Supercoiling-dependent sequence specificity of mammalian DNA methyltransferase.

Negative supercoiling of substrate DNA dramatically alters the in vitro sequence specificity of mammalian DNA methyltransferase (DNA MeTase). This result suggests that in vivo site selection by DNA MeTase could be regulated by conformational information in the form of alternative secondary structures induced in DNA by local supercoiling or by the binding of specific nuclear proteins. DNA in the left-handed Z-form is shown not to be a substrate for mammalian DNA MeTase. The sensitivity of DNA MeTase to DNA structure may also make it useful as a probe for sequences which undergo supercoiling-dependent structural transitions in vitro.     

Differential influence of DNA supercoiling on in vivo strength of promoters varying in structure and organisation in E. coli

DNA supercoiling is known to influence promoter activity in vitro and in vivo in a promoter-dependent manner in prokaryotes. In order to investigate how topology may influence promoter function, we have studied two kinds of promoter variants, (i) where only the spacer region is altered, and (ii) where the same promoter is tandemly repeated in either the same or opposite orientation. These promoters respond very differently to alterations in DNA supercoiling, suggesting that the overall structure of the promoter and its context contribute to the differential response to alterations in supercoiling in vivo.