Proportionality of Ribosome Levels and Growth Rates
المؤلف:
Robert Schleif
المصدر:
Genetics and Molecular Biology
الجزء والصفحة:
2nd Edition , p212-214
2025-05-27
548
The dramatic ability of bacteria to grow with a wide variety of rates prompts the question of how they manage to maintain balanced synthesis of their macromolecules. In a study of this question, Schaechter, Maaløe, and Kjeldgaard made the discovery that ribosomes are used at constant efficiency, independent of the cell growth rate. To appreciate their contribution fully, it will be helpful first to examine a related question: what is the average rate of protein synthesis per ribosome? As a first step we will estimate this value using typical cellular parameters, then we will calculate this value more carefully and include the effects of increase in the number of ribosomes during a cell doubling time.
An average bacterial cell with a doubling time of 50 minutes contains about 1 × 10-13 g protein and about 10,000 ribosomes. Approximating the molecular weight of amino acids to be 100,
1 × 10-13 g protein is 1 × 10-13/102 = 10-15 moles amino acid; 10-15 moles amino acid is 6 × 1023 × 10-15 = 6 × 108 molecules; 104 ribosomes polymerize these 6 × 108 amino acids in 50 min or 3 × 103 sec.
Thus the average rate of protein synthesis per ribosome is 6 × 108/3 × 103 = 20 amino acids per second per ribosome. Compared to the typical turnover number of enzymes, greater than 1,000 per second, this is a low number. We have seen already, however, the process of addition of a single amino acid to the growing polypeptide chain is complex and involves many steps.
To calculate accurately the rate of protein synthesis per ribosome during steady-state growth, we must include the growth of the cells in the calculation. This can be done in the following way. Define αr as the relative rate of synthesis of ribosomal protein, that is,
where Pr, is ribosomal protein and Pt is total protein. Let R(t) be the number of ribosomes in a culture at time t. Since ribosome number will increase like cell number, R(t) = R(0) eµt, and hence, 
= µR(t) . Also,
the rate of ribosome synthesis, dR/dt, equals the rate of ribosomal protein synthesis in amino acids per unit time divided by the number of amino acids in the protein of one ribosome, C:
Then dPt/dt equals the number of ribosomes times the average elongation rate per ribosome, K. That is,
We have the following two expressions for dR/dt
which yields K = µC/αr, our desired relation. Note that if αr is roughly proportional to the growth rate, as has been found for bacteria except at the slowest growth rates (Fig. 1), then the term µ/αr is a constant and hence K, average activity of a ribosome, is independent of the growth rate.
Fig1. The value of α, as a function of growth rate. Except at slow growth rates, αr is proportional to growth rate.
For E. coli B/r growing at 37°, with a doubling time of 48 minutes,
This value is close to the elongation rate of polypeptides, showing that most ribosomes in the bacterial cell are engaged in protein synthesis and are not sitting idle.
A measurement of αr at any time during cell growth can be accomplished by adding a radioactive amino acid to growing cells for a short interval (Fig. 2). Then an excess of the nonradioactive form of the amino acid is added and cells are allowed to grow until all the radioactive ribosomal proteins have been incorporated into mature ribosomes. The value of αr, is the fraction of radioactivity in ribosomal protein compared to total radioactivity in all the cellular protein. This fraction can be determined by separating ribosomal protein from all other cellular protein by electrophoresis or sedimentation and measuring the radioactivity in each sample.
Fig2. Schematic of the ribosomal protein regulation system. The mechanism deter mines the fraction, α, of total protein synthesis to be devoted to ribosomal protein synthesis.
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