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The Fis protein influences DNA supercoiling at a global level. The dimeric Fis protein represses the promoter of its own gene, fis, and those of the genes coding for the GyrA (gyrA) and GyrB (gyrB) subunits of the heterotetrameric DNA gyrase. Fis has a more complicated relationship with the gene coding for the monomeric DNA topoisomerase I (topA); here Fis acts positively when present at low concentrations but is a repressor at high concentrations. This dual input is indicated by the +/− symbol at the topA promoter. (The angled arrows upstream of each gene represent transcription start sites.) The Fis protein also binds to the chromosomal DNA where it can influence its availability as a substrate for DNA gyrase and DNA topoisomerase I. Thus Fis can influence the homeostatic control of global DNA supercoiling by modulating the expression of the genes coding for the main topoisomerases and by modulating their activities on the DNA substrate.