New results have been obtained on enzymes that alter DNA supercoiling, and on the role of supercoiling in controlling transcription. It was previously shown that ATP binding site of DNA gyrase can be labeled by an ATP affinity analog. However, two residues in the Gyr B subunit were equally labeled, lysine-103 and lysine-110. Mutations of these sites now show that lysine-103 is essential, but lysine-110 is not. A new topoisomerase has been isolated from a thermophilic archebacterium, with properties much closer to eukaryotic topoisomerases than to other bacterial enzymes, because it is able to relax positively as well as negatively supercoiled DNA, and to work in the absence of divalent metal ions. Further studies of DNA relaxation - stimulated transcription in E.coli have shown that promoter sites with this property also lead to transcription that reads through termination signals. This readthrough is not found when using purified RNA polymerase in vitro, indicating the involvement of other factors.
