Pock forming plasmids from antibiotic-producing Streptomyces.

Pock forming ability of fifteen plasmids isolated from antibiotic-producing Streptomyces was examined with polyethylene glycol mediated protoplast transformation. Four plasmids (pSF765, pSF689, pSF674 and pSF601) were found to have pock forming ability on Streptomyces lividans 66. Each putative transformants isolated from the center of pock regions harbored plasmids with same restriction enzyme cleavage sites as original plasmids. The plasmids from the transformants produced pock, which were morphologically identical to the original plasmids. Transformation frequency of S. lividans 66 with pSF689 from S. lividans 66 was at least 500-fold higher (5 x 10(5) transformants per 1 microgram DNA) than that with the plasmid isolated from the original strain (10(3) transformants per 1 microgram DNA). In case of pSF765 the transformation frequency was not changed (about 10(5) transformants per 1 microgram DNA) by replications in S. lividans 66. Expanded restriction enzyme maps for the four plasmids are reported.