Dissociation of Single Ribosomes as a Preliminary Step for Their Participation in Protein Synthesis.

32P-labelled derived 40-S ribosomal subunits and 80-S monoribosomes entered the polyribosome fraction of a rabbit-reticulocyte lysate active in protein synthesis by a process that was dependent on time, energy and temperature. Both 40-S and 60-S subparticles from the labelled monoribosome entered the polyribosomes at the same rate. The addition of excess unlabelled 40-S subparticles to the cell-free system containing labelled monoribosomes increased the ratio of 60-S to 40-S-subparticle radioactivity in the polyribosome fraction. This indicated a competition between added unlabelled and monoribosome-derived 40-S subparticles for envy into polyribosomes. From the evaluation of the translation time of mRNA, of the rate of monoribosomepolyribosome exchange, and of the equilibrium distribution of the cycling labelled monoribosomes and unlabelled subparticles with polyribosomes it is concluded that the dissociation of single ribosomes into subunits is an obligatory step for their entry into the polyribosomal fraction and that this process is coupled with initiation of mRNA translation. [ABSTRACT FROM AUTHOR]