CROSS-TALK BETWEEN TOPOISOMERASE-I AND HU IN ESCHERICHIA-COLI

In Escherichia coli about one half of the negative supercoiling of DNA is constrained by proteins, in contrast to the situation in eukaryoti c cells where most of the DNA is constrained by histones. The level of supercoiling in the unrestrained portion is controlled by a balance b etween the supercoiling activity of gyrase and the relaxing activity o f DNA topoisomerase I. In the present work we show, by disrupting one or both genes encoding the heterodimeric protein HU, that an interplay exists in bacteria between HU and topoisomerase I activity: a decreas e in the intracellular concentration of HU was accompanied by an incre ase in relaxing activity as measured in cell extracts. Conversely, a t opA10 mutant of topoisomerase I, which has low levels of relaxing acti vity, was unable to accept an HU deficiency introduced by transduction . Thus it appears that the ability to increase relaxing activity, or t o decrease an excess of supercoiling, is important for cells to surviv e in the absence of HU. These data can be explained in terms of HU con straining supercoiling in vivo as it does in vitro: the absence of HU would generate more unconstrained supercoiling, which in turn would re quire an increase in relaxing activity to maintain physiological level s. (C) 1996 Academic Press Limited