Allelopathic actions of the alkaloid 12-epi-hapalindole E isonitrile and calothrixin A from cyanobacteria of the genera Fischerella and Calothrix

The alkaloids 12-epi-hapalindole E isonitrile, isolated from the cyanobacterium Fischerella sp., and the indolophenanthridine calothrixin A, from Calothrix sp., were characterized in terms of their ability to kill several organisms and cell types, and their biochemical modes of action. Both compounds inhibited RNA synthesis, and consequently protein synthesis, in Bacillus subtilis. Calothrixin A also inhibited DNA replication, the hapalindole having little effect on this process. Measurements of in vitro RNA synthesis confirmed the in vivo results and suggested that both compounds inhibit RNA polymerase directly; the degree of inhibition was independent of the DNA concentration, but strongly dependent on the polymerase concentration.