Formation of positive supercoiled DNA by a nuclear factor from myeloma cells.

The formation of positive supercoiled DNA by an activity from a hypermutating myeloma line is reported. This activity forms positive supercoils from negative supercoiled DNA, it does not use positive supercoils to form negative ones and does not require an exogenous source of energy. The linking number changes by steps of 1, suggesting a type-I mechanism of action, and there seems to be an upper limit to the degree of positive supercoiling that can be achieved. Positive supercoiled DNA has to be taken into account as a possible structure of DNA in vivo for those functions where torsional stress is involved.