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

SUMMARY: 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 Bg/II 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 Bg/II 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.