Book of Abstracts: Albany 2003
June 17-21 2003
Translational Termination in E. coli by Class-I Release Factors: A Cryo-em Study
The mechanism of translational termination, the fundamental process whereby a finished protein dissociates from the peptidyl-tRNA and the ribosome, is still obscure. In prokaryotes, termination is triggered when translocation of a peptidyl-tRNA from the A to the P site of a ribosome also moves a stop codon into the decoding center (DC). The recognition of stop codons UAG or UAA by the release factor RF1 or stop codons UGA or UAA by RF2 mediates the release of the nascent polypeptide chain. Genetic and biochemical studies suggest that the action of class-I release factors (RFs) RF1/RF2 involves their interaction with both the DC on the 30S subunit and the peptidyl-transferase center (PTC) on the 50S subunit. These sites are separated by a distance of ~73Å on the ribosome. In contrast to these findings, the X-ray structure of RF2 places the residues implicated in these contacts, SPF (Ser-Pro-Phe) and GGQ (Gly-Gly-Gln) respectively, only 23 Å apart from each other(1). Our study of the RF2- (2) and RF1-ribosome complex using cryo-EM reveals that both class-I RFs assume a different, open conformation when bound to the ribosome in the presence of a stop codon. Altogether, our findings suggest how a stop codon-dependent signal is transmitted from the DC to PTC during translational termination, and enable us to address the question of why prokaryotes require two class-I RFs.
Urmila B. S. Rawat1
1Howard Hughes Medical Institute