DNA Topoisomerase Mutations in Bacteria. In: Gualerzi CO, Pon CL, editors. Bacterial Chromatin. Berlin: Springer Berlin Heidelberg; 1986. pp. 52-63. [DOI: 10.1007/978-3-642-71266-1_6]

Extensive regulation compromises the extent to which DNA gyrase controls DNA supercoiling and growth rate of Escherichia coli

As DNA gyrase is the only enzyme to supercoil DNA actively, we address here the question of whether it does play the expected dominant role in controlling the level of DNA supercoiling and growth rate in Escherichia coli. We modulated the expression of DNA gyrase around its wild-type level, and measured the effect on plasmid supercoiling and growth rate. As both the activity and the transcription rate of DNA gyrase are sensitive to DNA supercoiling we distinguish two types of control (with control defined as the percentage change observed on a 1% modulation of a parameter). The first type of control, here named inherent control, quantifies the effect of a sustained modulation of the transcription rate of gyrase. At its wild-type expression level this inherent control exerted by DNA gyrase on growth rate was very low, i.e. c mu/gyrase = 0.05 - 0.00, as was the inherent control on DNA supercoiling, c aLK/gyrase = 0.2. The second type of control, here named global control, quantifies the effect of a change in gyrase activity whilst allowing the cell to respond by readjusting gyrase transcription. Both types of control are linked via the sensitivity of gyrase transcription to DNA supercoiling, as determined from the inherent control by gyrase of the gyrase promoter activity using a chromosomal gyrB:lacZ fusion. As expected, the latter control was negative, but small, i.e. c gyr promoter/gyrase=-0.3. The global control by gyrase of active linking number was 0.1. These results show that although gyrase is an essential enzyme it does not have a high control, on either growth rate or DNA supercoiling. Homeostatic regulation of physiological DNA structure appears to dominate. At low degrees of DNA supercoiling, the control by DNA gyrase and by the other topoisomerases is much stronger.

Minor-groove binders are inhibitors of the catalytic activity of DNA gyrases

Non-intercalating DNA minor-groove binders may effectively inhibit the supercoiling activity of gyrases by influencing the enzyme recognition and cleavage site on DNA. For gyrase from Streptomyces noursei a wide range of inhibitory potency for different classes of ligands is observed. This can be explained by a number of structural and binding factors of the ligands competing with the gyrase on the target site of DNA, the mechanism of which is different from the classical gyrase inhibitors.

The Escherichia coli minB mutation resembles gyrB in defective nucleoid segregation and decreased negative supercoiling of plasmids

Nucleoid segregation in the Escherichia coli minB mutant and in cells that over-produce minB gene products appeared defective as measured from fluorescence micrographs. Electrophoretic resolution of topoisomers of plasmid isolates from the minB strain revealed a decreased level of negative supercoiling; in addition, multimerization was observed. Over-production of the minB gene product also resulted in a decreased level of negative supercoiling. This phenotype is typical of the gyrB(ts) mutant, which is known to be affected in chromosome decatenation and supercoiling. We propose that the minB mutation and over-production of the minB gene products cause a defect in nucleoid segregation, which may be related to the decrease in negative supercoiling. As in the gyrB(ts) mutant, retardation of nucleoid segregation is proposed to inhibit constriction initiation in the cell centre and to give rise to nucleoid-free cell poles. As a consequence, these cells divide between nucleoid and cell pole, resulting in minicell and (sometimes) in anucleate cell formation.

Microbial DNA topoisomerases and their inhibition by antibiotics

Supercoiling of bacterial DNA is regulated by topoisomerases and influences most of the metabolic processes involving DNA. The present review is devoted to a brief outline of the supercoiled state of DNA in bacteria and to all microbial topoisomerases hitherto described. Recent studies on topoisomerases of archaebacteria led to the discovery of a so-called reverse gyrase, the properties of which are also discussed. Special emphasis is given to a selective treatment of the effects of those antibiotics which act as gyrase inhibitors.