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

Abstract
Inducible expression of the aliphatic amidase operon in Pseudomonas aeruginosa is controlled by an antitermination mechanism which allows production of the fullā€length transcript only in the presence of smallā€molecule inducers, such as acetamide. Ligandā€regulated antitermination is provided by AmiC, the ligandā€sensitive negative regulator, and AmiR, the RNAā€binding positive regulator. Under nonā€inducing or repressing growth conditions, AmiC and AmiR form a complex in which the activity of AmiR is silenced. The crystal structure of the AmiCā€“AmiR complex identifies AmiR as a new and highly unusual member of the responseā€regulator family of bacterial signal transduction proteins, regulated by sequestration rather than phosphorylation. Comparison with the structure of free AmiC reveals the subtle mechanism of ligandā€induced release of AmiR.