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 control ling the level of DNA supercoiling and growth rate in Escherichia coli. We modulated the expression of DNA gyrase around its wild-type level, and meas ured the effect on plasmid supercoiling and growth rate. As both the activi ty and the transcription rate of DNA gyrase are sensitive to DNA supercoili ng we distinguish two types of control (with control defined as the percent age change observed on a 1% modulation of a parameter). The first type of c ontrol, here named inherent control, quantifies the effect of a sustained m odulation of the transcription rate of gyrase. At its wild-type expression level this inherent control exerted by DNA gyrase on growth rate was very l ow, i.e. c(gyrase)(mu) = 0.05 - 0.00, as was the inherent control on DNA su percoiling, c(gyrase)(aLk) = 0.2. The second type of control: here named gl obal control, quantifies the effect of a change in gyrase activity whilst a llowing the cell to respond by readjusting gyrase transcription. Both types of control are linked via the sensitivity of gyrase transcription to DNA s upercoiling, as determined from the inherent control by gyrase of the gyras e promoter activity using a chromosomal gyrB::lacZ fusion. As expected, the latter control was negative, but small, i.e. c(gyrase)(gyr promoter) = -0. 3. The global control by gyrase of active linking number was 0.1. These res ults show that although gyrase is an essential enzyme it does not have a hi gh control, on either growth rate or DNA supercoiling. Homeostatic regulati on of physiological DNA structure appears to dominate. At low degrees of DN A supercoiling, the control by DNA gyrase and by the other topoisomerases i s much stranger.