Transcription-factor binding and sliding on DNA studied using micro- and macroscopic models.
Transcription factors search for specific operator sequences by alternating rounds of 3D diffusion with rounds of 1D diffusion (sliding) along the DNA. The details of such sliding have largely been beyond direct experimental observation. For this purpose we devised an analytical formulation of umbrella sampling along a helical coordinate, and from extensive and fully atomistic simulations we quantified the free-energy landscapes that underlie the sliding dynamics and dissociation kinetics for the LacI dimer. The resulting potential of mean force distributions show a fine structure with an amplitude of 1 k(B)T for sliding and 12 k(B)T for dissociation. Based on the free-energy calculations the repressor slides in close contact with DNA for 8 bp on average before making a microscopic dissociation. By combining the microscopic molecular-dynamics calculations with Brownian simulation including rotational diffusion from the microscopically dissociated state we estimate a macroscopic residence time of 48 ms at the same DNA segment and an in vitro sliding distance of 240 bp. The sliding distance is in agreement with previous in vitro sliding-length estimates. The in vitro prediction for the macroscopic residence time also compares favorably to what we measure by single-molecule imaging of nonspecifically bound fluorescently labeled LacI in living cells. The investigation adds to our understanding of transcription-factor search kinetics and connects the macro-/mesoscopic rate constants to the microscopic dynamics.
Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, 75124 Uppsala, Sweden.
Erik G
EG
Marklund
Anel
A
Mahmutovic
Otto G
OG
Berg
Petter
P
Hammar
David
D
van der Spoel
David
D
Fange
Johan
J
Elf
D004247:DNA / Q000737:chemistry / Q000378:metabolism*; D004058:Diffusion; D005786:Gene Expression Regulation / Q000502:physiology*; D007700:Kinetics; D056968:Lac Repressors / Q000378:metabolism; D008954:Models, Biological; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D011485:Protein Binding; D011487:Protein Conformation; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*
D056968:Lac Repressors; D014157:Transcription Factors; D004247:DNA
facilitated diffusion; gene regulation; lac operon; lac repressors