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J. Biol. Chem., Vol. 276, Issue 4, 2562-2564, January 26, 2001

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Strengthened Arm-Dimerization Domain Interactions in AraC^*

Martin Wu and Robert Schleif

From the Biology Department, Johns Hopkins University, Baltimore, Maryland 21218

Constitutive mutations were sought and found in the N-terminal arm of the Escherichia coli regulatory protein of the arabinose operon, AraC protein. A new mutation, N16D, was of particular interest. Asn-16 is not seen in the crystal structure of the AraC dimerization domain determined in the absence of arabinose, because the N-terminal arm 18 residues are disordered, but in the presence of arabinose, residues 7-18 fold over the arabinose and make many interactions with it. In this state Asn-16 lies near two positively charged amino acids, Lys-43 and Arg-99. We propose that the introduction of the negatively charged aspartic residue at position 16 creates a charge-charge interaction network among Asp-16, Lys-43, and Arg-99 that holds the arm to the dimerization domain even in the absence of arabinose. This frees the DNA-binding domains and allows them to bind cis to I₁-I₂ half-sites and activate transcription. Mutating the two positively charged residues to alanines individually and collectively decreased or eliminated the constitutivity created by the N16D mutation.

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