DNA Supercoiling and Gene Expression

Abstract

The supercoiling of intracellular DNA and its influence on gene expression is briefly reviewed. In prokaryotes such as Escherichia coli, the degree of supercoiling is maintained by the actions of at least two DNA topoisomerases: an ATP-dependent enzyme DNA gyrase which reduces the linking number (negative supercoiling) and an ATP-independent enzyme DNA topoisomerase I which relaxes negatively supercoiled DNA. The expression of different genes is affected differently by supercoiling of the DNA; as the DNA becomes more negatively supercoiled, the expression of some genes is enhanced, while that of others is depressed or relatively unaffected. In eukaryotes, the existence of a gyrase-like enzyme that actively supercoils intracellular DNA is uncertain. Indirect evidence suggests, however, that DNA supercoiling may also occur and may have strong effects on gene expression. Some possibilities are raised that DNA supercoiling does not necessarily require a gyrase-type activity; in particular, the possibility that the transcription process itself in eukaryotes may cause supercoiling is discussed.