Recycling of Initiation Factors IF-1, IF-2 and IF-3.

Initiation of protein synthesis in Escherichia coli is a catalytic process, provided that conditions for hydrolysis of GTP are present. This means that the initiation factors IF-1, IF-2 and IF-3 are released at a certain stage of initiation complex formation, whereafter they recycle and are reutilized in another round of initiation. Experiments presented in this paper provide evidence for the mechanism of recycling, in which the hydrolysis of GTP and IF-1 both play a predominant role. We demonstrate that binding of fMet-tRNA, which is dependent on IF-2, is a stoichiometric process in a system devoid of IF-1. Addition of IF-1 converts it into a catalytic process, however, only if GTP can be hydrolysed. No such conversion takes place with 5′-guanylylmethylenediphosphonate. The same observation is valid for the uncoupled IF-2 dependent GTPase activity. In the process of initiation IF-1 acts as a recycling promoting factor, which displays analogous properties to elongation factor EF-Ts. We postulate that a ribosomal-bound IF-2 · GDP complex is converted into an IF-2 · GTP complex, which is released and is capable of starting a new cycle of initiation. Furthermore, kinetic evidence is presented that IF-3 recycles even at the level of 30-S initiation complex formation, irrespective of whether conditions for hydrolysis of GTP are provided. [ABSTRACT FROM AUTHOR]