  One of the major functions of RecA protein in the cell is to bind
single-stranded DNA exposed upon damage, thereby triggering the SOS repair
response.We present fluorescence anisotropy measurements at the binding onset,
showing enhanced DNA length discrimination induced by adenosine triphosphate
consumption. Our model explains the observed DNA length sensing as an outcome
of out-of equilibrium binding fluctuations, reminiscent of microtubule dynamic
instability. The cascade architecture of the binding fluctuations is a
generalization of the kinetic proofreading mechanism. Enhancement of precision
by an irreversible multistage pathway is a possible design principle in the
noisy biological environment.
