Control of gene expression
Cells can control any of the processes we have presented so far, transcription and translation, as well as other steps in protein synthesis. In this tutorial we will focus on a very simple version of gene expression control, control over the first step of transcription. This system is very important in the bacteria.
Our example system will be how some bacteria control the production of new enzymes to help obtain energy from the disaccharide, lactose. This enzyme is called "beta-galactosidase" and breaks down lactose into glucose and galactose, which then feed into the glycolysis pathway. The logic of the control system is straightforward:
- If there isn't any lactose around the cell doesn't make beta-galactosidase
- If there is lactose present the cell makes beta-galactosidase
This contol is effected by a repressor protein that can exclusively bind to either the DNA or lactose (but not both). When lactose is present the repressor protein binds to lactose and the genes for making beta-galactosidase are expressed. When there is no lactose present, the repressor protein binds to the DNA and blocks the transcription of mRNA for this enzyme.
Our working diagram of transcriptional control is:
John Rueter 11/25/96