A large (80kbp) plasmid designated as pSLL was isolated from Streptomyces laurentii ATCC 31255 (strain P0). Another covalently closed circular plasmid, pSLS (16kbp), was found in pSLL-cured P0 cells (named P1), which spontaneously formed pocks on a solid medium. The frequency of the generation of pSLL-loss P1 cells coordinated positively with the frequency of pock appearance. pSLS was also found as an integrated form in the chromosome of strains P0 and P1. The results indicated that strain P0 harbored two plasmids, pSLL and integrated form pSLS, and pSLS seemed to be generated as a free-form plasmid with the loss of pSLL. It was also suggested that pSLL may suppress the excision of the integrated pSLS from the chromosome and that the free form of pSLS may participate in pock formation.
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