The evaluation of the rates of biological processes from tracer kinetic data: II. RNA metabolism in growing bacteria

Abstract

Because messenger RNA continuously breaks down to form nucleotides that are reutilized for synthesis of all forms of RNA, analysis of the kinetics of uptake of precursors into the nucleic acids of cells is complicated. Our theoretical work ( Koch, 1962) is now extended to evaluate the effects of recycling for the case of growing cells. Equations are derived for several different assumptions about the turnover of the intermediate pool of RNA precursors. Even if one assumes the pool infinitesimal, the equations apply to uptake at times greater than a minute for Escherichia coli. The effective turnover rate constant of messenger is smaller than the true turnover rate constant by a factor which is the ratio of the rate at which molecules enter the intermediate pool from the exogenous source to the total rate at which molecules enter the pool from all sources. The initial lag in the incorporation of isotope into total nucleic acids can be used to measure the velocity of synthesis of the pool from all sources if the pool size is known. If the pool size is unknown, it may be estimated from the lag time extrapolated from uptake measurements at intermediate times. Short lag times, however, do not necessarily reflect a small pool. The pool may be large and it may communicate with a bypass or with the external medium but still give rise to a short lag because of the initial rapid uptake of precursor into messenger. The percentage of the isotope in particular classes of nucleic acid relative to the total nucleic acids is a quantity that can be measured. These measurements can be used to measure both the rate of synthesis and the amount of labile RNA in the cells.