Fig. 7.

Profiles, as a function of operator spacing, of the distribution of topoisomers in (a) 182-bp and (b) 318-bp Lac repressor-partitioned minicircles generated from eight modes of DNA loop attachment to the V-shaped protein assembly. Operator spacing is expressed in terms of the center-to-center distances of one of the two DNA lobes formed upon protein binding and DNA topology in terms of the differences in linking number ∆Lk = Lk – Lk₀ of the topoisomers with respect to the relaxed state. Values of Lk₀ are the linking numbers of the constructs of lowest torsional stress, Lk = 17 in (a) and Lk = 30 in (b). The fraction f _∆Lk of minicircles with a given value of ∆Lk is proportional to the sum of the Boltzmann factors of those constructs. The predictions in (b) mirror the effects of supercoiling on the stability of loops of the same size formed in 452-bp Lac repressor-divided minicircles (Krämer et al. 1988) if offset by 2 bp (note the shifted histogram of observed ∆Lk values above the predicted data). The discrepancy in phase disappears upon a slight increase in the assumed DNA helical repeat (see text)