Role of plasmids in producers of macrolides

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Abstract
Several strains of Streptomycetes producing macrolide antibiotics were found to contain extrachromosomal DNA, but it could be shown that the syntheses of macrolides are not necessarily plasmid‐determined. A plasmid‐bearing strain, Streptomyces reticuli, synthesising melanin and the macrolide leucomycin has been studied together with several variants arising after treatment with intercalating agents. Variants forming no melanin and (or) aerial mycelium possess altered extrachromosomal DNA and those which lack plasmid DNA do not produce aerial mycelium, melanin or the antibiotic on agar plates. In contrast with the wild‐type of S. reticuli, the plasmidless variants contain amplified nucleotide sequences within their chromosomal DNA. The significance of the changes within the chromosomal and extra‐chromosomal DNA in respect to the production of secondary metabolites is discussed.