Preparation of polyribosome aminoacyl-transfer ribonucleic acid from the muscle of chick embryos

Kinetics of initiating polypeptide elongation in an IRES-dependent system

The intergenic IRES of Cricket Paralysis Virus (CrPV-IRES) forms a tight complex with 80S ribosomes capable of initiating the cell-free synthesis of complete proteins in the absence of initiation factors. Such synthesis raises the question of what effect the necessary IRES dissociation from the tRNA binding sites, and ultimately from all of the ribosome, has on the rates of initial peptide elongation steps as nascent peptide is formed. Here we report the first results measuring rates of reaction for the initial cycles of IRES-dependent elongation. Our results demonstrate that 1) the first two cycles of elongation proceed much more slowly than subsequent cycles, 2) these reduced rates arise from slow pseudo-translocation and translocation steps, and 3) the retarding effect of ribosome-bound IRES on protein synthesis is largely overcome following translocation of tripeptidyl-tRNA. Our results also provide a straightforward approach to detailed mechanistic characterization of many aspects of eukaryotic polypeptide elongation.

Uncoupling of amino acid turnover on transfer RNA from protein synthesis in HeLa cells

The aminoacylation of tRNA has been investigated in relation to protein aynthesis in living HeLa cells. In cells growing normally, the rates of tRNA charing are compatible with the observed entry of amino acids into protein. In contrast, when protein synthesis is inhibited 95--98% by either reduced temperature or cycloheximide, aminoacylation of tRNA is relatively unaffected. We conclude that, under these conditions, the aminoacylation of tRNA is uncoupled from subsequent steps in protein synthesis. These results provide for the first time a possible biological role for the observed aminoacyl-tRNA hydrolase activities of the tRNA synthetases.