Lactose is a disaccharide of galactose and glucose. When the bacterium is in an environment that contains lactose as the sugar source it expresses the following structural genes.

1. Beta-galactosidase

2. Lactose permease

3. Thiogalactoside transacetylase

These enzymes are located adjacent to each other in the E. coli genome and coded by lacZ, lacY and lacA respectively. These enzymes are regulated by the sequence called lac Operon.

In order to metabolize lactose, the structural genes must be expressed and the protein products produced. Presences of Lactose turn on the operon by binding and changing conformation of repressor.

Questions

1.      Lactose permease is the enzyme that needs to transfer Lactose from outside to inside the membrane. If E.coli is in lactose free medium, on that time lac operon is turned off means no gene product of these enzymes. If the E.coli is shifted to the medium containing Lactose, how Lactose will come inside the membrane to turn on the Lactose operon.

2.      What will happen if Glucose and Lactose both are present in the medium

3.      All the three genes lacZ, lacY and lacA are transcribed in a single mRNA. After transcription how it is possible that only lacZ, and lacA are translated but not lacY.

4.      What will be the problem if these genes are in different place in the genome and are regulated by three different operon.