Limited regions of homology between linear and circular plasmids encoding methylenomycin biosynthesis in two independently isolated streptomycetes

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Abstract
PSV1 is a plasmid in Streptomyces violaceoruber SANK95570 that carries the methylenomycin biosynthetic (mmy) gene cluster. An ordered cosmid map and an EcoRI map have been constructed for pSV1, confirming that pSV1 is a 163 kb circular plasmid. The mmy gene cluster has been found on three different replicon structures; the circular plasmid pSV1, the 356 kb linear plasmid SCP1 and, via SCP1 integration, the linear chromosome of Streptomyces coelicolor A3(2). Comparison of pSV1 and SCP1 sequences revealed that the two plasmids have homology to each other only around the mmy and parAB regions, eliminating models in which pSV1 was generated by circularization of SCP1 or vice versa. It is likely that the mmy gene cluster was horizontally transferred as a set together with the parAB region in the comparatively recent evolutionary past.