[Isolation and preliminary characteristics of the extrachromosomal elements of Bacillus thuringiensis DNA]

Centrifugation of Bacillus thuringiensis 351 DNA in CsCl-ethidium bromide density gradient has revealed, besides the main band of chromosomal DNA (p = 1.56 g/cm2), some additional bands with higher density, which usually correspond cicle superhelix DNAs, are revealed. 6 discrete bands are observed under electrophoresis of total fraction of minor DNA bands, which suggests, that the preparation contains cicle DNAs of different size. The treatment with DNAse results in the appearance of 3 bands corresponding to opened cicle DNA forms, their molecular weight being 12-10(6)--4-10(6) daltons. Cicle super-helix and opened cicle DNA forms are found in minor fraction by means of electrone microscopy. Statistical analysis has revealed the presence of at least three types of cicle molecules of different size. The variant 351-10, free of extrachromosomal elements, is isolated after ethidium bromide treatment at high temperature. Possible mechanisms of cicle DNAs formation are discussed. Biological function of DNA extrachromosomes fragments in Bacillus thuringiensis is still obscure.