Multi-site internal modification of long DNA substrates for single-molecule studies
Single-molecule studies of protein-nucleic acid interactions frequently require site-specific modification of long DNA substrates. DNA isolated from bacteriophage {lambda} ({lambda}-DNA) is a convenient source of high quality long (48.5 kb) DNA. However, introducing specific DNA sequences, tertiary structures, and chemical modifications into {lambda}-DNA remains technically challenging. Most current approaches rely on multi-step ligations with low yields and incomplete products. Here, we describe a molecular toolkit for rapid preparation of modified {lambda}-DNA. A set of PCR cassettes facilitates the introduction of recombinant DNA sequences into {lambda}-DNA with 90-100% yield. Furthermore, various DNA structures and chemical modifications can be inserted at user-defined sites via an improved nicking enzyme-based strategy. As a proof-of-principle, we explore the interactions of Proliferating Cell Nuclear Antigen (PCNA) with modified DNA sequences and structures incorporated within {lambda}-DNA. Our results demonstrate that PCNA can load on both 5-ssDNA flaps and a 13xCAG triplet repeat. However, PCNA remains trapped on the 13xCAG structure, confirming a proposed mechanism for triplet repeat expansion. Although we focus on {lambda}-DNA, this method is applicable to all long DNA substrates. We anticipate that this molecular toolbox will be broadly useful for both ensemble- and single-molecule studies that require site-specific modification of long DNA substrates.