Reverse gyrase is essential for microbial growth at 95 °C

Reverse gyrase is an enzyme that induces positive supercoiling in closed circular DNA in vitro. It is unique to thermophilic organisms and found without exception in all microorganisms defined as hyperthermophiles, that is, those having optimal growth temperatures of 80 degrees C and above. Although its in vivo role has not been clearly defined, it has been implicated in stabilizing DNA at high temperatures. Whether or not it is absolutely required for growth at these high temperatures has yet to be fully determined. In a previous study with an organism that has an optimal growth temperature of 85 degrees C, it was shown that the enzyme is not a prerequisite for life at extreme temperatures as disruption of its gene did not result in a lethal phenotype at the supraoptimal growth temperature of 90 degrees C. Herein we show that the enzyme is absolutely required for microbial growth at 95 degrees C, which in this case is a suboptimal growth temperature. Deletion of the gene encoding the reverse gyrase of the model hyperthermophilic archaeon Pyrococcus furiosus, which has an optimal growth temperature of 100 degrees C, revealed that the gene is required for growth at 95 degrees C, as well as at 100 degrees C. The results suggest that a temperature threshold above 90 degrees C exists, wherein the activity of reverse gyrase is absolutely necessary to maintain a correct DNA twist for any organism growing at such temperature extremes.