Crystal structure and induction mechanism of AmiC-AmiR: a ligand-regulatedtranscription antitermination complex

Inducible expression of the aliphatic amidase operon in Pseudomonas aerugin osa is controlled by an antitermination mechanism which allows production o f the full-length transcript only in the presence of small-molecule inducer s, such as acetamide, Ligand-regulated antitermination is provided by AmiC, the ligand-sensitive negative regulator, and AmiR, the RNA-binding positiv e regulator. Under non-inducing or repressing growth conditions, AmiC and A miR form a complex in which the activity of AmiR is silenced. The crystal s tructure of the AmiC-AmiR complex identifies AmiR as a new and highly unusu al member of the response-regulator family of bacterial signal transduction proteins, regulated by sequestration rather than phosphorylation, Comparis on with the structure of free AmiC reveals the subtle mechanism of ligand-i nduced release of AmiR.