Sequence and organization of pXO1, the large Bacillus anthracis plasmid harboring the anthrax toxin genes

The Bacillus anthracis Sterne plasmid pXO1 was sequenced by random, "shotgu n" cloning. A circular sequence of 181,654 bp was generated. One hundred fo rty-three open reading frames (ORFs) were predicted using GeneMark and Gene Mark.hmm, comprising only 61% (110,817 bp) of the pXO1 DNA sequence. The ov erall guanine-plus-cytosine content of the plasmid is 32.5%. The most recog nizable feature of the plasmid is a "pathogenicity island," defined by a 44 .8-kb region that is bordered by inverted IS1627 elements at each end. This region contains the three toxin genes (cya, lef, and pagA), regulatory ele ments controlling the toxin genes, three germination response genes, and 19 additional ORFs. Nearly 70% of the ORFs on pXO1 do not have significant si milarity to sequences available in open databases. Absent from the pXO1 seq uence are homologs to genes that are typically required to drive theta repl ication and to maintain stability of large plasmids in Bacillus spp. Among the ORFs with a high degree of similarity to known sequences are a collecti on of putative transposases, resolvases, and integrases, suggesting an evol ution involving lateral movement of DNA among species. Among the remaining ORFs, there are three sequences that may encode enzymes responsible for the synthesis of a polysaccharide capsule usually associated with serotype-spe cific virulent streptococci.