Probing transcription factor dynamics at the single-molecule level in a living cell

Area of Science:

• Molecular Biology
• Genetics
• Biophysics

Background:

• Transcription factors are crucial proteins that control gene expression by binding to specific DNA sequences.
• Understanding the dynamic interactions of these proteins with DNA is fundamental to comprehending cellular processes.
• Previous studies often relied on in vitro methods, limiting insights into in vivo binding behaviors.

Purpose of the Study:

• To directly visualize and quantify the binding kinetics of a specific transcription factor (lac repressor) to its DNA operator in a living bacterial cell.
• To characterize the different modes of DNA interaction, including specific binding, nonspecific binding, and diffusion, at the single-molecule level.
• To elucidate the search mechanism employed by transcription factors for target DNA sites within the cellular environment.

Main Methods:

• Utilized single-molecule detection techniques, including fluorescent labeling of the lac repressor.
• Observed protein-DNA interactions within a living Escherichia coli (E. coli) cell.
• Measured binding and dissociation kinetics, nonspecific DNA binding, and 1D/3D diffusion of the repressor.

Main Results:

• Successfully visualized specific binding of the fluorescently labeled lac repressor to the chromosomal lac operator in live E. coli.
• Quantified the kinetics of repressor binding and dissociation in response to metabolic signals.
• Determined that lac repressor spends ~90% of its time nonspecifically bound to DNA, diffusing along it with residence times <5 milliseconds.

Conclusions:

• The study provides direct, in vivo evidence of transcription factor search mechanisms.
• The findings demonstrate the dynamic interplay between specific and nonspecific DNA binding in locating target sites.
• The developed single-molecule methods and insights are applicable to studying other nucleic acid-binding proteins.