5A;Materials and Methods). == Introduction == MutL homologs (MLH/PMS) are key components of mismatch repair (MMR). Mismatch recognition by MutS homologs (MSH) results in long-lived ATP-bound sliding clamps that recruit MLH/PMS, which Triptonide in turn stimulate the DNA transaction activities of several downstream effectors. InEscherichia coli(E. coli), these downstream effectors include MutH and UvrD. For example, theE. coliMutL stimulates the MutH endonuclease activity on a hemimethylated d(GATC) that directs excision repair to the newly replicated strand as well as enhances the UvrD helicase activity required for the strand excision process[1],[2]. MutL has been suggested to bind ssDNA in the presence of ATP; an activity that may play an important role in its interaction with downstream effectors such as UvrD[3],[4],[5]. Biochemical and structural studies suggest that the C-terminal region ofE. coliMutL forms a stable homodimer (LC20)[6],[7]while the N-terminal domain (LN40) contains a GHKL ATPase site[8]that dimerizes upon binding to ATP[9],[10]. Together, the resulting structure of the ATP-bound MutL appears to form a cavity via a flexible linker that contains a positively charged cleft[9]. This ATP-dependent MLH/PMS conformational change appears to be modulated by the ATP binding and hydrolysis cycle even in Triptonide the absence of DNA[11]. It is the positively charged cavity formed by ATP-bound MutL that appears to contain the ssDNA-binding domain[9],[11]. However, the properties and roles of MutL-ssDNA binding in MMR are poorly understood. Studies that appear to support a role for MutL-ssDNA binding in MMR include: 1) MutL appears to bind unmethylated ssDNA better than methylated ssDNA or unmethylated/methylated dsDNA[3], 2) ssDNA stimulates the MutL ATPase[9], and 3) the MutL(R266E) mutant protein that displays a weak ssDNA-binding affinity and that lacks ssDNA-stimulated ATPase activity genetically behaves like amutLnull mutant[5]. While these studies appear to correlate MutL-ssDNA binding with MMR, all of the ssDNA binding studiesin vitrowere performed at non-physiological ionic strengths[6],[12]. Moreover, there are reports that suggest MutL does not bind to DNA at physiological salt and that DNA binding is not required for MMR[13],[14]. We have developed single-molecule assays that examine the lengthening of random-coiled ssDNA, which results from MutL binding. These assays, single-molecule Frster Resonance Energy Transfer (smFRET)[15]and a single molecule flow-extension assay[16],[17], have allowed us to study the kinetics of the MutL-ssDNA interaction in real time. Our studies examined the interaction between MutL and ssDNA in the absence of other MMR proteins. Together the single-molecule analysis detailed both the heterogeneity and the physiological relevance of the MutL-ssDNA interaction. == Results == == E. coliMutL binds and stretches ssDNA == The ssDNA binding activity of MutL was examined using a partial duplex DNA that consisted of 15 bp double-stranded DNA (dsDNA) with a 33-deoxythymidine nucleotide (dT33) 5-overhang. An acceptor Cy5 at the ssDNA/dsDNA junction and a donor Cy3 on the end of the 5-overhang were used as FRET pairs. A 5-biotin anchored the DNA substrate to the quartz slide glass coated with PEG-biotin using a streptavidin linker Rabbit polyclonal to ACTN4 (Fig. 1A). Injection of MutL (50 nM) in 25 mM NaCl resulted in the abrupt decrease of the acceptor signal at 3 s that was maintained for 13 s Triptonide (Fig. 1B). This pattern and the anticorrelation signals between a donor and an acceptor were Triptonide repetitive for 85 s (Fig. 1B). == Figure 1. Single-molecule FRET analysis of MutL-ssDNA binding. == (A) Schematic representation of a single-molecule FRET system. The Evanescent field resulting from Total Internal Reflection excites a Cy3 donor fluorophore that may FRET with a Cy5 acceptor fluorophore. A partial duplex DNA attached to the PEG-biotin surface via a biotin-streptavidin linker contains a 5-dT33 Triptonide single-stranded tail labeled with the donor Cy3 and an acceptor Cy5 linked to the dsDNA/ssDNA junction. The distance between fluorophores changes with MutL-ssDNA binding affecting the FRET efficiency. (B) Representative single molecule trace of the Cy3 and Cy5 fluorescence intensity (top panel) and the resulting FRET (bottom panel) with.