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

  • 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