INITIATION-FACTORS OF PROTEIN-BIOSYNTHESIS IN BACTERIA AND THEIR STRUCTURAL RELATIONSHIP TO ELONGATION AND TERMINATION FACTORS

Initiation of protein biosynthesis in bacteria requires three initiati on factors: initiation factor 1, initiation factor 2 and initiation fa ctor 3. The mechanism by which initiation factors form the 70S initiat ion complex with initiator fMet-tRNA(fMet) interacting with the initia tion codon in the ribosomal P site and the second mRNA codon exposed i n the A site is not yet understood. Here, we present a model for the f unction of initiation factors 1 and 2 that is based on the analysis of sequence homologies, biochemical evidence and the present knowledge o f the three-dimensional structures of translation factors and ribosome s. The model predicts that initiation factors 1 and 2 interact with th e ribosomal A site mimicking the structure of the elongation factor G. We present data that extend the mimicry hypothesis to initiation fact ors 1 and 2, originally postulated for the aminoacyl-tRNA elongation f actor TU GTP ternary complex, elongation factor G and release factors.