An RNA polymerase inhibitor, cyclothiazomycin B1, and its isomer.

Novel cyclic thiopeptides, cyclothiazomycins B1 (1) and B2 (2), were isolated from Streptomyces sp. A307 as potent hyphal swelling inducing substances. They are stable in the solid state but slowly isomerize with one another in solution. Degradation experiments and spectroscopic analyses disclosed that they comprise unique tricyclic structures each containing a dehydroalanine, and two dehydrohomoalanine residues, along with three thiazolines, three thiazoles, and a trisubstituted pyridine. Cyclothiazomycin B1 (1) is expected to be a powerful tool for DNA-RNA transcription studies, because this cyclopeptide inhibits DNA-dependent RNA synthesis by bacteriophage RNA polymerases.