Molecular relationships among plasmids of Bacillus thuringiensis : conserved sequences through 11 crystalliferous strains

Screening for the plasmid content of 11 strains belonging to nine different serotypes of B. thuringiensis was carried out by electron microscopic examination and electrophoresis in agarose gels. All the strains contained at lest two covalently closed, circular (CCC) DNA species. In one strain (berliner 1715), 17 extrachromosomal elements could be distinguished with regard to their size, ranging from 3.9 to 180 Mdal. Southern hybridisation experiments showed that most of these plasmids fell into two categories (inferior to 15 Mdal and superior to 15 Mdal) which have no homology between them. Within these two size groups there is partial conservation of DNa sequences through various serotypes. Further relationships among the plasmids were investigated by a two dimensional version of the Southern's blotting technique. Possible homology between plasmids and the chromosomal DNA was studied. It was shown that the smaller plasmids from the berliner 1715 and kurstaki HD1 strains contained no sequence related to chromosomal DNA, whereas among the larger plasmids a few showed homologous sequences