Mechanism for the disassembly of the posttermination complex inferred from cryo-EM studies

Ribosome recycling, the disassembly of the posttermination complex after each round of protein synthesis, is an essential step in mRNA translation, but its mechanism has remained obscure. In eubacteria, recycling is catalyzed by RRF (ribosome recycling factor) and EF-G (elongation factor G). By using cryoelectron microscopy, we have obtained two density maps, one of the RRF bound posttermination complex and one of the 50S subunit bound with both EF-G and RRF. Comparing the two maps, we found domain I of RRF to be in the same orientation, while domain II in the EF-G-containing 50S subunit is extensively rotated (similar to 60 degrees) compared to its orientation in the 70S complex. Mapping the 50S conformation of RRF onto the 70S posttermination complex suggests that it can disrupt the intersubunit bridges B2a and B3, and thus effect a separation of the two subunits. These observations provide the structural basis for the mechanism by which the posttermination complex is split into subunits by the joint action of RRF and EF-G.