Translocation in Protein Synthesis: A Hybrid Structure Model

Abstract

POLYPEPTIDE chains are synthesized on 70S ribosomes which are composed of two macromolecular sub-units, the 30S and 50S particles^1,2. It is the 30S ribosome which carries the messenger RNA³ while the 50S ribosome has as one of its component proteins the peptide synthetase^4,5. The formation of a peptide bond occurs between a peptidyl-sRNA^6–8. and an amino acyl-sRNA^9,10 yielding a longer peptidyl-sRNA and a free sRNA molecule. This reaction (Fig. 1A) is catalysed by the peptide synthetase. The order in which amino-acyl-s-RNAs are introduced into the ribosome is determined by the messenger RNA^11,12. It is usually assumed that the 70S ribosome must, before the formation of a peptide bond, have two sites: a peptide, or p site, capable of holding a peptidyl-sRNA, and an amino-acid, or a site to hold the amino-acyl-sRNA¹³. Just after peptide bond formation the new peptidyl-sRNA must reside in the amino-acid site, as shown in Fig. 1B. In order to repeat the cycle of peptide bond formation, the peptidyl-sRNA is assumed to be translocated from the amino-acid site to the peptide site, with the concomitant advance of the messenger RNA through the 30S ribosome (Fig. 1). The whole process of peptide synthesis is catalysed by at least three enzymes¹⁴ and requires guanosine triphosphate (GTP)^15,16.