Chromosomal supercoiling in <i>Escherichia coli</i>

SummaryThe Escherichia coli chromosome is compacted into 40‐50 negatively supercoiled domains. It has been proposed that these domains differ in superhelical density. Here, we present evidence that this is probably not the case. A modified Tn10 transposable element was inserted at a number of locations around the E. coli chromosome. This element, mTn10‐plac‐lacZ+, contains the lac operon promoter, plac, whose activity increases with increasing superhelical density, fused to a lacZ+ reporter gene. Although mTn10‐plac‐lacZ+ fusion expression varies as much as approximately threefold at different insertion sites, the relative levels of expression from these elements are unaffected by replacing plac with the gyrA promoter, pgyrA, which has a reciprocal response to changes in superhelical density. Importantly, topoisomerase mutations and coumermycin, which inhibits DNA gyrase activity, alter mTn10‐plac‐lacZ+ and mTn10‐pgyrA‐lacZ+ fusion expression in expected ways, showing that the elements remain responsive to supercoiling and that topoisomerase activity is required for maintaining superhelical density. Fusion expression is not affected by anaerobic growth or osmotic shock, two physiol