Lac on, lac off

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The paper:

J. Elf et al., "Probing transcription factor dynamics at the single-molecule level in a living cell," Science, 316:1191–94, 2007. (Cited in 65 papers)

The finding:

Using fluorescence imaging, a Harvard team led by Sunney Xie quantified the kinetics of the lac operon repressor protein in Escherichia coli in real time. They showed that the protein spends a few milliseconds weakly and nonspecifically bound to DNA, diffusing along the chromosome, then dissociates for a fraction of a millisecond. This cycle of unbinding and rebinding various DNA segments repeats for a few minutes until the protein encounters its specific target.

The background:

Xie's team used a pair of techniques—developed in-house in 2006—to track fluorescently labeled proteins in living cells. In their method, DNA-bound proteins glow like bright dots, while the fluorescence of proteins diffusing in cytoplasm gets lost in...

The impact:

The Hot Paper reported the first direct observation of transcription factor dynamics in a living cell, and directly confirmed predictions made by in vitro work, says Peter von Hippel, a molecular biologist at the University of Oregon. "The whole field of looking at single living cells in a microscopic way is taking off."

The follow-up:

Last year, Xie's group used the technique to show that lac operon induction depends on whether the repressor dissociates partially or completely from its operator—a single-molecule stochastic event (Science, 322:442–46, 2008). In addition, Xie says, "we are working on experiments to make [the technique] possible in mammalian cells"—more complicated because of the added geometry of chromatin.

Quantifying lac repressor kinetics
3D diffusion rate in cytoplasm: 3 μm2/s
1D diffusion rate along DNA: 0.046 μm2/s

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