Question

In the absence of glucose E. coli can metabolize and grow on arabinose, a pentose sugar, using an inducible set of genes that are arranged in three groups on the chromosome (Fig shown). In one of these the araA, araB, andaraD genes encode enzymes for the metabolism of arabinose while the araC gene encodes a regulatory protein that binds adjacent to arabinose promoters and coordinates the expression of the genes in the arabinose operon. The other two groups of genes encode proteins involved in arabinose transport. To understand the regulatory properties of thearaC protein, you isolate a mutant bacterium with a deletion of the araC gene. As shown in Table 1, the mutant strain does not induce expression of the araA gene when arabinose is added to the medium.

Table1 Response of Normal and Mutant Bacteria to the Presence and Absence of Arabinose

	araA Gene Product
Genotype	Minus Arabinose	Plus Arabinose
araC+	1	1000
araC-	1	1

a. Do the results in Table 1 suggest that the araC protein is a positive regulator or a negative regulator of the arabinose operon? (3 pt)
b. What would the data in Table 1 have looked like if the araC protein were the opposite kind of gene regulatory protein? Redraw the table and include the data you would expect with explanation (5 pt)

Answer 1

a. Results of Table 1 suggests that araC⁺ is a positive regulator of araA gene product . This is because in the presence of Arabinose, araC⁺ gene induces the expression of araA gene 1000 folds. It is also evident that when araC gene is mutated to araC^-, the araA gene cannot be induced, even in the presence of arabinose. So definitely, AraC⁺ is a positive regulator of AraC gene.

b. If araC was a negative regulator, then after addition of arabinose the expression of araA gene was reduced 1000 times. The cells which has araC- has no effect on araA gene in either presence or absence of Arabinose.

The results is shown in the redrawn table below: