Transition period following a nutritional shift-up in the bacterium Escherichia coli B/r : Stable RNA and protein synthesis

Abstract

The previously observed patterns of stable RNA and protein synthesis during the transition period following a nutritional shift-up are analyzed and described by kinetic equations. (i) The kinetics of the stable RNA synthesis rate after a shift-up are diphasic; the first phase, a step-like increase occurring within minutes after the shift, reflects an induction of rRNA and tRNA transcription; the second phase, an exponential increase with a higher (post-shift) rate constant, reflects a change in the accumulation rate of functioning RNA polymerase. The ratio of the numbers of ribosomes and functioning RNA polymerase molecules is shown to be independent of growth rate or time after the shift. (ii) The protein synthesis rate increases exponentially throughout the transition period with a rate constant corresponding to that of post-shift steady-state growth, which reflects the exponentially increasing number of ribosomes. An initial step-like increase in the protein synthesis rate due to an increase in the ribosome efficiency occurs when the pre-shift growth is below 1·2 doublings/hr. (iii) The duration of the transition period is estimated from the time it takes for the stable RNA to protein ratio to reach its post-shift steady-state value. The period is approximately 3 hr but has no clearly defined end. (iv) When RNA or protein synthesis rates are measured in different units, for example as rate per unit of culture volume, rate per cell or per genome, these differently defined rates change in different patterns after a shift-up. The relationships between these units and the resulting kinetics are explained.