Figure 4. The S-DNA produced in the B-to-S transition does not promote RecA nucleoprotein filament formation.

(A) Time traces of the extension of an 876 bp DNA tether with two GC-rich ends recorded during B-to-S transition in force-increase scan and the reverse S-to-B transition in the subsequent force-decrease scan in 50 mM KCl, 10 mM MgCl₂, pH 7.4, at 24°C. The transitions are completely reversible, indicated by the same extensions recorded at the same forces during the force-increase and force-decrease scans. Inset on the top shows a sketch of the 876 bp DNA; (B–C) Results identical to those in (A) were obtained when the same experiments were repeated on the same DNA tether in 1 µM RecA and 1 mM ATP, 1x ATP regeneration system (B) or in 1 µM RecA and 1 mM ATPγS (C), indicating no RecA filaments formed on the S-DNA produced by the B-to-S transition within the experimental time scale.