RNA synthesis in Streptomyces antibioticus: in vitro effects of actinomycin and transcriptional inhibitors from 48-h cells.

Two forms of DNA-dependent RNA polymerase have been partially purified (about 100-fold relative to the crude extract) from 48-h old cells of Streptomyces antibioticus. The two forms show different Mg2+ optima for the incorporation of [3H]UMP into RNA. Substances inhibiting transcription have been isolated by ammonium sulfate precipitation from one of the fractions produced during the polymerase purification. Actinomycin can be shown to inhibit RNA synthesis catalyzed by the S. antibioticus polymerases to a similar extent regardless of the template used. When S. antibioticus DNA is the template, actinomycin inhibits transcription by S. antibioticus polymerase to a degree that is significantly less than the observed actinomycin inhibition of synthesis catalyzed by Escherichia coli polymerase or by either S. antibioticus or E. coli polymerase with calf thymus DNA as the template. Using an assay previously developed, it was shown that the association constant for the binding of actinomycin to S. antibioticus DNA was increased by the presence of RNA polymerase in the binding mixture, while the association constant for the binding to calf thymus DNA was decreased by RNA polymerase. RNA synthesis in crude, cell-free extracts of 12-h old S. antibioticus cells (not producing actinomycin) is less refractory to actinomycin inhibition than synthesis catalyzed by extracts of 48-h old (actinomycin producing) cells, and both extracts catalyze appreciable RNA synthesis at actinomycin concentrations that completely inhibit RNA synthesis catalyzed by E. coli extracts.