Question

Explain control of ara operon.

Answer 1

Answer:

The metabolism of the sugar arabinose is catalyzed by three enzymes encoded by thearaB, araA, and araD genes. Figure 11-15 depicts this operon. Expression is activated at araI, the initiator region. Within this region, the product of the araC gene, when bound to arabinose, can activate transcription, perhaps by directly affecting RNA polymerasebinding in the araI region, which contains the promoter for the araB, araA, and araDgenes (Figure 11-16a). This activation exemplifies positive control, because the product of the regulatory gene (araC) must be active for the operon to be expressed.

Figure 11-15

Map of the ara region. The B, A, and D genes together with the I and O sites constitute the ara operon.

Figure 11-16

Dual control of the ara operon. (a) In the presence of arabinose, the AraC protein binds to the araI region and, when bound to cAMP, the CAP protein binds to a site adjacent to araI. This binding stimulates the transcription of the araB, araA, and araD (more...)

An additional positive control is mediated by the same CAP-cAMP system that regulates lacexpression. In the presence of arabinose, both CAP and the binding of the araC product to the initiator region are required to allow RNA polymerase to bind to the promoter for the araB, araA, and araD genes. In the absence of arabinose, the araC product assumes a different conformation and represses the ara operon by binding both to araI and to anoperator region, araO, thereby forming a loop (Figure 11-16b) that prevents transcription. Thus, the AraC protein has two conformations that promote two opposing functions at two alternative binding sites. The conformation depends on whether the inducer, arabinose, is bound to the protein.