Mutants of Streptomyces atroolivaceus blocked in the biosynthesis of mithramycin were isolated both by natural selection and after treatment with mutagenic factors (UV and gamma rays, nitrous acid). Both physical factors were more effective than nitrous acid. The selection was complicated by a high instability of isolates, out of which 20-80% (depending on their origin) reversed spontaneously to the parent type. The primary screening (selection of morphological variants and determination of their activity using the method of agar blocks) made it possible to detect only potentially non-productive strains; however, the final selection had to be performed always under submerged conditions. Fifty-four stable non-productive mutants were divided, according to results of the chromatographic analysis, in five groups differing in production of six biologically inactive metabolites (compounds A-H). The mutants did not accumulate chromomycinone, chromocyclomycin and chromocyclin. On mixed cultivation none of the pairs of mutants was capable of cosynthesis of mithramycin or new compounds differing from standard metabolites. Possible causes of the above results are discussed.

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