Blasticidin S, an antibiotic^1,2) shows therapeutic effect on rice blast³⁾, which is economically the most important disease of the rice plants in Japan. The effect is considered to be due to high toxicity of blasticidin S to mycelial growth of Piricularia oryzae, pathogen of the rice blast³⁾. This high toxicity of blasticidin S to mycelial growth seems to be due to a strong inhibition on protein synthesis of Piricularia oryzae⁴⁾. This was indicated by the experiments of Misato et al.⁵⁾ They showed that blasticidin S inhibited ¹⁴C-glutamic acid incorporation into the protein fraction in Piric. oryzae in intact cells, whereas it did not inhibit glycolysis, succinic dehydrogenase activity and oxidative phosphorylation in cell-free extracts of Piric. oryzae and over-all incorporation of inorganic phosphate into nucleic acid fraction in intact mycelia of Piric. oryzae at the minimum growth inhibitory concentration (1 ppm).

The authors found that nitrogen metabolism in the mycelium of Piric. oryzae was remarkably disturbed, when blasticidin S was applied to a culture of log-phase growth. Trichloroacetic acid (5%)-insoluble fraction was decreased, while trichloroacetic acid-soluble fraction and RNA contents were rapidly increased. These results suggest the blasticidin S may exert its primarily fungicidal effect by inhibiting cellular nitrogen metabolism. Protein synthesis in the mycelium might be the primary action site. Therefore this possibility was tested in intact mycelial cells and in a cell-free system.

The present paper is concerned with ¹⁴C-amino acid incorporation into protein fraction in cell-free extracts and intact cells of Piric. oryzae.