12] Reverse gyrases from bacteria and archaea

Abstract

Among the enzymes that are responsible for shaping DNA, reverse gyrases are particularly interesting because of their structure and the unique reaction they catalyze: the formation of positive supercoils in circular DNA. Reverse gyrases are classified as type I topoisomerases because they change the DNA linking number by increments of one. Remarkably, this peculiar enzyme appears widely distributed in both the archaeal and bacterial kingdoms, but seems restricted to hyperthermophilic strains. Its presence in eukaryotes has been postulated, although without strong evidence so far. Furthermore, reverse gyrase activity is detectable in extracts of all hyperthermophilic species so far tested, provided that it is not masked by another prominent topoisomerase. This distribution makes the enzyme a good marker of thermophily. The biological function of reverse gyrases is still a matter of conjecture. However, since reverse gyrases seem essential in extremely thermophilic organisms, it was postulated that these enzymes participate in the stabilization of the DNA double helix. This chapter describes the purification of reverse gyrases from their natural hosts and from recombinant Escherichia coli strains, in addition to the various assays developed to test their biochemical properties. Finally, the main structural features of these enzymes and their putative mechanism are discussed, together with their possible in vivo functions.