CHARACTERIZATION OF A STREPTOMYCES-LIVIDANS-TYPE SITE-SPECIFIC DNA MODIFICATION SYSTEM IN THE AVERMECTIN-PRODUCER STREPTOMYCES-AVERMITILIS PERMITS INVESTIGATION OF 2 NOVEL GIANT LINEAR PLASMIDS, PSA1 AND PSA2

The degradation of Streptomyces avermitilis DNA samples analysed by conventional pulsed-field gel electrophoresis was shown to be due to Tris-dependent, double-strand cleavage. Using alternative electrophoretic conditions, separation of intact DNA molecules was achieved, permitting the identification of two novel giant linear plasmids: the 100 kb pSA1 and 250 kb pSA2. Use of pSA2 DNA as a probe showed that pSA1 does not cross-hybridize, indicating that the plasmids are not closely related. The site-specificity of the DNA modifications, which render the DNA susceptible to Tris-dependent cleavage, was found to be essentially identical to that of similar modifications found in the DNA of S. lividans.