Sequential Readout of mRNA by Ribosomes Polyribosomes

Throughout the course of protein synthesis, the ribosome is associated with a limited section of the template polyribonucleotide. Since the ribosome-bound sections of the template are protected from nuclease action, they may be isolated after nuclease treatment of the ribosome-template complexes. Such sections have been found to have a length of 40 to 60 nucleotide residues. It must be noted again that the length of the mRNA coding sequence usually exceeds 300 nucleotides. Therefore, in order to read the entire mRNA coding sequence, the ribosome should sequentially run over the template (or thread through itself) from the 5'-terminal part of the coding sequence to the 3'-terminal part. In other words, the ribosome should work as a tape-driving mechanism.

At what rate, then, would the ribosome move along the mRNA? In a bacterial cell (e.g. E. coli) a polypeptide with a length of 300 amino acids is synthesized for about 20 seconds at 37°C, i.e. one ribosome runs over about 40 to 50 nucleotides per second. The rate of mRNA readout in Eukaryotes can approach 30 nucleotides per second, but regulatory effects may reduce it to 5 to 10 nucleotides per second (see Chapter 13).

While moving along the template polynucleotide from the 5'-end to the 3'-end, the ribosome, after some time, moves away from the 5'-terminal section of the template. As a result, this section becomes exposed and is capable of binding with another free ribosome. The second ribosome will start the readout, and moving away from the 5'-terminus will give the third ribosome an opportunity to bind and start reading, etc. In this way, moving along the template one after another, a number of ribosomes simultaneously perform a readout of the same information and, hence, synthesize identical polypeptide chains (of course, at any given moment the chains on different ribosomes are at different stages of completion). This process is schematically presented in Fig. 4.4, where the ribosomes at the 3'-end of the template contain an almost completed polypeptide, the ribosomes located in the middle of the mRNA carry

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