Escherichia coli DNA Topoisomerase I Mutants Have Compensatory Mutations at or near DNA Gyrase Genes

Covalently closed DNA isolated from any natural source is negatively supercoiled. This topological state of the DNA apparently plays a crucial role during DNA replication, transcription, and recombination. In Escherichia coli the DNA is negatively supercoiled mainly due to the action of DNA gyrase (Gellert et al. 1976; Drlica and Snyder 1978). This protein is one of a group of enzymes called DNA topoisomerases, which catalyze the concerted breakage and rejoining of DNA phosphodiester bonds (Wang and Liu 1979). Gyrase is unique among the topoisomerases in its ability to put negative supercoils into DNA. Extensive studies with inhibitors and with conditional lethal mutants have shown that gyrase is essential for E. coli DNA replication and viability (for review, see Cozzarelli 1980; Gellert 1981). Presumably, negative supercoiling facilitates unwinding of the DNA duplex during replication, although a more direct role of gyrase in replication is also possible.