On the mode of action of lomofungin, an inhibitor of RNA synthesis in yeast.

Lomofungin is a potent inhibitor of RNA synthesis in yeast. Studies on the mode of action of the inhibitor were carried out using yeast RNA polymerases A and B and bacterial RNA polymerase. In vitro inhibition of RNA synthesis is independent of the nature and concentration of the template used and of the nucleoside triphosphate concentration. The extent of inhibition is strongly dependent upon the nature and concentration of divalent cations used to simulate transcription. The three RNA polymerases were inhibited to the same extent in the presence of Mn2+ ions whereas little inhibition was observed with Mg2+ ions. Spectrophotometric studies reveal the formation of different complexes between lomofungin and divalent cations (Mn2+, Mg2+, or Zn2+) with the respective stoichiometries of 0.5, 1, and 2 divalent cations per molecule of lomofungin. The complexes formed depend upon the nature of the divalent cation involved. No direct interaction between lomofungin and DNA could be observed in the presence of divalent cations but evidence is presented that lomofungin interacts with yeast RNA polymerase A. Inhibition of RNA synthesis occurs at the level of both chain initiation and elongation.