Time for Part 3 in the series examining how bacteria make decisions. Parts 1 and 2 focused on chemotaxis. Today, we will focus on how bacteria decide which genes need to be expressed and which need to be repressed. One of the most prevalent ways a bacterium decides this is by using a two component system, or TCS. TCSs are relatively simple compared to chemotaxis. As you would suspect from the name, TCSs are pathways consisting of only two protein members, the sensor histidine kinase and the response regulator. Histidine kinases are a major protein family in bacteria because they are able to sense many different factors in the bacterium’s environment including nutrients, toxins, fellow bacteria, etc. In case you are wondering, chemotaxis is a modified form of a TCS in which the histidine kinase CheA is regulated by the activity of a separate protein, the methyl-accepting chemotaxis protein.
What if you are a bacterium and you have been using a certain type of carbon source to generate energy and suddenly that carbon source isn’t as prevalent? In this case, you would want to shut down the enzyme factories that were converting the previous food source into energy and begin preparing new enzyme factories to convert other food sources into energy as you prepare for starvation. If these conditions persist, you might want to decide to hibernate in the form of a spore or cyst until conditions around you improve. Or, if other food sources are sensed in the environment, any special enzymes that would be needed to convert them into energy would need to be synthesized from their respective genes. All of these scenarios are controlled by TCSs. The conditions are used as input for the cell to decide the best strategy to survive and thrive. Histidine kinase activation leads to a hand-off event from the kinase to the response regulator of a molecule which acts as a green light for the response regulator to proceed with its job. This job may be to turn on gene expression to produce proteins needed in the cell. The response regulator’s job may be to shut down gene expression for proteins no longer needed by the cell. It is a carefully orchestrated balancing act evolved over millions of years to make sure only the proteins/enzymes needed by the cell at a given time are present assuring highly valuable energy molecules are not wasted.
Yet another way bacteria are Nature’s smallest 5000 ring circus.