A ratchet-like inter-subunit reorganization of the ribosome during translocation

The ribosome is a macromolecular assembly that is responsible for protein b iosynthesis following genetic instructions in all organisms. It is composed of two unequal subunits: the smaller subunit binds messenger RNA and the a nticodon end of transfer RNAs, and helps to decode the mRNA; and the larger subunit interacts with the amino-acid-carrying end of tRNAs and catalyses the formation of the peptide bonds. After peptide-bond formation, elongatio n factor G (EF-G) binds to the ribosome, triggering the translocation of pe ptidyl-tRNA from its aminoacyl site to the peptidyl site, and movement of m RNA by one codon(1). Here we analyse three-dimensional cryo-electron micros copy maps of the Escherichia coli 70S ribosome in various functional states , and show that both EF-G binding and subsequent GTP hydrolysis lead to rat chet-like rotations of the small 30S subunit relative to the large 50S subu nit. Furthermore, our finding indicates a two-step mechanism of translocati on: first, relative rotation of the subunits and opening of the mRNA channe l following binding of GTP to EF-G; and second, advance of the mRNA/(tRNA)( 2) complex in the direction of the rotation of the 30S subunit, following G TP hydrolysis.