Transcription factor as a topological homeostat.

Abundant prokaryotic chromatin architectural proteins often function also as global transcriptional regulators. In addition, some of this class of proteins modulate the activity of cellular topoisomerases and hence, the superhelical density of DNA. The relationships between the global effect of these proteins on DNA topology and their local effects exerted on particular promoter regions remain largely unexplored. One of the best-characterised examples of this class of proteins is the pleiotropic regulator of metabolism FIS, which reduces the activity of DNA gyrase and counteracts the increase of the overall superhelicity of DNA during early exponential growth phase. Binding of FIS to supercoiled DNA molecules in vitro leads to the formation of branched structures and consequent multiplication of apical loops, whereas on bending the upstream regions of stable RNA promoters FIS acts as a topological homeostat maintaining high local levels of supercoiling required for promoter activity. We argue that the coordinated effects of FIS on the global and local DNA architecture optimise gene expression by channelling the free energy of negative supercoiling to specific, biologically relevant sites.