The existing lac system is one that, through a long evolutionary process, has been selected to operate in an optimal fashion for the energy efficiency of the bacterial cell. Presumably to maximize energy efficiency, two environmental conditions have to be satisfied for the lactose metabolic enzymes to be expressed.
One condition is that lactose must be present in the environment. This condition makes sense, because it would be inefficient for the cell to produce the lactose metabolic enzymes if there is no substrate to metabolize. We have already seen that the cell's recognition that lactose is present is accomplished by a repressor protein.
The other condition is that glucose cannot be present in the cell's environment. Because the cell can capture more energy from the breakdown of glucose than it can from the breakdown of other sugars, it is more efficient for the cell to metabolize glucose rather than lactose. Thus, mechanisms have evolved that prevent the cell from synthesizing the enzymes for lactose metabolism when lactose and glucose are present together. The repression of the transcription of lactose-metabolizing genes in the presence of glucose is an example of catabo-lite repression. The transcription of proteins necessary for the metabolism of many different sugars is similarly repressed in the presence of glucose. We shall see that catabolite repression works through an activator protein.
Was this article helpful?