Purification and physical analysis of Bacillus anthracis plasmids pXO1 and pXO2

Virulent strains of Bacillus anthracis contain two large plasmids. pXO1 encodes the three component protein exotoxin and pXO2 is necessary for synthesis of the poly-D-glutamic acid capsule. A procedure for the isolation of these plasmids which yields high quantities of pure DNA is described. Restriction endonuclease analysis of these plasmids shows that they are not related. pXO1 is 174 kilobase pairs and pXO2 is 95 kilobase pairs. From their bouyant densities and melting temperatures we also determined their GC contents. pXO1 contains 31.1% GC base pairs and pXO2 is 31.4% GC. Both of these values are close to the GC content of B. anthracis genomic DNA which is 32.2%.