Are all nucleotidyl transferases metalloenzymes?

Previous studies (1,2) have shown that DNA polymerases (sea urchin and E. coli I) and E. coli RNA polymerases are zinc metalloproteins. We report here that DNA-dependent RNA polymerases I and II from rat liver, I, II and III from sea urchin, and RNA-dependent DNA polymerase (reverse transcriptase) from Rous sarcoma virus are inhibited by pre-incubation with the chelating agent, o-phenanthroline. These enzymes are not affected by pre-incubation with the related compound, m-phenanthroline, which is not a metal chelating agent. The data suggest that, in addition to Mg⁺⁺ or Mn⁺⁺, the enzymes require a tightly bound metal ion (possibly zinc) for catalytic activity. The DNA-terminal transferase is also inhibited by o-phenanthroline but not m-phenanthroline (3). On the other hand, we found that tRNA-nucleotidyl transferases from Rous sarcoma virions and yeast are not affected by either o- or m-phenanthroline. These observations suggest that many but not all nucleotidyl transferases are metallo-enzymes.