Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Polyribosomes enable rapid translation of a single template

If Ribosomes are carefully isolated from Tissues that actively synthesize Proteins, such as the Pancreas, they are frequently found clustered into groups consisting of several to many ribosomes, sometimes numbering up to 80 or even more. Such aggregates, termed polyribosomes, or Polysomes, have been investigated using Electron Cell/15.html">Microscopy and Biochemical Methods. Treatment with Ribonuclease causes polyribosomes to dissociate into individual ribosomes, indicating that they are held together by an RNA strand. Indeed, the strand connecting adjacent ribosomes can be visualized in electron micrographs (Fig. 29-18). It is none other than an mRNA molecule being simultaneously translated by multiple ribosomes positioned closely to one another (Fig. 29-18). Such simultaneous Translation of a single mRNA by many ribosomes significantly enhances the efficiency of template utilization.

METABOLISM/31.html">Transcription and Translation Processes in Bacteria are very tightly coupled. As shown in Fig. 29-19, ribosomes can initiate the translation of an mRNA even while it is still being synthesized by DNA-dependent RNA polymerase. Another feature of Protein Synthesis in bacteria is that the lifetime of mRNA molecules is extremely short, lasting only a few minutes, as they are rapidly degraded by Nucleases. For this reason, to maintain a steady level of protein synthesis, the mRNA for a given protein or group of proteins must be synthesized continuously and utilized with maximum efficiency. As we will see below, the short lifespan of mRNA in prokaryotes allows for the rapid shutoff of protein synthesis when a particular protein is no longer needed.