COINDUCTION OF DNA RELAXATION AND SYNTHESIS OF DNAK AND GROEL PROTEINS IN ESCHERICHIA-COLI BY EXPRESSION OF LETD (CCDB) PROTEIN, AN INHIBITOR OF DNA GYRASE ENCODED BY THE F-FACTOR

We examined the influence of overexpression of LetD (CcdB) protein, an inhibitor of DNA gyrase encoded by the F factor of Escherichia coli, on DNA supercoiling and induction of heat shock proteins. Cells were t ransformed with a plasmid carrying the structural gene for LetD protei n under control of the tac promoter, and LetD protein was induced by a dding isopropyl beta-D-thiogalactopyranoside (IPTG) to the culture med ium. Analysis by agarose gel electrophoresis in the presence of chloro quine revealed relaxation of plasmid DNA in cells depending on the con centration of IPTG employed for induction. Protein pulse-labeling expe riments with [S-35]methionine and cysteine revealed that synthesis of DnaK and GroEL proteins was also induced by IPTG, and concentrations n ecessary for DNA relaxation and induction of the heat shock proteins w ere much the same. Expression of mutant LetD protein lacking two amino acid residues at the C-terminus induced neither DNA relaxation nor th e synthesis of DnaK and GroEL proteins. Induction of wild-type LetD pr otein but not mutant LetD protein markedly enhanced synthesis of sigma (32). We interpret these results to mean that DNA relaxation in cells caused by the expression of LetD protein induces heat shock proteins v ia increased synthesis of sigma(32).