Initiation of DNA‐Dependent RNA Synthesis and the Effect of Heparin on RNA Polymerase

The kinetics of RNA synthesis, catalyzed by DNA‐dependent RNA polymerase from Escherichia coli , show a lag phase at high ionic strength or at low temperature when using a double‐stranded DNA as template. This lag phase increases with decreasing temperature at constant ionic strength or with increasing ionic strength at constant temperature. With single‐stranded DNA as template, no lag phase is observed. The length of the lag phase is greatly shortened by pre‐incubation of the enzyme with DNA at low Mg ++ concentration, independent of the presence of substrate. Heparin is a strong inhibitor of free and DNA‐bound polymerase. Two heparin molecules are required for the inactivation of one enzyme molecule. Enzyme engaged in RNA synthesis is instantly inhibited on single‐stranded DNA but not on double‐stranded DNA. With the aid of this inhibitor it is shown that 10 minutes after the start of incubation, at 16°, all enzyme molecules have initiated RNA chains on double‐stranded DNA.