Association factor of ribosomal subunits from Bacillus stearothermophilus

The isolation of a new factor, which can cause the in vitro association of 30S and 50S ribosomal subunits at low Mg(++) concentration, is described. The association factor is eluted together with the dissociation protein when ribosomes of Bacillus stearothermophilus are washed with salt solutions of high concentration. The association activity is heat-stable, whereas dissociation factor is inactivated after 10 min at 80 degrees C. This treatment allows the separation of both factors. Several properties rule out the possibility that uncharged, amino-acyl-, or peptidyl-tRNA are responsible for the association process described in this report. Digestion with trypsin shows that the association factor contains at least two components, one of which is a protein.