SUMMARY: The effects of the antibiotic validamycin A on the inositol content and hyphal branching of were investigated. Inositol was found associated with the cell wall and as a lipid extractable fraction composed of phospholipid, phosphosphingolipid and glycolipid. Validamycin A decreased the inositol content of the phospholipid fraction by 57% and the phosphosphingolipid fraction by 40%, but had no effect on the glycolipid fraction. The phosphatidylinositol (the only inositol-containing phospholipid detected) content of the biomass was reduced by 53 % in the presence of validamycin A and this decrease was compensated for by rises in the levels of phosphatidylserine and an unknown, minor phospholipid. Over the concentration range 1 to 5000 μ, validamycin A caused decreases in the colony radial growth rate, internode length (the distance between adjacent branches on a leading hypha) and inositol content of R. colonies, and a positive correlation was found between inositol content and internode length. Addition of inositol to the medium only slightly antagonized the effect of the antibiotic on mycelial morphology and did not restore phosphatidylinositol levels of the biomass to control values.


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