Giant linear plasmids in Streptomyces which code for antibiotic biosynthesis genes

Abstract

A number of examples of circular plasmids with specific functions are known in both prokaryotes and eukaryotes. Several linear plasmids have also been identified^1–6, but these are all relatively small: large linear plasmids cannot be separated from chromosomal DNA by conventional techniques. There are several cases where the genetic evidence suggests that a character is encoded by a plasmid but no plasmid can be physically detected. This has been the case for antibiotic synthesis genes in Streptomyces; in particular a plasmid SCP1 in Streptomyces coelicolor has been shown to be involved in methylenomycin production by genetic evidence^7,8. We report here the application of orthogonal-field-alternation gel electrophoresis⁹ to the isolation of linear plasmids from Streptomyces. We have discovered a large linear plasmid of around 520 kilobases in Streptomyces lasaliensis and subsequently similar giant linear plasmids in other Streptomyces strains. We have confirmed that genes for methylenomycin biosynthesis^10,11 are located on a series of giant linear plasmids in S. coelicolor. These observations may bear on the genetic variability and unstable genetic character of Streptomyces species.