Final answer:
The trp operon is a repressible system that is ON by default for tryptophan synthesis, while the lac operon is an inducible system that is OFF by default but turns ON in the presence of lactose and absence of glucose. Activator control further regulates the lac operon in response to glucose levels.
Step-by-step explanation:
The trp operon and the lac operon are both crucial regulatory mechanisms in bacteria, but they function in distinct ways according to the needs of the cell. The trp operon is considered repressible and is typically ON by default, as it is involved in the synthesis of tryptophan, an essential amino acid. This operon uses repressive control, wherein proteins bound to the operator sequence can prevent the binding of RNA polymerase when the cell has sufficient tryptophan, thus turning off transcription.
Conversely, the lac operon is an example of inducible control, and it remains OFF by default since it is required for the metabolism of lactose only when this sugar is present and glucose is limited. The presence of lactose induces the inactivation of the repressor, allowing transcription. Additionally, the lac operon is also regulated by activator control, with the Catabolite Activator Protein (CAP) increasing the binding efficiency of RNA polymerase to the promoter when glucose levels are low.
Thus, the trp operon is generally active, producing tryptophan when needed, and the lac operon activates only when lactose is available and glucose is scarce. This ensures efficient energy use and gene expression based on the metabolic requirements of the bacterium.