Question

3) Gene Control in Bacteria - Transcription Activators and Repressors You are interested in examining the regulation of the gene that encodes an enzyme, Tre- ase, important in metabolizing trehalose into glucose in bacteria. Trehalose is a disaccharide formed of two glucose units. It is known that two DNA-binding proteins, TreA and TreB, are important for binding to the promoter of the Tre-ase gene and are involved in regulating the transcription of the Tre-ase gene: TreA binds to the "A" site in the promoter region, and TreB binds to the "B" site. You make mutations in the TreA and TreB genes to create cells lacking these genes, observe what happens to transcription of the Tre-ase gene,and obtain the results in the table below: Transcription of Tre-ase in different media Glucose Trehalose normal cells lacking TreA cells lacking TreB cells lacking both TreA &TreB on OFF OFF OFF OFF on ON ON OFF OFF Glucose + Trehalose OFF ON OFF OFF a. What is the role for TreA in controlling Tre-ase expression? Explain. b. What is the role for TreB in controlling Tre-ase expression? Explain. c. From these data, what do you predict will happen to Tre-ase transcription (compared with that in normal cells) in the presence of trehalose if you were to create a version of the TreA protein that will constitutively bind to the "A" site in the Tre-ase promoter?

Answer 1

Ans a) Tre A is an activator protein. When it is present, in the absence of glucose, cells actively transcribe the trehalose operon (trehalose only, normal cells). However, when environmental conditions are the same but Tre A is absent (trehalose only, lacking Tre A), transcription continues [because the repressor is unbound] but only at a low level—cells cannot be activated to transcribe the operon at a high level.

Ans b)Tre B is a repressor protein that, when absent, results in cells that have constitutive expression (i.e., “always on”) of the Trehalose operon , even in the absence of trehalose (glucose only).

Ans c) Since the data presented support that Tre B is a repressor protein, if a version of the protein were always bound to the A element of the promoter, transcription of the trehalose operon will be able to proceed—even in the presence of the carbohydrate.