Physical characterization of plasmids isolated from Streptosporangium

Plasmids were isolated from two species of Streptosporangium by CsCl-ethidium bromide equilibrium density gradient centrifugation. A plasmid isolated from S. brasiliense, designated pSgB-1, was characterized by electron microscopy and agarose gel electrophoresis. The pSgB-1 plasmid is a closed circular DNA molecule of 9.4 X 10(6) Da. A restriction endonuclease map was generated and unique cleavage sites were found for EcoRI, ClaI, XbaI, and MstII. Another plasmid, pSgV-1, isolated from S. viridognriseum, has an estimated Mr of 54 X 10(6). The pSgB-1 plasmid is phenotypically cryptic but an unusual phenotypic trait, resembling phage plaques, may be associated with the S. viridogriseum plasmid pSgV-1.