Professional Education

  • Bachelor of Science, Louisiana State Univ And A.& M. Coll (2004)
  • Doctor of Philosophy, University of California Davis (2011)

Stanford Advisors

All Publications

  • RecA: Reculation one Mechanism of a Volecular Search Engine TRENDS IN BIOCHEMICAL SCIENCES Bell, J. C., Kowalczykowski, S. C. 2016; 41 (6): 491-507


    Homologous recombination maintains genomic integrity by repairing broken chromosomes. The broken chromosome is partially resected to produce single-stranded DNA (ssDNA) that is used to search for homologous double-stranded DNA (dsDNA). This homology driven 'search and rescue' is catalyzed by a class of DNA strand exchange proteins that are defined in relation to Escherichia coli RecA, which forms a filament on ssDNA. Here, we review the regulation of RecA filament assembly and the mechanism by which RecA quickly and efficiently searches for and identifies a unique homologous sequence among a vast excess of heterologous DNA. Given that RecA is the prototypic DNA strand exchange protein, its behavior affords insight into the actions of eukaryotic RAD51 orthologs and their regulators, BRCA2 and other tumor suppressors.

    View details for DOI 10.1016/j.tibs.2016.04.662

    View details for Web of Science ID 000376806400004

    View details for PubMedID 27156117

  • Mechanics and Single-Molecule Interrogation of DNA Recombination ANNUAL REVIEW OF BIOCHEMISTRY, VOL 85 Bell, J. C., Kowalczykowski, S. C. 2016; 85: 193-226


    The repair of DNA by homologous recombination is an essential, efficient, and high-fidelity process that mends DNA lesions formed during cellular metabolism; these lesions include double-stranded DNA breaks, daughter-strand gaps, and DNA cross-links. Genetic defects in the homologous recombination pathway undermine genomic integrity and cause the accumulation of gross chromosomal abnormalities-including rearrangements, deletions, and aneuploidy-that contribute to cancer formation. Recombination proceeds through the formation of joint DNA molecules-homologously paired but metastable DNA intermediates that are processed by several alternative subpathways-making recombination a versatile and robust mechanism to repair damaged chromosomes. Modern biophysical methods make it possible to visualize, probe, and manipulate the individual molecules participating in the intermediate steps of recombination, revealing new details about the mechanics of genetic recombination. We review and discuss the individual stages of homologous recombination, focusing on common pathways in bacteria, yeast, and humans, and place particular emphasis on the molecular mechanisms illuminated by single-molecule methods. Expected final online publication date for the Annual Review of Biochemistry Volume 85 is June 02, 2016. Please see for revised estimates.

    View details for DOI 10.1146/annurev-biochem-060614-034352

    View details for Web of Science ID 000379324700010

    View details for PubMedID 27088880

  • Condensing chromosome condensation NATURE CELL BIOLOGY Bell, J. C., Straight, A. F. 2015; 17 (8): 964-965


    Mitotic chromosome condensation has fascinated biologists since Flemming's early illustrations of mitosis in the late nineteenth century. Now--130 years later--chromatid condensation is reconstituted in vitro with the minimum components. The results are remarkably and beautifully simple, requiring only core histones, three histone chaperones, topoisomerase II and condensin I.

    View details for Web of Science ID 000358847700002

    View details for PubMedID 26239527

  • Imaging and energetics of single SSB-ssDNA molecules reveal intramolecular condensation and insight into RecOR function. eLife Bell, J. C., Liu, B., Kowalczykowski, S. C. 2015; 4


    Escherichia coli single-stranded DNA (ssDNA) binding protein (SSB) is the defining bacterial member of ssDNA binding proteins essential for DNA maintenance. SSB binds ssDNA with a variable footprint of ∼30-70 nucleotides, reflecting partial or full wrapping of ssDNA around a tetramer of SSB. We directly imaged single molecules of SSB-coated ssDNA using total internal reflection fluorescence (TIRF) microscopy and observed intramolecular condensation of nucleoprotein complexes exceeding expectations based on simple wrapping transitions. We further examined this unexpected property by single-molecule force spectroscopy using magnetic tweezers. In conditions favoring complete wrapping, SSB engages in long-range reversible intramolecular interactions resulting in condensation of the SSB-ssDNA complex. RecO and RecOR, which interact with SSB, further condensed the complex. Our data support the idea that RecOR--and possibly other SSB-interacting proteins-function(s) in part to alter long-range, macroscopic interactions between or throughout nucleoprotein complexes by microscopically altering wrapping and bridging distant sites.

    View details for DOI 10.7554/eLife.08646

    View details for PubMedID 26381353

  • Direct imaging of RecA nucleation and growth on single molecules of SSB-coated ssDNA NATURE Bell, J. C., Plank, J. L., Dombrowski, C. C., Kowalczykowski, S. C. 2012; 491 (7423): 274-U144


    Escherichia coli RecA is the defining member of a ubiquitous class of DNA strand-exchange proteins that are essential for homologous recombination, a pathway that maintains genomic integrity by repairing broken DNA. To function, filaments of RecA must nucleate and grow on single-stranded DNA (ssDNA) in direct competition with ssDNA-binding protein (SSB), which rapidly binds and continuously sequesters ssDNA, kinetically blocking RecA assembly. This dynamic self-assembly on a DNA lattice, in competition with another protein, is unique for the RecA family compared to other filament-forming proteins such as actin and tubulin. The complexity of this process has hindered our understanding of RecA filament assembly because ensemble measurements cannot reliably distinguish between the nucleation and growth phases, despite extensive and diverse attempts. Previous single-molecule assays have measured the nucleation and growth of RecA--and its eukaryotic homologue RAD51--on naked double-stranded DNA and ssDNA; however, the template for RecA self-assembly in vivo is SSB-coated ssDNA. Using single-molecule microscopy, here we directly visualize RecA filament assembly on single molecules of SSB-coated ssDNA, simultaneously measuring nucleation and growth. We establish that a dimer of RecA is required for nucleation, followed by growth of the filament through monomer addition, consistent with the finding that nucleation, but not growth, is modulated by nucleotide and magnesium ion cofactors. Filament growth is bidirectional, albeit faster in the 5'→3' direction. Both nucleation and growth are repressed at physiological conditions, highlighting the essential role of recombination mediators in potentiating assembly in vivo. We define a two-step kinetic mechanism in which RecA nucleates on transiently exposed ssDNA during SSB sliding and/or partial dissociation (DNA unwrapping) and then the RecA filament grows. We further demonstrate that the recombination mediator protein pair, RecOR (RecO and RecR), accelerates both RecA nucleation and filament growth, and that the introduction of RecF further stimulates RecA nucleation.

    View details for DOI 10.1038/nature11598

    View details for Web of Science ID 000310774300048

    View details for PubMedID 23103864

  • Fluorescent single-stranded DNA binding protein as a probe for sensitive, real-time assays of helicase activity BIOPHYSICAL JOURNAL Dillingham, M. S., Tibbles, K. L., Hunter, J. L., Bell, J. C., Kowalczykowski, S. C., Webb, M. R. 2008; 95 (7): 3330-3339


    The formation and maintenance of single-stranded DNA (ssDNA) are essential parts of many processes involving DNA. For example, strand separation of double-stranded DNA (dsDNA) is catalyzed by helicases, and this exposure of the bases on the DNA allows further processing, such as replication, recombination, or repair. Assays of helicase activity and probes for their mechanism are essential for understanding related biological processes. Here we describe the development and use of a fluorescent probe to measure ssDNA formation specifically and in real time, with high sensitivity and time resolution. The reagentless biosensor is based on the ssDNA binding protein (SSB) from Escherichia coli, labeled at a specific site with a coumarin fluorophore. Its use in the study of DNA manipulations involving ssDNA intermediates is demonstrated in assays for DNA unwinding, catalyzed by DNA helicases.

    View details for DOI 10.1529/biophysj.108.133512

    View details for Web of Science ID 000259393200025

    View details for PubMedID 18599625