7 DNA Topoisomerases

Abstract

Recent developments in the analysis of DNA structures clearly demonstrate that there is a rich repertoire of various structural motifs that DNA can assume and that the biological functions of DNA require a dynamic aspect in the transition among these structures. DNA topoisomerases play a critical role in mediating such structural transitions of DNA and thus are important in regulating many aspects of its biological functions. Numerous reviews (Liu 1983; Maxwell and Gellert 1986; Osheroff 1989; Cozzarelli and Wang 1990; Reece and Maxwell 1991; Hsieh 1992) and a chapter on DNA topoisomerases in the predecessor of this current volume (Wang 1982) give comprehensive accounts of the exciting developments in this field. This chapter summarizes the background of topoisomerases and highlights recent literature.

I. INTRODUCTION
DNA topoisomerase activity was first discovered with the ω protein (topoisomerase I) from Escherichia coli by virtue of its ability to relax covalently closed, supercoiled DNA (Wang 1971). The ω protein belongs to the type I DNA topoisomerase family of enzymes that can break and rejoin one DNA strand at a time. In 1976, E. coli DNA gyrase was identified (Gellert et al. 1976a); it defined the type II DNA topoisomerases, which act by passing a segment of DNA through a reversible double-strand break. Other reactions that can be catalyzed by topoisomerases include relaxation of supercoils, catenation and decatenation, and knotting/unknotting of DNA rings. Some DNA topoisomerases can also introduce superhelical turns into DNA by utilizing the energy from ATP hydrolysis: Eubacterial DNA gyrase can generate...