Bacillus anthracis produces two toxins composed of three proteins. Genetic tools were constructed to study the regulation of toxin synthesis. They included transcriptional fusions with various reporter genes, in replicative and integrative vectors. The reporter gene xyIE, encoding catechol 2,3-dioxygenase, may be valuable for screening of strong promoters, as expression of the gene can be visualized directly and the enzyme is not toxic for the cells. However, xyIE did not appear suitable for quantitative studies of regulation in B. anthracis. Therefore, transcriptional fusions between a lacZ reporter gene and the toxin genes were constructed. Experiments with a multicopy plasmid in trans suggested that the transcriptional activator(s) of the toxin genes were not titrated. B, anthracis strains, which contain pXO1 carrying multiple copies of fusions, were analysed. Expression of the reporter gene was proportional to the fusion copy number. indeed, single integration of a suicide plasmid can be distinguished from multiple integration according to the level of resistance to an appropriate antibiotic. Finally, recombination in B. anthracis was found to be very efficient (approximately 10(-2) recombinants per transconjugant cell).
