Not all genes need to be transcribed all of the time (nor should they be). This module looks at the strategy that bacteria use to regulate the transcription of their genes.
Objectives
The Need for Gene Regulation
Our consideration of gene expression (see the module on transcription) has until now focused on the actual mechanism of RNA synthesis, without regard to whether or not the mRNA (and the protein it encodes) is actually required. This is an important consideration. Think of it this way: in the winter, do you run your furnace nonstop, 24 hours a day? Of course not. Your house would get way too hot and your heating bill would probably force you into bankruptcy (in other words, there would unnecessary heat produced at great expense). The same thing is true of bacteria. Why should they go to the expense of producing the enzymes that metabolize a particular nutrient if that nutrient is not present in the environment? To put this another way, bacteria need to be able to respond to their environment by changing their patterns of gene expression. They accomplish this using strategies of gene regulation.
Operons
Genes that affect the same biochemical pathway in bacteria (for example, genes encoding proteins involved in metabolizing the sugar lactose) will all be expressed under the same conditions (such as when lactose is present). Therefore, it is economical to have all of these genes grouped together under the control of the same regulatory system. Such a grouping of similarly-regulated genes in bacteria is called an operon. Some operons are inducible, others are repressible. We will discuss the differences between these two types of operons later. An example of an inducible operon is the lactose operon (lac operon for short), which contains genes that encode enzymes responsible for lactose metabolism. The lac operon looks like this:
The three structural (protein-encoding ) genes of the lac operon are lac Z, which encodes the enzyme beta-galactosidase (which breaks down lactose into glucose and galactose), lac Y, which encodes a permease (that transports lactose into the cell), and lac A, which encodes a transacetylase (whose function we won't consider here). These three genes are under the control of the same promoter, designated P in the figure. As discussed elsewhere, the promoter is where RNA polymerase binds to the DNA and prepares to initiate transcription. The other regulatory element in an operon is the operator (designated O). As we'll see, this is the element that determines whether or not the genes of the operon are transcribed. In addition to all of the above, there is another gene, which is technically not part of the operon (it is controlled by a separate promoter, and is expressed all the time, or constitutively), but plays an important role in operon function. This is the lac I gene, which encodes a protein called the lac repressor. The lac repressor has two functional domains or regions: one that binds to the DNA of the operator region, and one that binds to lactose. When the repressor binds to the operator, it prevents RNA polymerase advancing along the operon, and transcription does not occur. The regulation of the operon depends on regulating whether or not the repressor binds to the operator. To understand how this works, we need to consider what happens when lactose is present, and what happens when lactose is absent.