SUMMARY: Strains carrying only one species of pock-forming plasmid, designated as pSK3, were isolated from two different derivative strains of MB273 which contained three species of plasmids, pSK1, pSK2 and pSK3. Single and double digestion of pSK3with seven restriction endonucleases yielded fragments identical with those of pSK3 and assignable to those obtained from pSK1and pSK2. In particular, digestion with alone or in combination with other restriction endonucleases afforded the same size fragments as those of pSK1and pSK2. Strains containing pSK3induced pocks on lawns of strains carrying pSK1or pSK2and resisted pock formation by the latter strains. Therefore, it was concluded that pSK3was a pSK3 derivative with elevated pock-forming ability and represented a composite plasmid consisting of two elements, pSK1and pSK2, without any loss of their plasmid functions. Deletion derivative plasmids constructed from the /II fragments of pSK3provided evidence supporting the above conclusion. Pock formation by a pSK3-containing strain against strains carrying pSK1, or pSK2or no plasmid accompanied the transfer of pSK3from the former to the latter. Segregation of pSK1and pSK2from pSK3was observed in mycelium from pocks caused by pSK3-containing strains and on subculture of pSK3-containing strains.


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