Unused protein synthetic capacity of Escherichia coli grown in phosphate-limited chemostats.

Escherichia coli was grown in phosphate-limited chemostat cultures at dilution rates corresponding to 8 to 12-hour doubling times. The RNA content expected in such cultures based on the “constant efficiency” hypothesis was six to eight-fold lower than that actually found. Measurement of the growth kinetics, pulse incorporation of guanine and tryptophan following nutritional shift-up, and the determination of delay time for the completion of the first polypeptide of β -galactosidase after addition of inducer lead to the following conclusions. (1) The “extra” RNA corresponds to unused protein synthetic capacity that is not in use in phosphate-limited growth. (2) However, those ribosomes actually engaged in protein synthesis do operate as efficiently as in faster growing cells. (3) Immediately (less than five minutes) on being shifted-up, the previously unused protein synthetic capacity does function. (4) The rate of RNA accumulation after a nutritional shift-up from phosphate-limited growth accelerates quickly, then decelerates, and then accelerates more slowly to the steady-state rate.