Regulation of gene Expression | Lac Operon | Biology
The regulation of gene expression
Gene regulation is the process of controlling which genes in a cell’s DNA are expressed (used to make a functional product such as a protein).
The lac operon
• The lac operon of E. coli contains genes involved in lactose metabolism. It’s expressed only when lactose is present and glucose is absent.
• Two regulators turn the operon “on” and “off” in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP).
• The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose.
What makes the lac operon turn on?
E. coli should express the lac operon only when two conditions are met:
• Lactose is available, and
• Glucose is not available
Structure of the lac operon
The lac operon contains three genes: lacZ, lacY, and lacA. These genes are transcribed as a single mRNA, under control of one promoter.
The lacZ gene encodes an enzyme called β-galactosidase, which is responsible for splitting lactose (a disaccharide) into readily usable glucose and galactose (monosaccharides).
The lacY gene encodes a membrane protein called lactose permease, which is a transmembrane “pump” that allows the cell to import lactose.
The lacA gene encodes an enzyme known as a transacetylase that attaches a particular chemical group to target molecules.