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Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription
Dual Positive And Negative Control: the Arabinose Operon

Prokaryotic control of transcription seems to mix and match different aspects of positive and negative regulation in different ways. Thus the general themes from our detailed examination of the lactose operon should be on how positive and negative regulation can work through the activity of sequence-specific, allosterically regulated DNA-binding proteins. Even in the metabolism of other sugars, we see transcriptional regulatory mechanisms that are certainly different in detail. The arabinose operon is such an example (Figure 14-16).

The structural genes (araB, araA, and araD) that encode the metabolic enzymes that break down arabinose are transcribed as a multigenic mRNA. Transcription is activated at araI, the initiator region, which contains both an operator site and a promoter. The araC gene encodes an activator protein that, when bound to arabinose, activates transcription of the ara operon, perhaps by helping RNA polymerase bind to the promoter, located within in the araI region. An additional activation event is mediated by the same CAPcAMP catabolite repression system that regulates lac operon expression.

In the presence of arabinose, both the CAPcAMP complex and the AraCarabinose complex must bind to the initiator region in order for RNA polymerase to bind to the promoter and transcribe the ara operon (Figure 14-17a). In the absence of arabinose, the AraC protein assumes a different conformation and represses the ara operon by binding both to araI and to a second operator region, araO, thereby forming a loop (Figure 14-17b) that prevents transcription. Thus, the AraC protein has two conformations, one that acts as an activator and the other that acts as a repressor. The two conformations, dependent on whether the allosteric effector has bound to the protein, also differ in their abilities to bind a specific target site in the araO region of the operon.

MESSAGE
Operon transcription can be regulated by both activation and repression. Operons regulating the metabolism of similar compounds, such as sugars, can be regulated in quite different ways.

© 1999 by W. H. Freeman and Company. All rights reserved.