Passage III

In the human gut, E. coli bacteria break down lactose (milk sugar) into 2 sugars: glucose and galactose. Th reaction is catalyzed by the enzyme β-galactosidase.

The E coli gene lacZ contains information needed to synthesize β-galactosidase. lacZ is located within the lac operon (a portion of DNA). The lac operon also contains the lac operon operator (LOO), a DNA region that regulates lacZ (see Figure1).

If LOO is active, lacZ is active, so β-galactosidase is synthesized. If LOO is inactive, lacZ is inactive, so β-galactosidase is not synthesized. The activity of LOO is regulated by protein 1 (P1), a protein found in E. coli cells.

Three students propose models to describe the regulation of LOO by P1 in E. coli.

Student 1

If lactose is absent from a cell, P1 binds to LOO, inactivating LOO. If lactose is present, it binds to P1, causing a conformational change (shape change) in P1. As a result, P1 cannot bind to LOO. When P1 is not bound to LOO, LOO is active (see Figure 2).

Student 2

If lactose is absent from a cell, P1 cannot bind to LOO, so LOO is inactive. When lactose is present, it binds to P1, causing a conformational change in P1. As a result, P1 binds to LOO. When P1 is bound to LOO, LOO is active (see Figure 3).

Student 3

If lactose is absent from a cell, then galactose is also absent. If galactose is absent from a cell, then P1 cannot bind to LOO, so LOO is inactive. If lactose is present in a cell, a few lactose molecules break down into glucose and galactose without the presence of a catalyst. The galactose then binds to P1, causing a conformational change in P1. As a result, P1 binds to L OO. When P1 is bound to LOO, LOO is active (see Figure 4).

Figures adapted from Nell.A Campbell, Biology, 3rd ed. @1993 by Benjamin/Cummings Publishing Company, Inc.