No abstract

Purified DNA-dependent RNA polymerases A, B and C isolated from calf thymus contain a significant amount of zinc. Atomic absorption spectroscopy revealed the presence of 6.7, 5.35 and 2.6--4.1 g-atoms of zinc per mole of polymerase A, B and C, respectively. These enzymes are inhibited by treatment with 1,10-phenanthroline at concentrations varying from 10(-5) to 10(-4) M. However, the addition of zinc ions does not restore fully the activity of 1,10-phenanthroline treated enzymes. Exogenous zinc ions reduce in vitro an overall RNA synthesis catalysed by RNA polymerases from calf thymus. In addition to the sites which bind zinc in a specific and stoichiometric way these enzymes possess other classes of binding sites with high and low affinity. Occupancy by exogenous zinc of these additional binding sites inhibit polymerase activity.