Chain elongation rate of messenger and polypeptides in slowly growing Escherichia coli

Abstract

The kinetics of completion of the polypeptide chains of β-galactosidase after addition of inducer to cultures of Escherichia coli were studied with an extremely sensitive and accurate fluorometric assay. The length of time after induction is initiated before the first polypeptide chains are completed (delay time) is essentially the same (95 ± 5 seconds) over a wide range of growth rates (50 minutes to 12.6 hours doubling time). Even during chemostat growth with a doubling time of 24 hours, the delay time is only slightly prolonged. From control experiments and the results of others it can be concluded that neither the chain elongation rate for transcription nor for translation is appreciably slowed in cells whose steady-state rate of production of β-galactosidase and most other proteins is slowed by the chemostat limitation. The average lag (the time until messenger RNA translation comes to steadystate) is also essentially invariant with growth rate. It follows that the life span of the messenger molecule is also not markedly altered by changing the growth rate. This is also shown by pulse induction experiments. It is shown in the Appendix that the shape of the induction curves when inducer is added and then quickly removed or when the inducer is added and not removed is consistent with either a model of messenger decay resulting from the first-order loss of capacity to initiate translation or a model of decay random in time and place on the messenger RNA chain. It is argued that induction experiments, when inducer is added and not removed, are decisive in excluding models in which each messenger molecule is read a fixed number of times, since in this case the conclusion does not depend on the assumption that internal inducer can be instantaneously removed by dilution or by removing external inducer, or that certain drugs affect only initiation.