Single-molecule fluorescence techniques have been successfully applied to uncover the structure, dynamics and interactions of DNA, RNA and proteins at the molecular scale. While the structure and function of these biomolecules are imposed by their sequences, single-molecule studies have been limited to a small number of sequences due to constraints in time and cost. To gain a comprehensive understanding on how sequence influences these essential biomolecules and the processes in which they act, a vast number of sequences have to be probed, requiring a high-throughput parallel approach. To address this need, we developed SPARXS: single-molecule parallel analysis for rapid exploration of sequence space. This platform enables simultaneous profiling of millions of molecules, covering thousands of distinct sequences, at the single-molecule level by coupling single-molecule fluorescence microscopy with next-generation high-throughput sequencing. Here we describe how to implement SPARXS and give examples from our study into the effect of sequence on Holliday junction kinetics. We provide a detailed description of sample and library design, single-molecule measurement, sequencing, coupling of sequencing and single-molecule fluorescence data, and data analysis. The protocol requires experience with single-molecule fluorescence microscopy and a basic command of Python to use our Papylio package for SPARXS data analysis. Familiarity with the underlying principles of Illumina sequencing is also beneficial. The entire process takes ~1–2 weeks and provides a detailed quantitative picture of the effect of sequence on the studied process.

At the core of molecular biology lies the intricate interplay between sequence, structure, and function. Single-molecule techniques provide in-depth dynamic insights into structure and function, but laborious assays impede functional screening of large sequence libraries. We introduce high-throughput Single-molecule Parallel Analysis for Rapid eXploration of Sequence space (SPARXS), integrating single-molecule fluorescence with next-generation sequencing. We applied SPARXS to study the sequence-dependent kinetics of the Holliday junction, a critical intermediate in homologous recombination. By examining the dynamics of millions of Holliday junctions, covering thousands of distinct sequences, we demonstrated the ability of SPARXS to uncover sequence patterns, evaluate sequence motifs, and construct thermodynamic models. SPARXS emerges as a versatile tool for untangling the mechanisms that underlie sequence-specific processes at the molecular scale.

Next-generation sequencing techniques have led to a new quantitative dimension in the biological sciences. In particular, integrating sequencing techniques with biophysical tools allows sequence-dependent mechanistic studies. Using the millions of DNA clusters that are generated during sequencing to perform high-throughput binding affinity and kinetics measurements enabled the construction of energy landscapes in sequence space, uncovering relationships between sequence, structure, and function. Here, we review the approaches to perform ensemble fluorescence experiments on next-generation sequencing chips for variations of DNA, RNA, and protein sequences. As the next step, we anticipate that these fluorescence experiments will be pushed to the single-molecule level, which can directly uncover kinetics and molecular heterogeneity in an unprecedented high-throughput fashion. Molecular biophysics in sequence space, both at the ensemble and single-molecule level, leads to new mechanistic insights. The wide spectrum of applications in biology and medicine ranges from the fundamental understanding of evolutionary pathways to the development of new therapeutics.

Single-molecule FRET is a versatile tool to study nucleic acids and proteins at the nanometer scale. However, currently, only a couple of FRET pairs can be reliably measured on a single object, which makes it difficult to apply single-molecule FRET for structural analysis of biomolecules. Here, we present an approach that allows for the determination of multiple distances between FRET pairs in a single object. We use programmable, transient binding between short DNA strands to resolve the FRET efficiency of multiple fluorophore pairs. By allowing only a single FRET pair to be formed at a time, we can determine the pair distance with subnanometer precision. The distance between other pairs are determined by sequentially exchanging DNA strands. We name this multiplexing approach FRET X for FRET via DNA eXchange. Our FRET X technology will be a tool for the high-resolution analysis of biomolecules and nanostructures.

Detection of specific nucleic acid sequences is invaluable in biological studies such as genetic disease diagnostics and genome profiling. Here, we developed a highly sensitive and specific detection method that combines an advanced oligonucleotide ligation assay with multicolor single-molecule fluorescence. We demonstrated that under our experimental conditions, 7-nucleotide long DNA barcodes have the optimal short length to ascertain specificity while being long enough for sufficient ligation. Using four spectrally separated fluorophores to label DNA barcodes, we simultaneously distinguished four DNA target sequences differing by only a single nucleotide. Our single-molecule approach will allow for accurate identification of low-abundance molecules without the need for target DNA preamplification.