DMA Topoisomerases

Abstract

In the past few years, much progress has been made in the study of DNA topoisomerases. The quickened pace of the field is reflected by the number of recent reviews on this subject (Bauer 1978; Champoux 1978; Wang and Liu 1979; Cozzarelli 1980; Gellert 1981a,b; Wang 1981; Wang and Kirkegaard 1981). The purpose of this chapter is not to provide an additional comprehensive review of the literature but to give a brief account of the present status of the field and to comment on a few selected topics.

REACTIONS THAT ARE CATALYZED BY DNA TOPOISOMERASES
Some of the reactions that are catalyzed by DNA topoisomerases are illustrated in Figure 1 and are listed in Table 1. All these topoisomerizations involve the transient breakage of DNA phosphodiester bonds and the reformation of the very same linkages; reactions that involve bond exchange will be discussed in a later section.

The forward reaction of Figure 1A, the relaxation of a supercoiled DNA, has been observed for all DNA topoisomerases. The various subclasses of the enzymes differ, however, in their dependence on the sense of DNA superhelicity. The major bacterial type-I enzyme, the top A gene product, is effective only when the DNA is negatively supercoiled (Wang 1971). The major type-I enzyme from eukaryotes, on the other hand, relaxes negatively and positively supercoiled DNA with equal efficiency (Champoux and Dulbecco 1972). Similarly, the known ATP-dependent type-II enzymes coded by phage T4 and eukaryotes are all insensitive to the sense of DNA superhelicity (Liu et...