A temperature sensitive mutant affecting the B-subunit of DNA gyrase was isolated. The mutation leads to the thermolability of DNA gyrase in vitro and to the plasmid DNA relaxation in vitro. The immediate stop of the increase in mutant culture titre has been observed under nonpermissive conditions. However, DNA synthesis does not cease, though its efficiency is reduced by a factor of three. The transcription rate is also reduced two - three times, but more quickly than the rate of replication. This would mean that the transcription apparatus is the first to react to the change of DNA supercoiling in the cell. This suggestion is supported by the facts that small amounts of rifampicin increase the viability of ts mutant cells under nonpermissive conditions and also by the previously obtained results concerning the change in sensitivity of RNA polymerase mutants to DNA gyrase inhibitors.
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