Saccharomyces cerevisiae mismatch repair complex MLH1‐PMS1 binds and rapidly diffuses along DNA in one dimension

DNA mismatch repair is a conserved pathway that corrects replication errors introduced by DNA polymerase. Formation of an MSH‐MLH ternary complex at a mismatch results in the initiation of downstream repair events that act to excise the mismatch so that the template strand information is maintained. Previously we showed that the S. cerevisiae MSH2‐MSH6 complex can travel along the helical axis of undamaged DNA. This property is believed to allow the complex to efficiently locate lesions within the genome. Subsequent interactions with the MLH1‐PMS1 complex are poorly understood. To begin to investigate MSH‐MLH interactions, we have analyzed the behavior of MLH1‐PMS1 on DNA using bulk assays and total internal reflectance fluorescence microscopy (TIRFM) of single molecules. We observed a robust and stable DNA binding activity for MLH1‐PMS1 in the absence of MSH2‐MSH6. TIRFM experiments revealed populations of MLH1‐PMS1 that differ by the speed at which they travel along DNA. Some molecules move roughly the same speed as MSH2‐MSH6 molecules whereas on average others moved 10‐fold faster. Our data suggests that MLH1‐PMS1 can diffuse along DNA via hopping or stepping while topologically linked in a ring‐like configuration. We also show that MLH1‐PMS1 can freely bypass nucleosomes, providing the first direct demonstration that a passively diffusing protein can traverse a stationary nucleosome while traveling along a chromatin lattice. We are currently testing our hypotheses that the unstructured linker arms of MLH1 and PMS1 that connect their globular N and C terminal domains are important for rapid diffusion along DNA and traversing nucleosomes. This work was supported by the Howard Hughes Medical Institute, the NIH, and SUNY.