Characterization of a unique type IA topoisomerase in Bacillus cereus.

Bacillus cereus topoisomerase III beta (bcTopo III beta) has been cloned, overexpressed and biochemically characterized. This enzyme exhibits 64% and 33% sequence identity to Bacillus subtilis topoisomerase III (bsTopo III) and Escherichia coli topoisomerase III (ecTopo III) respectively. The enzymatic properties of bcTopo III beta differ substantially from other bacterial type IA topoisomerases, including E. coli type IA topoisomerases and B. cereus topoisomerase I (bcTopo I) and III alpha (bcTopo III alpha). bcTopo III beta only partially relaxes negatively supercoiled DNA and appears incapable of generating fully relaxed topoisomers. In contrast to ecTopo III and bcTopo III alpha, bcTopo III beta is not a decatenase. bcTopo III beta is unable to compensate the loss of ecTopo III in vivo. Therefore, bcTopo III beta is a unique prokaryotic type IA topoisomerase that is different from previously characterized topoisomerases.