Functional modes of the regulatory arm of AraC.

One of the two crystal structures of the arm-dimerization domain determined in the absence of arbinose fails to show the arm, whereas the other structure does show it. The two structures lead to different pictures for the regulatory behavior of the arms. Trypsin digestion, fluorescence anisotropy, and NMR experiments presented here were designed to resolve the issue and show that in arm-dimerization domain, the arms are structured, although differently, in the presence and absence of arabinose. The arms have also been shown to interact with the DNA binding domains of AraC by their requirement for the immobilization of the DNA binding domains that is necessary for DNA looping and repression. The binding of arabinose has been shown to release the DNA binding domains and looping ceases. The picture resulting from the new experiments and the crystal structures of the arm-dimerization domain is that in the absence of arabinose, the arm adopts one structure on the dimerization domain and that the DNA binding domain then binds to this complex. Upon binding arabinose, the arm restructures and as a result, no longer serves as a gasket between the DNA binding domain and dimerization domain. The DNA binding domain is then released, subject only to the constraints imposed by the flexible linker connecting it to dimerization domain, and the protein relocates on the DNA and activates transcription.