Key points:
Bacterial genes are often found in operons. Genes in an operon are transcribed as a group and have a single promoter.
Each operon contains regulatory DNA sequences, which act as binding sites for regulatory proteins that promote or inhibit transcription.
Regulatory proteins often bind to small molecules, which can make the protein active or inactive by changing its ability to bind DNA.
Some operons are inducible, meaning that they can be turned on by the presence of a particular small molecule. Others are repressible, meaning that they are on by default but can be turned off by a small molecule.
Introduction
We tend to think of bacteria as simple. But even the simplest bacterium has a complex task when it comes to gene regulation! The bacteria in your gut or between your teeth have genomes that contain thousands of different genes. Most of these genes encode proteins, each with its own role in a process such as fuel metabolism, maintenance of cell structure, and defense against viruses.
Some of these proteins are needed routinely, while others are needed only under certain circumstances. Thus, cells don't express all the genes in their genome all the time. You can think of the genome as being like a cookbook with many different recipes in it. The cell will only use the recipes (express the genes) that fit its current needs.
How is gene expression regulated?
There are various forms of gene regulation, that is, mechanisms for controlling which genes get expressed and at what levels. However, a lot of gene regulation occurs at the level of transcription.
Bacteria have specific regulatory molecules that control whether a particular gene will be transcribed into mRNA. Often, these molecules act by binding to DNA near the gene and helping or blocking the transcription enzyme, RNA polymerase. Let's take a closer look at how genes are regulated in bacteria.
In bacteria, genes are often found in operons
In bacteria, related genes are often found in a cluster on the chromosome, where they are transcribed from one promoter (RNA polymerase binding site) as a single unit. Such a cluster of genes under control of a single promoter is known as an operon. Operons are common in bacteria, but they are rare in eukaryotes such as humans.
