How can repression of P_BAD be generated from the araO2 site that is located more than 200 base pairs upstream? There are three possibilities (Fig. 1). A signal could be transmitted through the DNA, for example, by changing the angle of the base tilt; something could polymerize along the DNA starting at araO2 and finally cover araI; or the DNA could bend back and bring the araO2 region near P_BAD. This latter possibility was shown to be the case by a series of experiments in which the spacing between araO2 and the promoter region was varied. If five base pairs, which is half a helical turn, are added between these two sites, the ability to repress is greatly diminished, but if 11 base pairs are added, the ability to repress is restored. An addition of 15 base pairs eliminates repression, and a longer addition of 31 base pairs restores repression.

Fig1. Three possible mechanisms for repression from a site several hundred nucleotides from a promoter (a) properties of the DNA can be altered, (b) protein can polymerize along the DNA, or (c) loops can form.

These results show that the absolute distance between the two sites is not greatly important but that the angular orientation between the two is. This implies that the DNA loops back to form contacts between AraC protein bound at araO2 and something in the promoter region. If the protein at araO2 is on the opposite side of the DNA (Fig. 2), the additional energy required to twist the DNA half a turn so the sites again face one another is too great and the loop required for repression does not form. The isolation of repression deficient point mutations located the sites necessary for repression. As expected, some were in araO2. Additional repression negative mutations were in araI, thereby identifying the other end of the loop.

Fig2. How a five-base-pair insertion can introduce half a helical turn and prevent formation of a loop.

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مواضيع ذات صلة

How AraC Protein Loops and Unloops

Binding Sites of the ara Regulatory Proteins

Endoplasmic Reticulum-Associated Degradation and the Unfolded Protein Response

Protein Folding and Modifications in the Endoplasmic Reticulum

Protein Modifications

Regulating AraC Synthesis

Repression by AraC

Detection and Isolation of AraC Protein

Genetics of the Arabinose System

The Eukaryotic Genome

Nucleic Acids and Their Organization in Eukaryotic, Prokaryotic, and Viral Genomes

The Sugar Arabinose and Arabinose Metabolism