DNA intercalating drugs inhibit positive supercoiling induced by novobiocin in halophilic archaea.

The two DNA intercalators, actinomycin D and 2-methyl-9-hydroxy-ellipticine, and the DNA minor groove ligant DAPI inhibited the growth of the haloarchaeon Halobacterium sp. GRB and bind to its plasmid pGRB-1. In contrast to specific DNA topoisomerase II inhibitors, they produced neither double-stranded breaks nor relaxation of plasmidic DNA. The two DNA intercalators inhibited positive supercoiling induced by novobiocin, suggesting that positive supercoiling in haloarchaea is due to transcription, as in the domain Bacteria. Plasmids from haloarchaea could thus be used to prescreen for DNA intercalators and to discriminate between different drug families via their mode of action.