HOW THE RIBOSOME MOVES AMONG THE MESSENGER-RNA DURING PROTEIN-SYNTHESIS

The movement of a ribosome along the mRNA was assessed by the followin g experimental strategy. mRNAs were synthesized which contained a shor t coding sequence with at least four codons and a P-32 label at one en d and an oligo(C) sequence at the other end. When these mRNAs were fix ed on the ribosome with tRNAs specific for the defined codons, the oli go(C) stretches were partially outside of the ribosome, whereas the la beled ends were inside the ribosome and thus protected. The overhangin g oligo(C) regions were trimmed with the cytidyl-specific RNase CL3 id entifying the nucleotides of the mRNAs, which emerged from the ribosom e. An mRNA enters the ribosome at nucleotide 18 +/- 1, when counting s tarts at the first nucleotide of the P-site codon, and leaves the ribo some at nucleotide -21 +/- 2. The ribosome does not move at either sid e upon A-site occupation but does so at both sides simultaneously upon translocation. The results further indicate that most of the mRNA str etches downstream and upstream of the coding region are not required f or the translocation reaction. It is therefore likely that the tRNAs a re pulling the mRNA through the ribosome via codon-anticodon interacti ons in the course of translocation.