Activation of a recombinase-deficient mutant recA protein with alternate nucleoside triphosphate cofactors

We recently described two mutant recA proteins, (G160N)recA and (H163A)recA, which have full single-stranded DNA-dependent ATP hydrolysis activity but which are unable to promote the ATP-dependent strand exchange reaction under standard reaction conditions (pH 7.5). These mutant proteins, however, are able to promote strand exchange at pH 6.0 to 6.8. Here we show that this activation correlates with a pH-dependent decrease in the S_0.5 value for ATP, with the (H163A)recA protein becoming active in strand exchange at pH values where the S_0.5(ATP) decreases below 100 μM. We also show that the (H163A)recA protein is active in strand exchange over the range of pH 6.0-8.2 if dATP (or ddATP) is used in place of ATP as a cofactor; dATP is hydrolyzed by (H163A)recA protein at the same rate as ATP but has an S_0.5 value lower than 100 μM across this pH range. These results are discussed with regard to the general significance of the S_0.5 value in determining whether a nucleoside triphosphate will be able to stabilize the recA-single-stranded DNA filament in the strand exchange active conformational state.