A SMALL (2.4 MB) BACILLUS-CEREUS CHROMOSOME CORRESPONDS TO A CONSERVED REGION OF A LARGER (5.3 MB) BACILLUS-CEREUS CHROMOSOME

We have determined the sizes of the chromosomes of six Bacillus cereus strains (range 2.4-4.3 Mb) and constructed a physical map of the smal lest B. cereus chromosome (2.4 Mb). This map was compared to those of the chromosomes of four B. cereus strains and one B. thuringiensis str ain previously determined to be 5.4-6.3 Mb. Of more than 50 probes, 30 were localized to the same half of the larger B. cereus and B. thurin giensis chromosomes. All 30 were also present on the small chromosome. Twenty of the probes present an the other half of the larger chromoso mes were either present on extrachromosomal DNA, or absent from the B. cereus strain carrying the small chromosome. We propose that the geno me of B. cereus/B. thoringiensis has one constant part and another les s stable part which is more easily mobilized into other genetic elemen ts. This part of the genome is localized to one region of the chromoso me and may be subject to deletions or more frequent relocations betwee n the chromosome and episomal elements of varying sizes up to the orde r of megabases.