Interaction of Escherichia coli MutS and MutL at a DNA mismatch

MutS and MutL are both required to activate downstream events in DNA mismat ch repair. We examined the rate of dissociation of MutS from a mismatch usi ng linear heteroduplex DNAs or heteroduplexes blocked at one or both ends b y four-way DNA junctions in the presence and absence of MutL. In the presen ce of ATP, dissociation of MutS from linear heteroduplexes or heteroduplexe s blocked at only one end occurs within 15 s. When both duplex ends are blo cked, MutS remains associated with the DNA in complexes with half-lives of 30 min. DNase I footprinting of MutS complexes is consistent with migration of MutS throughout the DNA duplex region. When MutL is present, it associa tes with MutS and prevents ATP-dependent migration away from the mismatch i n a manner that is dependent on the length of the heteroduplex. The rate an d extent of mismatch-provoked cleavage at hemimethylated GATC sites by MutH in the presence of MutS, MutL, and ATP are the same whether the mismatch a nd GATC sites are in cis or in trans. These results suggest that a MutS-Mut L complex in the vicinity of a mismatch is involved in activating MutH.