Single-Stranded DNA Curtains for Studying the Srs2 Helicase Using Total Internal Reflection Fluorescence Microscopy.

Helicases are crucial participants in many types of DNA repair reactions, including homologous recombination. The properties of these enzymes can be assayed by traditional bulk biochemical analysis; however, these types of assays cannot directly access some types of information. In particular, bulk biochemical assays cannot readily access information that may be obscured in population averages. Single-molecule assays offer the potential advantage of being able to visualize the molecules in question in real time, thus providing direct access to questions relating to translocation velocity, processivity, and insights into how helicases may behave on different types of substrates. Here, we describe the use of single-stranded DNA (ssDNA) curtains as an assay for directly viewing the behavior of the Saccharomyces cerevisiae Srs2 helicase on single molecules of ssDNA. When used with total internal reflection fluorescence microscopy, these methods can be used to track the binding and movements of individual helicase complexes, and allow new insights into helicase behaviors at the single-molecule level.