ALTERED TOPOISOMERASE ACTIVITIES MAY BE INVOLVED IN THE REGULATION OF DNA SUPERCOILING IN AEROBIC-ANAEROBIC TRANSITIONS IN ESCHERICHIA-COLI

This study uncovers a new mechanism of regulation of DNA supercoiling operative in vivo upon an aerobic-anaerobic transition in Escherichia coli. Exponentially growing aerobic batch cultures were subjected to a shift to anaerobic conditions. The ratio [ATP]/[ADP] remained essentially constant at 8.5 in the aerobic culture and after a transition to anaerobiosis while DNA supercoiling increased noticeably upon anaerobiosis. This result indicated that the mechanism of regulation of DNA supercoiling by the [ATP]/[ADP] ratio was not operative. The increase in DNA supercoiling was followed by a large decrease in the DNA-relaxing activity of topoisomerase I while gyrase activity remained relatively constant. This decrease in the activity of topoisomerase I is likely to be responsible for the increase in DNA supercoiling.