Genetic regulation in Bacteria
Hanna Salman, Ph.D.
The biological cell is a miniature chemical reactor with many interacting molecules. Many of the biochemical reactions that take place in the cell involve a small number of molecules of a given type and therefore can fluctuate from cell to cell. In a dividing population of cells, large fluctuations in the proteins content of the cells can emerge as a result. This in turn, can lead to significant effects of functionality, such as flipping a genetic switch and causing a split in the population into distinct phenotypic subgroups. The phage γ lysis-lysogeny decision circuit is a classic example, where the concentration of the repressor protein (CI) determines if the phage will stay dormant or becomes active. My work focuses on studying the protein distribution across a genetically homogeneous bacterial population, how this distribution evolves under environmental changes, meaning studying the population adaptation. We also would like to know if the protein distribution in a population gives an evolutionary advantage to the population, when subjected to a selection pressure.
The first project focuses on the long term dynamics of the expression distribution of a protein down stream of a Lactose operon (LacO) promoter and under constant and changing environmental conditions. To monitor the state of the LacO promoter in each individual cell, the promoter was inserted in a low copy plasmid upstream of a GFP gene. The plasmid was transferred into DH5α E. coli. The work is carried out in a chemostat; a system which maintains constant conditions (temperature, chemicals and bacterial concentration) for indefinite time. The concentrations of bacteria and other chemicals in the reactor are maintained constant by allowing a constant flow of fresh medium into the reactor and flushing out old mix (medium and bacteria) at the same rate. The change of the environment is applied after the system reaches equilibrium (constant concentration of bacteria). The medium, or the feeding solution, is simply switched from glucose into lactose rich medium (and back again to glucose) and the expression distribution is measured at different times by flow cytometry. In this work, my aim is to understand the dynamics of switch adaptation to new environmental conditions and how the variability of expression in the population changes with it. I would like to see if the noise level is optimized at any point of the process and what are the conditions and benefits of it, if there are any.
The second project aim is to find out whether the expression distribution across a genetically homogeneous population plays any role in the population fitness and survival. For this purpose I will study two bacterial populations, in which two different plasmids are inserted. The difference is that those plasmids express anti-biotic resistance but with two different promoters, which lead to the same mean expression level of the anti-biotic resistance but with very different variability. Evidently the measuring device will be GFP expressed with the anti-biotic resistance gene. The bacteria will be grown under the same conditions and the growth rate of each culture will be measured as a function of the antibiotics concentration, which represents the selection pressure. Furthermore, I will also be able to see how the selection pressure affects the various distributions of gene expression (measured again as before by flow cytometry techniques), when the survival of the cell depends on that specific gene.
For further information: E. Braun, N. Brenner, Transient responses and adaptation to steady state in a eukaryotic gene regulation system, Physical Biology 1, 67 (2004).