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The recA protein catalyses the ATP-driven homologous pairing and strand exchange
of DNA molecules. It is an allosteric enzyme: the ATPase activity is
DNA-dependent, and ATP-bound recA protein has a high affinity for DNA, whereas
the ADP-bound form has a low affinity. In the absence of ATP hydrolysis, recA
protein can still promote homologous pairing, apparently through the formation
of a triple-stranded intermediate. The exact role of ATP hydrolysis is not
clear, but it presumably drives the triplex intermediate towards products. Here
we determine the position of bound ADP diffused into the recA crystal. We show
that only the phosphates are bound in the same way as in other NTPases
containing the G/AXXXXGKT/S motif. We propose that recA protein may change its
conformation upon ATP hydrolysis in a manner analogous to one such protein, the
p21 protein from the ras oncogene. A model is presented to account for the
allosteric stimulation of DNA binding by ATP. The mechanism by which nucleoside
triphosphate hydrolysis is coupled to the binding of another ligand in recA
protein and p21 may be typical of the large class of NTPases containing this
conserved motif.
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