EFFECT OF THE DELETION OF THE SIGMA(32)-DEPENDENT PROMOTER (P1) OF THE ESCHERICHIA-COLI TOPOISOMERASE-I GENE ON THERMOTOLERANCE

Topoisomerase I and DNA gyrase are the major topoisomerase activities responsible for the regulation of DNA supercoiling in the bacterium Es cherichia coli. The pi promoter of topA has previously been shown to b e a sigma(32)-dependent heat-shock promoter. A mutant strain with a de letion of P1 was constructed. This mutant is >10-fold more sensitive t o heat treatment (52 degrees C) than the wild type. After brief treatm ent at 42 degrees C, wild-type Escherichia coli acquires an enhanced r esistance to the effects of a subsequent 52 degrees C treatment. This is not the case for the P1 deletion mutant, which, and under these con ditions, is about 100-fold less thermotolerant than the wild type. The presence of a plasmid expressing topoisomerase I restored the heat-su rvival level of the mutant to that of the wiles type. During heat shoc k, the superhelical density of a plasmid with the heat-inducible rpoD promoter is increased in the P1 deletion mutant. We also note that the pulse-labelling pattern of proteins at 42 degrees C (displayed on SDS -polyacrylamide gels) is different in the mutant, and, most notably, t he amounts of DnaK and of GroEL protein are reduced. A model is propos ed in order to unify these observations.