RESUMEN
The assembly of RecA onto a torsionally constrained double-stranded DNA molecule was followed in real time using magnetic tweezers. Formation of a RecA-DNA filament on the DNA tether was stalled owing to different physical processes depending on the applied stretching force. For forces up to 3.6 pN, the reaction stalled owing to the formation of positive plectonemes in the remaining DNA molecule. Release of these plectonemes by rotation of the magnets led to full coverage of the DNA molecule by RecA. At stretching forces larger than 3.6 pN, the twist induced during filament formation caused the reaction to stall before positive supercoils were generated. We deduce a maximum built-up torsion of 10.1 +/- 0.7 k(b)T. In vivo this built-up torsion may be used to favor regression of a stalled replication fork or to free the chromosomal DNA in E.coli from its condensing proteins.