| Abstrakt: |
Like RecA, Saccharomyces cerevisiae Rad51p promotes strand exchange between circular single-stranded DNA (ssDNA) and linear double-stranded DNA (dsDNA). We have investigated several parameters characteristic of the interaction of Rad51p with ssDNA and dsDNA, particularly the effects of the nucleotide cofactors ATP and ADP and the analogs adenosine 5'-O-(thiotriphosphate) (ATPgammaS) and adenylyl-imidodiphosphate (AMP-PNP). Rad51p binding to both 1-N6-ethenoadenosine and 3-N4-ethenocytidine ssDNA (epsilonDNA) and dsDNA requires the presence of Mg2+ and ATP; no binding occurs in the presence of ADP, AMP-PNP, or ATPgammaS. Binding of Rad51p to dsDNA also requires ATP; ADP is ineffective, whereas ATPgammaS and AMP-PNP are considerably less able to promote binding and only at elevated concentrations of Rad51p. ATP binding, not ATP hydrolysis, is required for Rad51p binding to DNA. The Kd values for ATP for promoting binding of Rad51p to ssDNA and dsDNA are 1 and 3 microM, respectively. Rad51p binding occurs with a stoichiometry of one monomer of Rad51p per approximately 6.3 nucleotides of epsilonDNA and approximately 3.3 base pairs of dsDNA. Once formed, Rad51p. ssDNA complexes are stable so long as sufficient ATP levels are maintained. ATP hydrolysis causes dissociation of Rad51p from DNA. Moreover, the preformed complex is stable in the presence of a 10-fold excess of ADP or AMP-PNP over ATP. ATPgammaS, however, in the same -fold excess over ATP causes dissociation of the Rad51p. ssDNA complex. |
