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   Rad51, the human recombinase, is an ATP-dependent catalyst of homologous pairing and strand exchange. The protein accumulates at sites of double strand breakage during homologous recombination and in response to DNA damage. It works in concert with various recombination mediator proteins, including RPA and Rad52, which are essential for its catalytic efficiency. Rad51 has been described as a tumor-associated antigen due to its high overexpression in cancer cells. It has direct interactions with both p53 and BRCA2, which are proposed to moderate its cellular availability and localization. Rad51 is functionally analogous to E.coli RecA, which is known for both catalysis of recombination and mediation of the SOS response to DNA damage. A biochemical comparison of the two proteins reveals that they differ greatly in their rates of ATP hydrolysis, extent of strand transfer, and orientation of strand exchange. These variations could arise from structural differences in individual domains, altered domain organization within subunits, or induced differences upon interaction with their cognate ssDNA binding proteins.