5 - FUNCTION OF PLASMIDS IN PRODUCERS OF MACROLIDES

Abstract

Extrachromosomal elements have been identified in several strains of Streptomycetes producing different macrolide antibiotics. Streptomyces reticuli which produces melanin and the macrolide antibiotic leucomycin has been studied in most detail. This strain contains one large plasmid with a molecular weight of 48–49 Mdal. The strain is genetically unstable. Several different variants have been characterized which lack the ability to produce melanin and (or) aerial mycelium. The variants synthesize normal, no or reduced amounts of the macrolide. Most of the variants still contain plasmid DNA and dependinq on the variant tested, the plasmid DNA consists of a homoqeneous or a very heteroaeneous population of covalently closed circular (CCC) DNA molecules. Genetical and biochemical studies have revealed that genetic instability of this species results from chanqes within both the extrachromosomal and chromosomal DNA.