Characterization of a natural cointegrate of the pock-forming plasmids pSK1 and pSK2 of Streptomyces kasugaensis MB273

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 carrying 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.