Sequence of Events in Initiation of Protein Synthesis

Initiation factors recycle during the assembly of a 70‐S initiation complex. At early stages the factors are present, but they are absent in the mature 70‐S initiation complex. Experiments described in this paper deal with the sequence of events and the contribution of each factor in the assembly process. Initiation factors IF‐1 and IF‐2 bind to 30‐S subunits independent of the presence of the other components involved in initiation. The fate of IF‐2 during 70‐S initiation complex formation has been studied in a system devoid of IF‐1 and in a system in which GTP is replaced by 5′‐guanylylmethylenediphosphonate (Guo‐5′‐ P 2 ‐CH 2 ‐ P ). In both cases it is found that IF‐2 is not released from the 70‐S initiation complex. This leads to the inhibition of the elongation factor EF‐Tu dependent binding of the second aminoacyl‐tRNA, coded for by the messenger. The data are interpreted by assuming partly overlapping binding sites for EF‐Tu and IF‐2.