title: Escherichia coli DNA topoisomerase I mutants: increased supercoiling is corrected by mutations near gyrase genes.
abstract: Bacterial chromosomes and plasmid (pBR322) DNA from topoisomerase I-defective Escherichia coli strains have been characterized with respect to superhelical density. The topoisomerase I defect results in increased negative superhelical density of both the bacterial chromosome and pBR322. Thus topoisomerase I is involved in determining the level of supercoiling in bacteria. Three of the topoisomerase I-defective strains were studied carry secondary mutations that decrease superhelical density; these additional mutations are closely linked to the gyrB locus in two of the strains and to the gyrA locus in the third strain.
mesh_terms: D002872:Chromosome Deletion; D002876:Chromosomes, Bacterial / Q000502:physiology; D004264:DNA Topoisomerases, Type I / Q000235:genetics*; D004250:DNA Topoisomerases, Type II / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics; D004278:DNA, Superhelical / Q000235:genetics*; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D005796:Genes; D005798:Genes, Bacterial; D008040:Genetic Linkage; D009154:Mutation; D010957:Plasmids
chemical_list: D004269:DNA, Bacterial; D004278:DNA, Superhelical; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II