Gene Regulation

Gene regulation is the control of the amount of gene product (the expression level) generated by a cell or organism and the control of changes in expression level under different conditions. The ability to regulate gene expression level is critical since a cell must control internal conditions so that things such as necessary structures and biochemical reactions occur and that they occur to the appropriate degree. Since the appropriate degree can change as conditions change, the cell needs to be able to adjust expression levels as conditions change. Gene regulation also allows a cell to avoid wasteful expenditure of energy and molecular components generating products when they are not needed. If a cell expressed every single gene all of the time it would probably run out of building blocks such as nucleotides (for transcription) and amino acids (for translation) very quickly. Even if id did not run out of building blocks the internal biochemistry would be far from optimal. In short, a cell would not survive if every gene was expressed at the same level all of the time. Gene regulation does not just involve the ability to 'turn off' a gene when the product is not needed and to 'turn on' the gene when it is. When a gene is being expressed, the cell regulates how MUCH of the product is being expressed, which is why we refer to the level of expression. In this section we will examine the various ways that cells control expression levels by regulating gene expression.

Before discussing regulation further we should consider what is meant by different conditions in the section above. Basically, what changes in condition does a cell or organism need to respond to? It is difficult to answer this with anything other than a very vague 'a lot'! However, you can get a basic sense by considering two things. First, an organism must respond to external conditions. These are things such as temperature or light, the presence of certain food molecules, other organisms, etc. At the level of the gene an organism can respond by changing the expression level of one or more genes. In the case of a change in temperature an organism might start to express genes that aid protein stability since temperature can affect how a protein folds. In the case of the presence of a certain food molecule it could start to express a set of genes to allow it to metabolize that food. Hopefully these simple examples give you a basic idea. Second, multicellular organisms (primarily multicellular eukaryotes) also respond to internal conditions, in particular signals from other cells. As one cell changes or responds to an external change it can send out a signal such as a hormone or a steroid that other cells respond to through gene regulation. This is of particular importance during development. In fact, given the enormous number of important external and internal conditions a cell or organism responds to it is probably the case that, although some genes might remain fairly constant, gene expression overall is in a state of constant flux.

What you will find is that there are many mechanisms by which genes are regulated (and we are discovering new mechanisms every year). Not only are there many different mechanisms that we observe across genes, but each gene is regulated by a variety of mechanisms. Therefore, when we study any specific gene we have to figure out the many ways in which it is regulated. This is a difficult task so there are few genes for which we think we have a complete understanding of how they are regulated. You will also find that there are some interesting differences between how prokaryotes and eukaryotes regulate gene expression. Therefore, we cover them separately.

Despite the variation there are some general concepts or mechanisms that we can learn. The idea is to develop an understanding of some of these general mechanisms so that we can use them as a basis for understanding any specific gene. In order to study some general mechanisms we often study specific genes as illustrations so keep this in mind as we move back and forth between specific genes (and the details specific to that gene) and the general mechanisms that we see repeated from one gene to another.

Gene regulation can occur at any stage of expression. In fact, a specific gene can be regulated at various stages which means that the amount of gene product generated is a result of a complex set of mechanisms acting at the different stages. Different mechanisms occur at the different stages but at any stage from initiation of transcription through the final stage of protein folding and processing the cell can regulate the process in such a way as to affect how much product is being made. The major stages at which we study regulation in our course (which is not an exhaustive coverage), and the general mechanisms that are involved in regulating those stages, are summarized in this figure.

• Gene Regulation in Prokaryotes


• Gene Regulation in Eukaryotes