Characterization of a Natural Cointegrate of the Pock-forming Plasmids pSK1*and pSK2*of Streptomyces kasugaensis MB273

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Abstract
Strains carrying only one species of pock-forming plasmid, designated as pSK3*, were isolated from two different derivative strains of Streptomyces kasugaensis MB273 which contained three species of plasmids, pSK1, pSK2 and pSK3. Single and double digestion of pSK3* with seven restriction endonucleases yielded fragments identical with those of pSK3 and assignable to those obtained from pSK1* and pSK2*. In particular, digestion with BglII alone or in combination with other restriction endonucleases afforded the same size fragments as those of pSK1* and pSK2*. Strains containing pSK3* induced pocks on lawns of strains carrting pSK1* or pSK2* and resisted pock formation by the latter strains. Therefore, it was concluded that pSK3* was a pSK3 derivative with elevated pock-forming ability and represented a composite plasmid consisting of two elements, pSK1* and pSK2*, without any loss of their plasmid functions. Deletion derivative plasmids constructed from the BglII fragments of pSK3* provided evidence supporting the above conclusion. Pock formation by a pSK3*-containing strain against strains carrying pSK1*, or pSK2* or no plasmid accompanied the transfer of pSK3* from the former to the latter. Segregation of pSK1* and pSK2* from pSK3* was observed in mycelium from pocks caused by pSK3*-containing strains and on subculture of pSK3*-containing strains.