Operons: The Lac Operon
Operons: Bacterial genes coding for proteins whose functions are related may be organised in a cluster that is transcribed into a polycistronic mRNA from a single promoter. Control of this promoter regulates all the genes. Unit of regulation (contains structural genes and cis-acting elements) called the operon.
The Lac Operon: Lactose pathway in E. coli enables bacteria to take up and metabolise β-galactoside sugars (e.g. lactose). 3 lac structural genes: • lacZ encodes the enzyme β-galactosidase (breaks down β-galactosides). • lacY encodes permease (transports β-galactosides into cell). • lacA encodes transacetylase (transfers acetyl groups). 3 genes regulated by single promoter and operator, which overlap. LacI is upstream and codes for lac operon repressor. It is on a separate gene and has its own promoter. Lac operon has 3 operators. Tetramer can bind 2 operators. Full repression requires binding to primary operator (lacO1) and one additional (lacO2/O3).
The Lac Operon: Lactose pathway in E. coli enables bacteria to take up and metabolise β-galactoside sugars (e.g. lactose). 3 lac structural genes: • lacZ encodes the enzyme β-galactosidase (breaks down β-galactosides). • lacY encodes permease (transports β-galactosides into cell). • lacA encodes transacetylase (transfers acetyl groups). 3 genes regulated by single promoter and operator, which overlap. LacI is upstream and codes for lac operon repressor. It is on a separate gene and has its own promoter. Lac operon has 3 operators. Tetramer can bind 2 operators. Full repression requires binding to primary operator (lacO1) and one additional (lacO2/O3).
