Microbiology

Gene expression is regulated at many levels in bacteria. Figure 14.1 in your textbook displays a summary of regulatory mechanisms used to control gene expression. An overview of gene expression in bacteria can be found at (http://www.fda.gov/AnimalVeterinary/SafetyHealth/AntimicrobialResistance/ucm134359.htm). We have been discussing antibiotic resistance in bacteria; now let’s look at how this works on a molecular level. An operon is a group of genes that are regulated by one promotor. The expression of bacterial genes is controlled by the action of diffusible repressors and activators. Tetracycline is an antibiotic used to treat respiratory infections, infections of the skin, including acne, and infections of the genitourinary system. Over time however, many bacteria have become resistant to tetracycline and this antibiotic is no longer as effective as it once was. The mechanism often utilized by resistant strains is the synthesis and use of an efflux pump. While there are multiple types, in general, this pump consists of a membrane‐spanning protein that extrudes the antibiotic from the cell. Since efflux pumps are energetically expensive to make, they are only produced when tetracycline is present. A repressor protein, TetR, has been identified and characterized in resistant strains. The efflux‐pump is negatively controlled by TetR. TetR binds to the operator region when there is no tetracycline present. When tetracycline is present in the cell, it will bind to TetR and change the structure of TetR. TetR is released from the gene and the cell can produce the transmembrane protein needed to get rid of the antibiotic from inside the cell. When the antibiotic is gone, the repressor sits back on the gene. TetR gene transcription is inhibited by the repressor protein. Transcription and translation are controlled by regulatory proteins that can bind to activator binding sites in the presence of a key nutrient or other chemical in the cell. The cell is “sensing” the presence of the nutrients and the activator can then turn on genes in response. Let’s take a look at another example of operons. This video shows how the lac operon works. Combination_of_Switches_The_Lac_Operon.swf (675.139 KB)