Analytical equilibrium ultracentrifugation indicates that Escherichia coli MutS exists as an equilibrating mixture of dimers and tetramers. The association constant for the dimer-to-tetramer transition is 2.1 ؋ 10 7 M ؊1 , indicating that the protein would consist of both dimers and tetramers at physiological concentrations. The carboxyl terminus of MutS is required for tetramer assembly because a previously described 53-amino acid carboxyl-terminal truncation (MutS800) forms a limiting species of a dimer (Obmolova, G., Ban, C., Hsieh, P., and Yang, W. MutS homologs participate in multiple genetic stabilization pathways by virtue of their ability to recognize a spectrum of DNA lesions. Recognition of base pair mismatches by MutS homologs has been implicated in the rectification of DNA biosynthetic errors and in the dissolution of recombination events involving diverged sequences (1-5). Recognition of damaged base pairs by MutS homologs is also involved in the transcription-coupled repair of DNA damage (2, 4) and has been implicated in the activation of checkpoint and apoptotic responses to certain types of DNA damage in mammalian cells (6). Members of the MutS family can thus be viewed as molecular sentinels that respond to subtle variations in DNA structure or dynamics and then communicate presence of that lesion to downstream activities (1, 7-9).Events downstream of MutS recognition have been delineated for the Escherichia coli pathway responsible for correction of DNA biosynthetic errors, and this reaction has been reconstituted in vitro using near homogeneous components. Although a number of mechanistic details remain to be established, basic roles of the individual activities have been defined. MutS recruits MutL to the heteroduplex in a reaction requiring ATP (10 -12). Assembly of this ternary complex is sufficient to activate the latent endonuclease activity of MutH, which incises the unmethylated strand at a hemimethylated d(GATC) strand signal (13). In addition, this complex activates the unwinding activity of DNA helicase II, which loads at the MutH incision with an orientation bias so that helix unwinding proceeds toward the mismatch (14, 15). The unwound portion of the incised strand is hydrolyzed by a single-strand exonuclease. If unwinding proceeds from a strand break located 5Ј to the mispair, the displaced single strand is degraded by the 5Ј-to-3Ј hydrolytic activity of ExoVII or RecJ exonuclease (16,17). When helicase unwinding initiates at a nick 3Ј to the mismatch, the unwound strand is degraded by the 3Ј-to-5Ј activity of ExoI, ExoVII,. Excision terminates at a number of sites centered about 50 base pairs beyond the mismatch, the ensuing gap is repaired by DNA polymerase III holoenzyme in the presence of single-strand binding protein, and DNA ligase restores covalent integrity to the repaired strand (18,20).In addition to its mismatch recognition activity, bacterial MutS harbors a weak ATPase (21) that is stimulated by DNA (22). Because the rate-limiting step for ATP hydrolytic turnover is se...