The DNA dependence of the ATPase activity of DNA gyrase

We have studied the ATPase activity of DNA gyrase both in the absence and presence of DNA. In the absence of DNA we show that the gyrase B protein alone has a very low level of ATPase activity which can be increased many-fold by pretreatment of the B protein with heat or urea. When both the gyrase A protein and linear DNA are also present, the ATPase activity of the untreated B protein is greatly stimulated. We find that the extent of stimulation is dependent upon the length of the DNA but largely independent of DNA sequence. DNA molecules greater than 100 base pairs in length are much more effective in stimulating the gyrase ATPase than those of 70 base pairs or less, although short DNA molecules will stimulate the ATPase at high concentrations. The behavior of long and short DNA molecules with respect to ATPase stimulation is also reflected in their abilities to bind DNA gyrase. To account for these data we propose a model for the interaction of gyrase with ATP and DNA in which ATP hydrolysis requires the binding of DNA to two sites on the enzyme.