A temperature-sensitive mutant of E. coli with a defective DNA gyrase B subunit has been obtained. The mutation is expressed in the thermolability of DNA gyrase in vitro and in DNA relaxation in vivo. DNA replication in the mutant does not stop under non-permissive conditions; its rate gradually falls by a factor of 2 to 3. The transcription rate also drops by a factor of 2 to 3, but before replication. Small concentrations of rifampicin, an inhibitor of bacterial RNA polymerase, make for a partial survival of the mutant cells under non-permissive conditions. The results suggest the conclusion that DNA supercoiling is mainly required to ensure the optimum transcription level in the cell.

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To investigate the interaction of subunits A and B of DNA gyrase during DNA supercoiling, a Cour mutant of Escherichia coli was obtained and the effect of nalidixic acid on the supercoiling of DNA by wild-type and mutant enzymes was assayed. The enzyme of the Cour strain proved to be more sensitive to nalidixic acid than the wild-type DNA gyrase. Hence the mutation affecting the B subunit can also change the properties of the A subunit, which fact suggests that the two subunits of DNA gyrase are in contact during DNA supercoiling.

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