Ansamitocins, maytansinoid antitumor antibiotics producing organism, fermentation, and antimicrobial activities.

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Abstract
Ansamitocins are new maytansinoid antitumor antibiotics produced by an actinomycete strain No. C-15003 (N-1). The organism is characterized by coremia formation on solid media, release of the rod-shaped motile spores with peritrichous flagella in liquid media, the formation of some branched mycelia at a later stage of growth in liquid culture which break up into motile elements, lysozyme resistance, the occurrence of meso-diaminopimelic acid in the cell wall and guanine-cytosine content of 71 .+-. 1 mol%. From these results, the organism was designated as Nocardia sp. No. C-15003 (N-1). In the fermentation fluids, activity against eukaryotic microorganisms was detected. Three of the purified materials, which have the activity against Tetrahymena pyriformis strain W and Hamigera avellanea IFO 7721, were new ansamycin antibiotics with antileukemic activities and were named ansamitocins P-3, P-3'' and P-4. Ansamitocins show growth inhibitory activity against several eukaryotic microorganisms but no activity against prokaryotic microorganisms. The acyl moieties at the C-3 position of ansamitocins are essential for their antifungal activities.