Final answer:
E. coli prefers glucose over lactose as a carbon source due to the regulation of the lac operon, which gene expression is controlled by the presence of glucose. The repression of the operon by a repressor protein and activation by CAP-cAMP are key to switching between the use of glucose and lactose.
Step-by-step explanation:
The bacterium Escherichia coli (E. coli) prefers glucose as its primary source of carbon and energy over lactose. The mechanism underlying this preference involves the regulation of the lac operon, which consists of genes necessary for the uptake and metabolism of lactose. The lac operon is typically repressed in the presence of glucose due to the binding of a repressor protein to the operator sequence, which blocks transcription of the operon. When glucose levels are low, the repression is lifted, and the operon can be transcribed.
Researchers Francois Jacob and Jacques Monod made significant discoveries regarding this phenomenon. They found that, in the absence of glucose, cyclic AMP (cAMP) concentration increases, which then binds to the catabolite activator protein (CAP). The cAMP-CAP complex then binds to the promoter of the lac operon, stimulating transcription of the genes required for lactose utilization.