Plasmids of Streptomyces kasugaensis MB273: their pock formation, their dispensable endonuclease cleavage sites for pock formation, and transformation of S. kasugaensis MB273 by them.

Plasmid-free strains of Streptomyces kasugaensis MB273 were isolated. In mating experiments S. kasugaensis MB273 was found to cause a lethal zygosis (pock) phenotype in a plasmid-free host. The pock-forming plasmids were identified as either pSK1* or pSK2* on the basis of their endonuclease cleavage-sites. The strain carrying pSK1* was found to induce pocks on the strain bearing pSK2*, and vice versa. The endonuclease cleavage-sites in pSK1* and pSK2* that were nonessential for pock formation were determined in deletion or insertion derivatives. The single sites for Bcl I and Sal I in pSK1* and for Bgl II in pSK2*, respectively, could be useful for DNA cloning without destroying pock-forming ability. Protoplasts of S. kasugaensis MB273-derivatives prepared in stationary phase of mycelial growth were competent for transformation, however, regeneration frequencies decreased during this phase.