Formation of 70S ribosomes: large activation energy is required for the adaptation of exclusively the small ribosomal subunit.

Association of ribosomal subunits is an essential reaction during the initiation phase of protein synthesis. Optimal conditions for 70S formation in vitro were determined to 20 mM Mg2+ and 30 mM K+. Under these conditions, the association reaction proceeds with first order kinetics, suggesting a conformational change to be the rate-limiting step. 70S formation separates into two sub-reactions, the adaptation of the ribosomal subunits to the association conditions and the association step itself. The activation energy of the process was determined to 78 kJ/mol and revealed to be required exclusively for the adaptation of the small subunit, rather than the large subunit or the association step. The presence of mRNA [poly(U)] together with cognate AcPhe-tRNA, accelerates the association rate significantly, forming a well-defined 70S peak in sucrose gradient profiles. mRNA alone provokes an equivalent acceleration, however, the resulting 70S couple impresses as an ill-defined, broad peak, probably indicating the readiness of the ribosome for tRNA binding, upon which the ribosome flips into a defined state.