Expectations for Ribosome Regulation
The final topic we consider here is the regulation of the level of the protein synthesis machinery itself. Not surprisingly, we will find that the synthesis of this machinery is regulated by the intensity of its use.
A ribosome is a large piece of cellular machinery. It consists of about 55 proteins, two large pieces of RNA, and one or two smaller RNAs.
Hence it is natural to expect a cell to regulate levels of ribosomes so that they are always used at the highest possible efficiency. Since bacterial cells and some eukaryotic cells may grow with a wide variety of rates, a sophisticated regulation mechanism is necessary to ensure that ribo-somes are fully utilized and synthesizing polypeptides at their maximal rate under most growth conditions.
In an earlier section we discussed the finding that proteins in bacteria are elongated at about 16 amino acids per second. In the next sections we will find that the rate of cellular protein synthesis averaged over all the ribosomes is also about 16 amino acids per second. This means that virtually no ribosomes sit idle. All are engaged in protein synthesis.