From Gene Expression to Behavior: A Study in Bacterial Cells

Gene expression plays an important role in cell metabolism, motility, and replications. Using fluorescence microscopy, I found that the expression profile of LamB, an Escherichia coli outer membrane protein, is unimodal in cell cytosol. The bimodal distribution of lambda phage, which binds to LamB receptors on the outer membrane, should result from the heterogeneity in protein export and assembly processes. The study also estimates the burst-like LamB expression to have a mean bursting rate of 9.4 bursts per cell-cycle and a mean bursting number of 82 copies per burst. In another study on bacterial growth curves, I unexpectedly discovered that depletion of 4 amino acids (arginine, methionine, isoleucine, and glycine) can cause diauxie-like growth arrests, which last for 5 to 30 minutes depending on the amino acids used. As a quantitative measure of response times of gene regulations upon starvation, this technique could potentially provide a simple and robust means to determine the time constants in metabolic networks of a variety of bacteria. Normal expression of genes is also significant for robust bacterial swimming and chemotaxis. I observed strikingly that very small Vibrio alginolyticus cells are able to reorient by a pi-flip (180 degrees) in around 60 ms, but this ability is gradually lost when the cell size becomes larger. I also observed that in a cheY deletion mutant of V. alginolyticus, over expression of CheY, or its phosphorylated form CheY-p, significantly diminishes the mutant's ability to change swimming directions. To understand how Vibrio alginolyticus reorients during a flick, H. Fu came up with a dynamic model incorporating flagellar off-axial rotation during a flick and I performed the calculation. Comparing the model prediction with the experiment, my observations are consistent with the physical picture that V. alginolyticus' flagellum follows some closed path on the surface of a unisphere characterized by the polar and azimuthal angles. Fitting the data of cheY deletion mutant using this model suggests the possibility of motor remodeling and "stiffening" when CheY concentration is increased