Complexes of DNA gyrase with defined DNA fragments from 127 to 256 base pairs in length have been studied by the method of electro-dichroism. The results suggest that DNA is wrapped around the enzyme in a single loop of 110 base pairs, with the entry and exit points close together. The rest of the DNA extends outward in two tails with an angle of about 120 degrees between them. In the presence of ATP or a nonhydrolyzable ATP analog, the tails are wrapped back and bound to the enzyme core. This process is considered to represent an intermediate stage in the supercoiling reaction of DNA gyrase. The complexes are also being studied by neutron scattering methods. Preliminary results are consistent with the model of a single turn of DNA bound to the enzyme.
