Reassociation of eukaryotic ribosomal subunits. Kinetics of reassociation; role of deacylated transfer ribonucleic acid; effect of cycloheximide.

The reassociation of ribosomal subunits to form 80 S monomers is catalyzed by a rat liver cytosol fraction (G-25) and by the initiation factor MI (formerly called 40 S binding factor). Reassociation requires a template (we have generally used poly(U)) and either aminoacyl-tRNA or deacylated tRNA. Initiation factor-catalyzed formation of 40 S60 S couples can occur in two ways. The first pathway of reassociation has faster kinetics, requires aminoacyl-tRNA, is insensitive to cycloheximide, and leads to the formation of 80 S ribosomes active in the synthesis of protein. The second pathway has slower kinetics, uses deacylated-tRNA, is sensitive to inhibition by cycloheximide, and the 80 S ribosomes formed do not synthesize protein. Although cycloheximide inhibits reassociation by the second pathway it does not interfere with the binding of deacylated tRNA to 40 S subunits (or of aminoacyl-tRNA, for that matter). That reassociation which occurs in the absence of initiation factors probably proceeds by the second pathway since it has slow kinetics and is inhibited by cycloheximide. Reassociation of ribosomal subunits catalyzed by purified MI is also inhibited by cycloheximide, which suggests that a second factor is normally required.