An active role for a structured B-linker in effector control of the sigma(54)-dependent regulator DmpR

The activities of many prokaryotic sigma (54)-dependent transcriptional act ivators are controlled by the N-terminal A-domain of the protein, which is linked to the central transcriptional activation domain via a short B-linke r. It used to be thought that these B-linkers simply serve as flexible teth ers. Here we show that the B-linker of the aromatic-responsive regulator Dm pR and many other regulators of the family contain signature heptad repeats with regularly spaced hydrophobic amino acids. Mutant analysis of this reg ion of DmpR demonstrates that B-linker function is dependent on the heptad repeats and is critical for activation of the protein by aromatic effecters . The phenotypes of DmpR mutants refute the existing model that the level o f ATPase activity directly controls the level of transcription it promotes. The mutant analysis also shows that the B-linker is involved in repression of ATPase activity and that allosteric changes upon effector binding are t ransduced to alleviate both B-linker repression of ATP hydrolysis and A-dom ain repression of transcriptional activation. The mechanistic implications of these findings for DmpR and other family members are discussed.