Relationship of the physical and enzymic properties of Escherichia coli recA protein to its strand exchange activity

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Abstract
We have shown that performing the recA protein catalyzed strand exchange reaction in the presence of acetate anions, rather than chloride which is commonly used, greatly increases the rate of the reaction. The initial rate of the reaction in an acetate-based buffer is approximately 3-4 times higher in the presence of Escherichia coli single-stranded DNA binding protein (SSB protein) and 2 times higher in its absence than the initial rate in chloride. To determine the enzymatic basis for this stimulatory effect of acetate buffer, we investigated the relationship between a number of physical and enzymatic properties of recA protein and the strand exchange reaction. We have found that although the acetate anion has some effect on the aggregation properties and the single-stranded DNA-dependent ATPase activity of recA protein, these effects cannot explain the enhanced strand exchange activity in an acetate-based buffer. We do find, however, that two aspects of recA protein activity closely parallel the ability of this protein to catalyze strand exchange. The first is the ability of recA protein to displace SSB protein from single-stranded DNA, an event critical to presynaptic complex formation. RecA protein is able to resist displacement by SSB protein at a lower magnesium concentration in acetate than in chloride buffer. The magnesium ion concentration dependence of strand exchange coincides exactly with this behavior. The second activity correlated to strand exchange is the duplex DNA-dependent ATPase activity of recA protein. We find that over a wide variety of sodium chloride and sodium acetate concentrations, this duplex DNA-dependent ATPase activity is linearly related to the amount of product formed in the strand exchange reaction. We postulate that this duplex DNA-dependent ATPase activity is important in the denaturation of the duplex DNA during the branch migration step of strand exchange and have also determined that this reaction is quite efficient, with the number of ATP molecules hydrolyzed per base pair exchanged being 0.75 .+-. 0.25. In addition, recA protein catalyzed strand exchange between circular single-strand and linear duplex DNA molecules is shown to be irreversible, and a possible explanation for this irreversibility is presented.