal Ala tail (“Ala tailing”). Nascent-chain release is accompanied by 50S recycling (“Ribosome recycling”, dotted line). Right side: mRNA cleavage can result in mRNA decay (dotted line) or in trailing ribosomes becoming stalled at the mRNA 3’end, which are sensed by SsrA/tmRNA. The SsrA reaction leads to ribosome recycling (“Ribosome recycling”, dotted line) and to nascent-chain modification with a C-terminal SsrA tag (“SsrA tagging”). Both Ala-tails and the SsrA tag act as degrons, recognized by ClpXP and other proteases (“Proteolysis”). See the main text for additional details. Objects: the mRNA is shown in red, with a stalling site within the open reading frame represented by ‘!’ within a triangle and the mRNA stop codon shown as a ‘Stop’ traffic sign; the direction of translation is indicated by arrows; the stalled ribosome is shown in orange (light, 50S subunit; dark, 30S subunit); trailing and collided ribosomes are shown in green (dark, 50S subunit; light, 30S subunit). Quality control factors are indicated by their names.