KINETICS OF RIFAMPICIN - RNA-POLYMERASE COMPLEX - DIFFERENCES BETWEEN CRUDE AND PURIFIED ENZYME FRACTIONS

The antibiotic rifampicin forms a very tight complex with DNA-dependent RNA polymerase of Escherichia-coli. The rate constants of association and dissociation of this complex are dependent on the purity of the enzyme. Thus a crude RNA polymerase (fraction-3 enzyme) has rate constants different from those of an enzyme further purified by DEAE-cellulose chromatography (fraction-4 enzyme). The complex produced by the antibiotic and the fraction-3 enzyme is about 10 times more stable and is formed about 10 times more slowly than the complex with fraction-4 enzyme. The RNA present in the crude enzyme and removed by chromatography on DEAE-cellulose is the cause of the change in the kinetics of the complex. tRNA of rat liver and crude rat liver RNA added to purified RNA polymerase have a similar effect. Mg2+, which has no intrinsic influence, augments the effect of the nucleic acids; monovalent cations do not. Since nucleic acids increase the stability of the complex, but at the same time decrease the rate of its formation, the equilibrium constant. Keq, remains almost the same. The possible effects of nucleic acids on the rifampicin binding site are discussed.