6.1.1(b) - Regulation of gene expression

Control Points In Gene Expression

Gene expression means using the information in a gene to make a functional product, usually a protein. Control of gene expression means regulating this process so that a cell does not make every protein all the time.

For this lesson, think of gene expression as a route with three main control points:

1. Transcriptional control: regulation of whether, or how often, a gene is transcribed into RNA.
2. Post-transcriptional control: regulation after the primary RNA transcript has been made, but before mature mRNA is translated.
3. Post-translational control: regulation after a polypeptide has been made, often by activating or modifying the protein.

Cells regulate this route so that a gene product is made or activated only when and where it is needed.

[DIAGRAM: three_levels_gene_expression_control: Lesson 137: Regulation of gene expression - diagram 02; asset_slug: 137_m06_1_1_regulation_of_gene_expression__diagram_02; recommended_method: drawn_biology; description: A clean 16:9 flow diagram showing DNA transcription to primary mRNA, processing to mature mRNA, translation to inactive protein and cyclic AMP-linked activation to active protein, with labels for transcriptional, post-transcriptional and post-translational control.]

The diagram shows why the level matters. If a mechanism changes whether RNA polymerase makes RNA, it is transcriptional. If it changes the RNA after transcription, it is post-transcriptional. If it changes a protein after translation, it is post-translational.

The Lac Operon Switch

In this simplified lactose comparison, glucose is low in both conditions, so the diagrams isolate the role of the repressor.

The lac operon is a prokaryotic example of transcriptional control. It is found in bacteria such as E. coli and controls genes involved in using lactose as an energy source.

The important parts are:

[DIAGRAM: lac_operon_off_on: Lesson 137: Regulation of gene expression - diagram 01; asset_slug: 137_m06_1_1_regulation_of_gene_expression__diagram_01; recommended_method: drawn_biology; description: A two-panel 16:9 drawn biology diagram of the lac operon with lactose absent and lactose present, showing regulatory gene, repressor protein, promoter, operator, lacZ/lacY/lacA structural genes, RNA polymerase blocked in the OFF state and transcription allowed in the ON state.]

The regulatory gene has its own promoter and produces repressor protein whether lactose is present or absent. This avoids spending resources making large quantities of lactose-use enzymes when their substrate is unavailable. When lactose is absent, the repressor is active. The repressor protein binds to the operator. This prevents RNA polymerase from transcribing the structural genes, so the enzymes for lactose use are not made in significant amounts.

When lactose is present, its isomer allolactose acts as an inducer and binds to the repressor, changing its shape. This is often summarised as lactose binding to the repressor. The repressor protein can no longer bind to the operator. RNA polymerase can then bind at the promoter and transcribe the structural genes. The mRNA can be translated into proteins such as beta-galactosidase, which helps break down lactose, and permease, which helps lactose enter the bacterial cell.

Now apply that classification to the operon mechanism when lactose is present.