Disruption of an ATP-dependent isomerization of the recA protein by mutation of histidine 163

We have used site-directed mutagenesis to replace histidine 163 of the recA polypeptide with an alanine residue. The new [Ala-163]recA protein catalyzes single-stranded (ss) DNA-dependent ATP hydrolysis with a turnover number that is similar to that of the wild-type recA protein. Despite being proficient in ssDNA-dependent ATP hydrolysis, the [Ala-163]recA protein is unable to promote the ATP-dependent three-strand exchange reaction under standard reaction conditions, pH 7.5. The [Ala-163]recA protein does exhibit three-strand exchange activity at pH 6.0-7.0, however, and the induction of strand exchange activity at low pH correlates directly with the activation of an ATP-dependent isomerization of the mutant protein. Thus, the [Ala-163]recA protein is functionally similar to our previously described mutant [Asn-160]recA protein (Bryant, F. R. (1988) J. Biol. Chem. 263, 8716-8723; Muench, K. A., and Bryant, F. R. (1990) J. Biol. Chem. 265, 11560-11566). Trypsin proteolysis studies indicate that the [Ala-163]recA and [Asn-160]recA proteins, like the wild-type recA protein, are organized into carboxyl-terminal and amino-terminal domains of nearly equal size. According to this structural model, the [Ala-163]recA and [Asn-160]recA mutations may lie in a linker region joining these two domains. We speculate that the [Ala-163]recA and [Asn-160]recA mutations interfere with an ATP-dependent conformational change of the recA protein that perhaps involves a change in the relative orientation of the carboxyl-terminal and amino-terminal domains.