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

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Abstract
Plasmid-free strains of S. kasugaensis MB273 were isolated. In mating experiments S. kasugaensis MB273 causes a lethal zygosis (pock) phenotype in a plasmid-free host. The pock-forming plasmids were identified as pSK1* or pSK2* on the basis of their endonuclease cleavage-sites. The strain carrying pSK1* induces 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 of 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; regeneration frequencies decreased during this phase.