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DNA topoisomerases are enzymes that control and modify the topological states of DNA in cells. Bacterial DNA gyrase is a type II DNA topoisomerase which catalyses the negative supercoiling of prokaryotic DNA , utilising the free energy released by the hydrolysis of ATP. DNA gyrase is the target of two classses of antibiotic drugs: the quinolones and the coumarins. DNA gyrase consists of two proteins (A and B), with the active species being a heterotetramer (A2B2). Topoisomerase IV is another type II enzyme found in bacteria. Unlike DNA gyrase, it is unable to catalyse the supercoiling of DNA, merely its relaxation. It appears the role of this enzyme, in vitro, is to decatenate the daughter chromosomes in the final stages of DNA replication. The enzyme shows a great deal of sequence homology with DNA gyrase. The enzyme comprises two subunits, ParC and ParE. The ParC protein is homologous to the gyrase A protein, while the ParE subunit is homologous to the gyrase B protein.

We have determined the structures of intact gyrase B protein and the ParE subunit of topoisomerase IV. The structure of the N-terminal region of either protein is similar to the equivalent region of E.coli protein. However, the 25 kDa C-terminal domain of the protein is disordered in both the structures. Efforts are underway to crystallise the intact complex.

The gyrase B protein is the target of the coumarin antibiotics. We have solved the structures ofthe complexes of the 24 kDa N-terminal domain of the B protein with novobiocin (Fig. 1) and chlorobiocin at 2.7 Å and 1.9 Å respectively. The gyrase B protein is also inhibited by a group of naturally occuring cyclic peptides called cyclothialidines. We have solved the structure of the 24 kDa B protein fragment complexed with a cyclothialidine (Fig. 2) at 2 Å resolution.