Type II DNA topoisomerases (topoII) control DNA topology during replication, transcription and chromosome segregation. Essential for cell proliferation, topoII are targets of antibacterial and antineoplasic compounds of prime importance for human health. The emergence of antibiotic resistant bacterial strains and the selection of resistance clones during cancer treatment urge for the development of new drugs and for a better understanding of their mode of action. This proposal aims at the functional and structural study of eukaryotic and prokaryotic DNA topoisomerases to provide knowledge on their mechanisms and on drug interference. Structural research in the field is to date limited to individual domains which do not recapitulate the enzymatic functions. Solving the atomic structure of a full length topoII is key to understand their mechanism and the molecular basis for inhibition by drugs binding at domain interfaces. Our primary goal is to conduct structural studies on a full length bacterial topoII, DNA gyrase, in complex with molecules of therapeutic interest such as the leading quinolone antibiotic family. The success of our project is bound to our ability to stabilize a defined conformation of the enzyme. The human topoII shares high sequence conservation and a similar structural organization with the bacterial enzyme. It is a biomarker for cancer and a key chemotherapy target for anti-tumour drugs such as the etoposides. Results obtained on DNA gyrase will provide valuable information to study the structure of human topoII, its drug binding mechanisms, and the effect of mutations found in resistant tumors. Finally, we plan to investigate the role of topoII in a more integrated context and in particular its interactions with the transcriptional apparatus. A combination of X-ray crystallography, electron microscopy, and other biophysical methods will be used for structure determination in conjunction with functional assays and clinical studies.