Gene Expression and Transcriptional Regulation

Gene expression is the process by which the information in a gene is used to make a functional product, usually a protein, and regulation of gene expression controls which genes are active, in which cells and at which times. In brief: every cell in your body has the same DNA, but a muscle cell and a nerve cell use different genes. Most regulation takes place at the stage of transcription, where regulatory proteins called transcription factors switch genes on or off. In bacteria, groups of genes are controlled together in operons such as the lac operon.

Regulation explains how one genome can produce hundreds of different cell types, and how cells respond to changes in their environment. This guide explains why genes must be regulated, describes the lac operon in bacteria, explains regulation in eukaryotes, works through a lac operon example step by step and corrects common mistakes.

What is gene expression?

Gene expression includes transcription (DNA to RNA) and translation (RNA to protein), and the processing steps between. A gene is “expressed” or “switched on” when its product is being made, and “off” when it is not. Some genes, called housekeeping genes, are expressed in almost all cells all the time, such as those for ribosomal proteins. Others are used only in particular cell types or under particular conditions.

Why is regulation needed?

• Efficiency: making proteins that are not needed wastes energy and materials. A bacterium that does not have lactose in its environment does not need enzymes to digest it.
• Specialization: in multicellular organisms, cells have the same DNA but differ in function. Insulin is made only in beta cells of the pancreas, and hemoglobin is made in developing red blood cells.
• Development: genes are switched on and off in a precise sequence while an embryo forms.
• Response to the environment: cells increase or decrease the production of proteins in response to nutrients, hormones, heat and stress.

How does the lac operon work in bacteria?

In bacteria, genes with related functions are often grouped together and controlled as a unit called an operon. The lac operon in E. coli, described by François Jacob and Jacques Monod, controls the genes needed to use the sugar lactose. It has these parts:

• Structural genes: lacZ (codes for beta-galactosidase, which breaks lactose into glucose and galactose), lacY (codes for permease, which brings lactose into the cell) and lacA.
• Promoter: where RNA polymerase binds to start transcription.
• Operator: a DNA sequence next to the promoter where a repressor protein can bind.
• Regulatory gene (lacI): located elsewhere, it makes the repressor protein.

Without lactose

The repressor binds to the operator and blocks RNA polymerase, so the structural genes are not transcribed. The system is off, and the cell does not waste resources.

With lactose

A small amount of lactose is converted into allolactose, which binds to the repressor and changes its shape. The repressor can no longer attach to the operator, so RNA polymerase can transcribe the genes. The enzymes are made and lactose is digested. When lactose is used up, the repressor attaches again and transcription stops. Lactose acts as an inducer, and the lac operon is called an inducible system.

There is a second level of control. If glucose is present, the cell prefers it to lactose. High glucose lowers the level of a signal molecule, cyclic AMP, so the activator protein CAP cannot help RNA polymerase, and the operon is expressed at only a low level even when lactose is present. The operon is fully active only when lactose is present and glucose is scarce.

How is gene expression regulated in eukaryotes?

Regulation in eukaryotes has many levels, and is more complex.

• Chromatin structure: DNA wound tightly around histones is not accessible. Chemical tags, such as methyl groups on DNA or acetyl groups on histones, change how tightly the chromatin is packed, and so whether genes are available for transcription. This is part of epigenetics, which is covered in a later article.
• Transcription factors: proteins that bind to specific DNA sequences. Activators bind to enhancers (regions that may be far from the gene) and help RNA polymerase to start. Repressors block transcription. Genes usually need a combination of several factors, which allows precise control.
• Hormones: steroid hormones such as estrogen enter cells and bind to receptor proteins, and the complex acts as a transcription factor.
• RNA processing: alternative splicing allows different mRNAs from the same gene.
• RNA stability and interference: small RNAs, such as microRNAs, bind to mRNA and block translation or cause degradation.
• Translation and protein control: the rate of translation, protein modification and breakdown.

Regulation in prokaryotes and eukaryotes compared

Worked example: predicting lac operon activity

Predict whether the lac operon is on or off in each condition, and what happens in a mutant.

1. Normal bacteria in a lactose-only medium (low glucose, lactose present): the repressor is released, CAP is active and the operon is highly expressed. The bacteria grow on lactose.
2. A mutant that makes a repressor that cannot bind to the operator: the operon is always on (constitutive expression), even without lactose, so the enzymes are made all the time. The cell wastes energy, but the mutant would still grow on lactose.
3. A mutant with a repressor that cannot bind allolactose: the repressor stays on the operator in all conditions. The operon is permanently off, so the bacteria cannot grow on lactose.

The reasoning applies a simple logic: the operon is on only when the repressor is off the operator, and the activator is working to give the highest level of expression. Applying a similar approach to any regulatory problem helps to identify which component is missing or changed.

How was gene regulation discovered?

François Jacob and Jacques Monod proposed the operon model in 1961 based on genetic experiments with E. coli, and they shared the 1965 Nobel Prize in Physiology or Medicine with André Lwoff. In the 1960s and 1970s, researchers discovered that eukaryotic cells use more complex mechanisms. The discovery of homeotic (Hox) genes in fruit flies by Edward Lewis, Christiane Nüsslein-Volhard and Eric Wieschaus showed how regulatory genes control body plans, and they received the 1995 Nobel Prize. Andrew Fire and Craig Mello discovered RNA interference in 1998.

Why does regulation matter in medicine and technology?

Many diseases involve faulty regulation. Cancer often results from mutations that switch on genes that promote cell division (oncogenes) or switch off genes that stop it (tumor suppressors). Drugs can target regulation: some hormone therapies for breast cancer block estrogen receptors, and certain drugs change histone modification in blood cancers. In biotechnology, inducible promoters, similar to the lac promoter, are used to make bacteria produce a protein only when an inducer is added, which gives control over production.

Which mistakes are common?

Mistake 1: Saying that cells in one organism have different genes

They have the same DNA. They differ in which genes are expressed.

Mistake 2: Thinking that the repressor destroys the gene

It only blocks transcription by binding to the operator. It does not change the DNA.

Mistake 3: Mixing up the inducer and the repressor

The inducer (allolactose) binds to the repressor and stops it from working. The repressor blocks the operator.

Mistake 4: Applying the operon model to all organisms

Operons are typical of bacteria. Most eukaryotic genes are controlled individually.