Saccharomyces cerevisiae DNA-dependent RNA polymerase III: a zinc metalloenzyme

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Abstract
Yeast nuclear RNA polymerase III was purified by batch adsorption to phosphocellulose, followed by ion-exchange chromatography on DEAE-Sephadex and affinity chromatography on DNA-Sepharose. Polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band which contained polymerase activity. The MW estimated by sedimentation velocity centrifugation in a glycerol gradient was 380,000. Enzyme activity was inhibited 50% at 0.1 mM 1,10-phenanthroline and 100% at 1.0 mM, but was restored when 1,10-phenanthroline was removed by dialysis. Enzyme activity was not inhibited by 7,8-benzoquinoline, a nonchelating structural analog of 1,10-phenanthroline. These results strongly suggest that inhibition of enzyme activity occurs by the formation of a reversible enzyme-zinc-phenanthroline ternary complex. The zinc content, measured by atomic absorption spectroscopy, was 2 g-atoms per mol of enzyme. Zn was not removed from the enzyme by gel filtration on Sephadex G-25, by passage through Chelex-100 resin, or by dialysis against buffer containing 1,10-phenanthroline. Enzyme-bound Zn was removed by dialysis after denaturation of the enzyme with heat and sodium dodecyl sulfate. Enzyme-bound Zn did not exchange with free Zn. These results establish yeast nuclear RNA polymerase III as a Zn metalloenzyme.