A yeast ribosomal subunit association factor (AF) has been purified from a high-salt ribosomal wash. The purified enzyme is a thermostable protein that associates ribosomal subunits at low Mg2 + concentration without requiring energy. It appears to be an aggregate of trimers or dimers (molecular mass 125 or 79 kDa) which on sodium dodecyl sulfate gels shows the presence of a major protein band whose estimated molecular mass is 43 kDa. Evidence also indicates the existence of a 50-kDa polypeptide which seems to be unstable since with freezing and thawing it gives rise to the 43-kDa polypeptide. It was shown that the labelled factor interacts with 80s ribosomes and with 40s ribosomal subunits. The purified polypeptide reacts with antibodies directed against EF-la, this last protein recognizing the antibodies raised against AF. Likewise, both EF-la and AF associate ribosomal subunits in the same way. When EF-1 is heated, it not only maintains its association activity, but also behaves like a 43-kDa polypeptide in an SDS electrophoresis run. These observations strongly suggest that AF originates from EF-la, which implies that the well-known elongation factor may also play a role in the initiation step of protein synthesis.In the ribosomal cycle, when the ribosomal subunits are released at chain termination they may bind factors which prevent their reassociation and channel them into another cycle of synthesis, or they may recombine and enter the pool of monomers. In prokaryotic and eukaryotic cells, the monomer ribosomes accumulate at the expense of polysome degradation that takes place when protein synthesis diminishes under a variety of metabolically adverse conditions [l]. These single ribosomes can revert to polysomes when the rate of overall protein synthesis increases and in this way function as a reserve of potential subunits. This fluctuation in the rate of protein synthesis and the corresponding monomer level, which are not necessarily related to growth, may occur normally in eukaryotic cells. Such regulation may reflect variations in the activity of some factor or group of factors that participates either in the ribosomal subunit association or in the dissociation step. The interdependence between these two kinds of factors will determine whether subunits, released at peptide chain termination, will immediately reassociate with mRNA to initiate another round of synthesis or will couple and enter the pool of inactive monomers.Earlier studies have established that ribosomes are in dynamic equilibrium with their subunits [2-51. Vast amounts of information accumulated in the last two decades point to the existence of initiation factors that prevents the association of ribosomal subunits into single ribosomes but does not actively promote their dissociation. Dissociation of single ribosomes can take place spontaneously at low magnesium concentration, thereby generating subunits which are unable to revert to monosomes because binding of eIF-3 and eIF-4C to 40s subunits and binding of eIF-6 to 60s subunits block ...
