I was reading DNA replication in E.coli in Lehninger Principles of Biochemistry 6th Ed. and stumbled upon the following --
The tight right handed wrapping of DNA around DnaA oligomeric complex induces an effective positive supercoil. The associated strain in the nearby DNA leads to denaturation of AT rich DUE region.
Yet again I have come across the following in in Indian Molecular Biology books which certainly contradict Lehninger.
Binding of DnaA protein to oriC initiates unwinding of the DUE by introduction of negative superhelical turns.
I've done a little more research and found this paper, which clarifies the problem I previously had:
As a right-handed ATP-DnaA spiral would be expected to wrap DNA into a positive supercoil, adjacent DNA regions would be subjected to compensatory negative writhe (Fig. 6d, left). Although any superhelical strain generated by DnaA-mediated wrapping would be expected to spread throughout the topological domain that con- tains the origin, it may preferentially affect the nearby DUE, which is unstable and prone to spontaneous unwinding (Fig. 6d, top arrow).
Changed question: Why is this negative supercoiling generated in a positively supercoiling ccDNA?