Molecular Biology of the Gene 7th Edition by Richard Losick, James Watson, Michael Levine, Tamara Baker, Alexander Gann

Exercise 9

The ara operon controls the expression of several genes including araC. In the presence of the sugar arabinose and the DNA-binding protein AraC, gene expression is turned on. The AraC protein is involved in positive autoregulation.
As depicted below, researchers constructed a circuit consisting of a series of artificial promoters (each containing an AraC binding site and a lac operator). Remember that the presence of arabinose and AraC promotes transcription of downstream genes, lacl encodes the LacI repressor, and binding of LacI to the lac operator turns off transcription (even in the presence of arabinose and AraC).

A. If arabinose is added to a culture of bacteria containing all three constructs, an oscillatory pattern of YFP expression is observed. Assume that a small amount of AraC is present. Briefly explain how YFP expression is turned on following addition of arabinose.
B. After the initial increase in YFP production following addition of arabinose, how is YFP turned off?
C. Once YFP expression is off, how is it turned on again?
D. How do you think the oscillation would be affected if a small amount of IPTG (an inducer of the lac operon) was added to the medium along with arabinose?
E. Draw the three-node circuit, labeling the nodes (AraC, LacI and YFP) and including the appropriate edges (?. to represent activation and ? to represent repression).