Quantitative influence of macromolecular crowding on gene regulation kinetics

We introduce macromolecular crowding quantitatively into the model for kinetics of gene regulation in Escherichia coli. We analyse and compute the specific-site searching time for 180 known transcription factors (TFs) regulating 1300 operons. The time is between 160 s (e. g. for SoxS M-w = 12.91 kDa) and 1550 s (e. g. for PepA(6) of M-w = 329.28 kDa). Diffusion coefficients for one-dimensional sliding are between 0.003 mu m(2)s(-1) for large proteins up to 0.4 mu m(2)s(-1) for small monomers or dimers. Three-dimensional diffusion coefficients in the cytoplasm are 2 orders of magnitude larger than 1D sliding coefficients, nevertheless the sliding enhances the binding rates of TF to specific sites by 1-2 orders of magnitude. The latter effect is due to ubiquitous non-specific binding. We compare the model to experimental data for LacI repressor and find that non-specific binding of the protein to DNA is activation-and not diffusion-limited. We show that the target location rate by LacI repressor is optimized with respect to microscopic rate constant for association to nonspecific sites on DNA. We analyse the effect of oligomerization of TFs and DNA looping effects on searching kinetics. We show that optimal searching strategy depends on TF abundance.