The mapping of DNA topoisomerase sites in vivo: a tool to enlight the functions of topoisomerases.

The possibility to record a trace of the precise sites of topoisomerase action has been exploited for almost 12 years in many laboratories. The large majority of the studies were performed in vitro, giving a good picture of sequence specificities of topoisomerases, and of the preference of various drugs for some sequences. Only a relatively small number of reports concern in vivo studies. Their main conclusions are the following: i) topoisomerase II sites are often found near replication origins and termini, where they are supposed to play a role in the decatenation of daughter DNA molecules, and possibly in the initiation of replication; ii) topoisomerase II sites are found in the promoter region of many genes, but they seem related to the condensation state of chromatin in this region, rather than to transcription per se; iii) some topoisomerase II sites, resistant to high salt, are found in or near matrix associated regions (MARs), suggesting a role in loop anchorage or (and) in the control of topology of individual chromatin loops; iv) topoisomerase I sites appear less localized, acting all along the transcription units, where they seem directly involved in transcription; and v) topoisomerase I sites are possibly connected with replication fork progression and (or) with the termination of replication. Despite these advances, the precise role of topoisomerases in vivo is still poorly understood, especially in recombination and chromatin condensation and decondensation during the cell cycle. Future attempts should take into account the possible specialization of the multiple topoisomerases found in a given cell, and the use of highly synchronized systems.