Extraction, cloning and physical maps of plasmid DNAs from Streptomyces noursei

Three plasmids, pSCY 2, pSCY 3 and pSCY 4, were detected in Streptomyces noursei B3, a producer of cycloheximide and nystatin. pSCY 3 and pSCY 4 had molecular sizes of 15.1 megadaltons (Md) and 3.2 Md, respectively. When covalently closed circular (ccc) DNAs were extracted from the mycelia during the course of cultivation, the yield of ccc DNA extracted per mycelium reached the highest value at the starting point of the exponential growth phase and decreased rapidly during exponential phase; the yield increased gradually in stationary phase. The smallest plasmid, pSCY 4, could not be detected after 42 hours of cultivation. To purify and characterize DNA of the major plasmid pSCY 3, it was cloned into Escherichia coli using pACYC 184 as a vector, and the physical maps of the composite plasmids were constructed.