A study of the topological change of DNA in Escherichia coli induced by heat shock.

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Heat treatment of wild-type Escherichia coli cells leads to relaxation of negatively supercolied plasmid DNA, followed by a re-supercoiling reaction to the original level of the linking number of DNA.There was no evidence of the recovery of the DNA torsional strain in DNA of gyrA mutant cells. After heat treatment, DnaK and GroEL proteins were synthesized continuously in gyrA mutant cells but only transiently in wild-type cells. Thus, change in superhelical density of the DNA correlates with temperature -induced expression of heat shock proteins.Inhibitors of DNA gyrase (nalidixic acid, novobiocin), an organic solvent (ethanol) and a psychotropic drug (chlorpromazine) all stimulated relaxation of cellular DNA, over the same concentration range that induced DnaK and GroEL proteins, As DNA relaxation was induced by heat treatment or chemicals in an rpoH, mutant, the process is not the result of any induced synthesis of heat shock proteins.In rpoH and dnaK mutants, a re-supercoiling reaction did not occur, hence DnaK protein and other heat shock proteins presumably participate in the reaction. We propose that DNA relaxation induced by heat treatment stimulates synthesis of heat shock proteins involved in the re-supercoiling of DNA, which in turn shuts off the induction of these proteins.