Bacteria have many different kinds of control at its disposal. This article will examine three of these controls. The three controls in this article are global, negative, and positive.

Global control systems are the ways that the environment effects gene expression. A good example of these processes is food consumption. When faced with many types of sugars, which one does bacteria generally consume first? The answer is glucose, because it is the easiest to digest. All other carbon sources have to be converted into glucose before it can be digested.

The wide variety of enzymes that help make sure glucose is consumed first is called catabolite repression. Catabolite repression can also be called glucose effect, because glucose was the first discovered way to start this repression. By use of this repression, the organism will always be consuming the best food first. This increases it chances of survival. Catabolite repression can cause diauxic growth. Diauxic growth is a lag in the growth of cells while it changes food sources. This happens because it takes time for the cells to adapt to the new source of food.

The way that catabolite repression works is by controlling the transcription of an activator protein. This means that it is a positive control (more about this below). First cyclic adenosine monophosphate, also known as cyclic AMP, binds to a protein called catabolite activator protein (CAP). Then afterwards it can binds to RNA polymerase. When the polymerase is bound, it can start transcription. Cyclic AMP is necessary for many metabolic controls. Glucose stops cyclic AMP from forming which is why no other food sources are used, until all the glucose is gone. Catabolite repression is called global control, because it controls a great many processes. Every sugar that the organism could digest is repressed by this process.

Negative control in transcription is used as a means to stop transcription. This means that transcription will take place unless something is done to prevent it. The enzyme that stops transcription is called a repressor. In order for the repressor to stop transcription a corepressor has to bind. The corepressor is usually the final product of the transcription that it stops. This occurrence is very useful when there is plenty of a final product around, and it would just be wasting energy to keep producing more.

A repressor could be bound to the process and need to be released. This is done by an inducer. The inducer is added to promote transcription of certain genes. This is used as a way to control certain expressions to when they are actually needed.

The next thing to look at is how this is done. The first stage to be looked at is the repression process. The corepressor binds to the repressor, and changes the conformation of the repressor itself. The new conformation can then bind to the operon. The operon is a group of genes that are put in a linear and consecutive arrangement controlled by the operator. An operator is an area of DNA that controls if the genes are expressed or not. So if the repressor binds here, then transcription is blocked. The induction process is similar. The transcription is blocked unless an inducer is bound to it. Then the repressor becomes inactive.

Positive control is the other way that genes can be controlled. This means that no transcription happens unless something is done to activate it. An activator protein is used to promote the transcription of the genes. The activator protein is activated by an inducer, which allows transcription to occur. They do not bind to an operator like negative control, but instead by an activator-binding site, which is called an operon. The activator protein lets the transcription protein bind easier to the DNA.