BACILLUS-ANTHRACIS PXO1 VIRULENCE PLASMID ENCODES A TYPE-1 DNA TOPOISOMERASE

The virulence plasmid pXO1 is responsible for toxin production in Baci llus anthracis. A DNA fragment from pXO1 was isolated and was shown, b y sequence analysis, to contain part of a type 1 DNA topoisomerase gen e. Attempts to clone the entire wild type gene, designated topX, in Es cherichia coli, were unsuccessful. In order to obtain the complete gen e, it was first insertionally inactivated and then cloned in the mutat ed form. The deduced amino acid sequence of Topo X1 shows similarities to that of the two E. coil type 1 DNA topoisomerases. The N-terminal two-thirds of the putative B. anthracis protein exhibits strongest seq uence similarity to topoisomerase III, whereas the C-terminal portion contains cysteine residues that could form three zinc-binding domains, as they do in topoisomerase I. The suggested active-site tyrosine Is conserved in all three proteins. The regulation of expression from the topX promoter is modified by addition of a gyrase inhibiting antibiot ic. The Topo X1 protein is likely to be involved in the stability of p XO1.