The arabinose regulon provides a good example of a regulatory protein that acts as both a repressor and an activator. Furthermore, positive regulation by AraC protein was the paradigm that led to our understanding of the mechanism of transcriptional activation. The results described below refer to the arabinose regulon in E. coli and Salmonella.
The arabinose regulon allows bacteria to transport L-arabinose (via the unlinked araE and areFGH gene products) then convert the arabinose to D-xylulose-5-phosphate in three enzymatic steps catalyzed by the araBAD gene products. The D-xylulose-5-phosphate is then further catabolized via the pentose phosphate pathway.
The organization of the divergent araC - araBAD operon is shown below. The genes encoding arabinose transport systems, araE and araFGH, are located at distant positions on the chromosome. Each of these genes is transcriptionally regulated by the AraC protein.
The AraC protein has two discrete domains, a DNA-binding domain and a dimerization/arabinose-binding domain. The two domains are connected by a flexible linker. In the absence of arabinose, the linker is fixed into an extended conformation, allowing distal AraC molecules to interact, but prevent interaction between AraC molecules bound to adjacent sites on the DNA. This facilitates formation of a loop, with AraC bound to an operator site within the araC structural gene (O2) and a site in front oof the araB gene (I1) as shown in the figure below. Under these conditions, expression of the araC and araBAD genes is repressed.
In the presence of arabinose, the arabinose binds to the dimerization domain of AraC, and allosterically changes the conformation of the linker. In this conformation, an AraC molecule cannot effectively interact with another AraC molecule bound to a distal site, but two AraC molecules bound to adjacent sites dimerize. Thus, AraC binds to the adjacent I1 and I2 sites in front oof the araB gene as shown in the figure below. Under these conditions, AraC contacts RNA Polymerase and activates expression of the araC and araBAD genes. In the presence of arabinose, AraC also binds upstream of araE and araFGH and activates transcription of the arabinose transport genes.
SOME USEFUL REVIEWS ON AraC regulation:
A brief summary of the mechanism of the AraC activator can also be found on Bob Schleif's website.
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Last modified October 26, 2005