Figure 2.

Ca²⁺-stabilized Rad51-dsDNA-ATP complexes get locked in the stretched form, whereas Rad51-dsDNA-ADP filaments can freely pass between non-stretched and stretched forms of the filament. (A) Rad51 in a buffer containing ATP and Ca²⁺ is added to the DNA that is prevented from rotation and thus maintains its natural twist. (B and D; the lowest profile) After elimination of unbound Rad51, the preformed complex progressively increases its length on DNA unwinding towards a structure with ca 18.6 bp/turn. (C and D) Rewinding of stretched complexes does not convert them to a non-stretched form, although some shortening is observed. (E) Extension profiles of Rad51-dsDNA-ADP filaments formed in the presence of Mg²⁺ as the helicity of DNA is changed from 10.5 bp/turn (σ = 0) to 18.6 bp/turn (σ = −0.43).