The lac operon is an operon, or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source. coli normally metabolise glucose, but if glucose is not available in a new environment, the lac operon can allow E. coli to metabolise lactose, the sugar found in milk using the structural genes beta-galactosidase and lactose permease. The lac operon of E. coli contains genes involved in lactose metabolism. ... 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.
