Balancing Synthesis of Ribosomal Components
Although the synthesis of the individual components of ribosomes may be rather well regulated, a slight imbalance in the synthesis of one component could eventually lead to elevated and potentially toxic levels of that component. Synthesis of the ribosomal RNAs in bacteria are kept in balance by a simple mechanism. As we have already seen, these RNAs are synthesized in one piece by an RNA polymerase that initiates at a promoter and transcribes across the genes for the three RNAs. Different mechanisms are used to maintain balanced synthesis of some of the ribosomal proteins. In one case, one of the proteins encoded in a ribosomal protein operon reduces translation of all the proteins in that operon. This effect is called translational repression.
The finding that a ribosomal protein represses translation of proteins only from the same operon provides an efficient means for the cell to maintain balanced synthesis of all the ribosomal proteins. Suppose that some ribosomal proteins began to accumulate because their synthesis is a little faster than the other proteins and the rRNA. Then, as the level of these proteins begins to rise in the cytoplasm, they begin to repress their own synthesis, and the system rapidly comes back to a balanced state.
How do we know about translational repression? The main clue came from careful measurements on cells with an increased number of genes coding for some of the ribosomal proteins. The increased copy number might have been expected to increase the synthesis of the corresponding proteins, but it did not. The synthesis of the mRNA for these proteins did increase as expected, and therefore it appeared that extra ribosomal proteins in the cell inhibited translation of their own mRNA. Proof of the idea of translational repression came from in vitro studies in which levels of individual free ribosomal proteins could be adjusted at will. The addition of DNA containing genes for some of the ribosomal proteins and properly prepared cell extract permits transcription and translation to yield ribosomal proteins synthesized in vitro. Nomura found that addition of the appropriate free ribosomal proteins to such a system repressed synthesis of the proteins encoded by the same operon as the added protein.
Figure 7.28 Determination of the pool size of ribosomal protein by the kineticsof a pulse of radioactive amino acids into ribosomal protein.
Not surprisingly, a ribosomal protein that regulates synthesis of a group of proteins binds to the mRNA to effect the repression. The structure of the binding region on the mRNA for some of these proteins is the same as the structure the protein binds to in the rRNA in the ribosome.
Global limits can be placed on the accuracy with which the synthesis of ribosomal components is balanced. A short pulse of radioactive amino acid is provided to the cells, and the total pool of all ribosomal proteins can be determined by measuring the kinetics of incorporation of label into mature ribosomes (Fig. 7.28). The results show that the pool contains less than a five-minute supply of ribosomal proteins. Similarly, the pool size of each individual ribosomal protein can be measured. The results of these experiments show that most of the ribosomal proteins also have very small intracellular pools.
We have seen that the mechanisms regulating ribosome synthesis have good reason to be sophisticated, and, indeed those aspects that have been investigated have turned out to be complicated. Much of the biochemistry and perhaps even much of the physiology of ribosome regulation remain to be worked out. In bacteria and other single-celled organisms such as yeast, it is likely that most of the regulation mecha - nism can be dissected by a combination of physiology, genetics, and biochemistry. It will be interesting to see if analogous problems in higher organisms can also be solved without the availability of genetics.
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