30S initiation complex

Suttle, D. P. and Ravel, J. M. (1974). The effects of initiation factor 3 on the formation of 30S initiation complexes with synthetic and natural messengers. Biochem. Biophys. Res. Commun., 57, 386-93. 

The complex of mRNA, fMet-tRNAfMet, IF1, IF2 and the 30S ribosomal subunit is called the 30S initiation complex. 

The first step of RNA translation begins with the initiation of polypeptide synthesis (Fig. 17-10). GTP is bound into the 30S initiation complex and is subsequently hydrolyzed and released upon binding to the 50S subunit. The fMet-tRNA eI occupies what is known as the peptidyl (P) site of the ribosome (Fig. 17-9) another site (A), capable of accommodating an aminoacyl-tRNA, is empty at this stage. It is aligned with the next codon (shown as xxx in Fig. 17-10) in the mRNA. 

Figure 30.22 Translation initiation in prokaryotes. Initiation factors aid the assembly first of the 30S initiation complex and then of the 70S initiation complex.

Protein synthesis takes place m three phases initiation, elongation, and termination. In prokaryotes, mRNA, formylmethionyl-tRNAf (the special initiator tRNA that recognizes AUG), and a 30S riboso-mal subunit come together with the assistance of initiation factors to form a 30S initiation complex. A SOS ribosomal subunit then joins this complex to form a 70S initiation complex, in which fMet-tRNAf occupies the P site of the ribosome. 

Two initiation factors (IFl and IF3) bind to a VOS ribosome. IF3 and IFl appear to promote the dissociation of VOS ribosomes into free 30S and SOS subunits. mRNA and a third initiation factor (IF2), which carries a molecule of GTP and the charged initiator tRNA bind to a free 30S subunit. IF2 is one of a class of G proteins (see here). After these have all bound, the 30S initiation complex is complete. 

The SOS subunit binds to the 30S initiation complex. It contains three sites for tRNA binding, called the P site (peptidyl), the A site (aminoacyl), and the E site (exit). When the two ribosomal subunits join, the AUG initiator codon with its bound tRNAf t aligns with the P site. 

ACTIVE FIGURE 12.10 The formation of an initiation complex. The 30S ribosomal subunit binds to mRNA and fmet-tRNA in the presence of GTP and the three initiation factors, IF-1, IF-2, and IF-3, forming the 30S initiation complex. The 50S ribosomal subunit is added, forming the 70S initiation complex. Sign in at www.thomsonedu.com/iogin to expiore an interactive version of this figure. 

SOS initiation complex The 30S subunit (assisted by IF-3) is complexed with the mRNA such that the initiation codon is aligned within the P-site. IF-2 (GTP) delivers the initiator fMet-tRNA, to the P-site to form the 30S initiation complex. 

S initiation complex IF-3 is released and the joining of 508 to the 30S initiation complex is accompanied by the IF-2 mediated GTP hydrolysis, which results in the release of IF-1 and IF-2 and the formation of the 70S initiation complex. Initiation results in the formation of the 70S initiation complex, IMet-tRNAf mRNA ribo-some complex in which the fMet-tRNAf " occupies the P-site while the A-site is poised to accept an incoming aa-tRNA. 

N. f laaktgeboren, A. Vermaas and 0. Vorma, The Joining of the 30S-initiation complex with the SOS subunits, the main target for thiostrepton, Eur. J. Biochem. 57, 493 (1975),... 

Each factor plays a particular role in translation initiation, acting together to form 30S initiation complexes that can proceed toward elongation. The precise function of IFl is not known, although it does stimulate the binding and activities of IF2 and IF3. It binds to 30S subunits directly, and in combination with the other factors promotes the formation of 30S initiation complexes. Once the 30S complex is formed, IFl is ejected along with IF3. To date there is no conclusive evidence that the IFs affect the Shine-Dalgarno interaction between the mRNA and rRNA. 

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