SrmB Rescues Trapped Ribosome Assembly Intermediates.

RNA helicases play various roles in ribosome biogenesis depending on the ribosome assembly pathway and stress state of the cell. However, it is unclear how most RNA helicases interact with ribosome assembly intermediates or participate in other cell processes to regulate ribosome assembly. SrmB is a DEAD-box helicase that acts early in the ribosome assembly process, although very little is known about its mechanism of action. Here, we use a combined quantitative mass spectrometry/cryo-electron microscopy approach to detail the protein inventory, rRNA modification state, and structures of 40S ribosomal intermediates that form upon SrmB deletion. We show that the binding site of SrmB is unperturbed by SrmB deletion, but the peptidyl transferase center, the uL7/12 stalk, and 30S contact sites all show severe assembly defects. Taking into account existing data on SrmB function and the experiments presented here, we propose several mechanisms by which SrmB could guide assembling particles from kinetic traps to competent subunits during the 50S ribosome assembly process.