Summary: Butyrate and isobutyrate (after isomerization to -butyrate) are specific precursors for the biosynthesis of monensin A in . High concentrations of both butyrate and isobutyrate (greater than 20 and 10 mm, respectively) were toxic to plated on solid medium. Spontaneous mutants resistant to these substances were isolated. These new strains produced monensins at even higher concentrations of butyrate or isobutyrate, with an increased yield of monensin A. produced an anti-isobutyrate (AIB) factor, which was originally found to be excreted by some isobutyrate-resistant strains growing on solid medium containing isobutyrate. On plates, the AIB factor efficiently counteracted toxic concentrations not only of isobutyrate, but also of acetate, propionate, butyrate, 2-methylbutyrate, valerate and isovalerate against as well as other species. Although the AIB factor enabled normal growth, sporulation and monensin production on plates, it did not have positive effects on submerged cultures of with isobutyrate. The partial purification of the AIB factor was achieved. The role of the AIB factor during spore germination on solid medium containing isobutyrate or its homologues is discussed.


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