Late steps in bacterial translation initiation visualized using time-resolved cryo-EM

Bacterial translation initiation entails the tightly regulated joining of the 50S ribosomal subunit to an initiator transfer RNA (fMet-tRNA(fMet))-containing 30S ribosomal initiation complex (IC) to form a 70S IC that subsequently matures into a 70S elongation-competent complex (70S EC). Rapid and accurate 70S IC formation is promoted by 30S IC-bound initiation factors (IFs), which must dissociate before the resulting 70S EC can begin translation elongation(1). Although comparison of 30S(2–5) and 70S(4,6–8) IC structures have revealed that the ribosome, IFs, and fMet-tRNA(fMet) can acquire different conformations in these complexes, the timing of conformational changes during 70S IC formation, structures of any intermediates formed during these rearrangements, and contributions that these dynamics might make to the mechanism and regulation of initiation remain unknown. Moreover, the absence of a 70S EC structure obtained directly from a 70S IC formed via an IF-catalyzed initiation reaction has precluded an understanding of ribosome, IF, and fMet-tRNA(fMet) rearrangements that occur upon maturation of a 70S IC into a 70S EC. Using time-resolved cryogenic electron microscopy (TR cryo-EM)(9) we report the first, near-atomic-resolution view of how a time-ordered series of conformational changes drive and regulate subunit joining, IF dissociation, and fMet-tRNA(fMet) positioning during 70S EC formation. Our results demonstrate the power of TR cryo-EM to determine how a time-ordered series of conformational changes contribute to the mechanism and regulation of one of the most fundamental processes in biology.