Journal
NUCLEIC ACIDS RESEARCH
Volume 35, Issue 5, Pages 1737-1749Publisher
OXFORD UNIV PRESS
DOI: 10.1093/nar/gkm073
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The parD operon of Escherichia coli plasmid R1 encodes a toxin-antitoxin system, which is involved in plasmid stabilization. The toxin Kid inhibits cell growth by RNA degradation and its action is neutralized by the formation of a tight complex with the antitoxin Kis. A fascinating but poorly understood aspect of the kid-kis system is its autoregulation at the transcriptional level. Using macromolecular (tandem) mass spectrometry and DNA binding assays, we here demonstrate that Kis pilots the interaction of the Kid-Kis complex in the parD regulatory region and that two discrete Kis-binding regions are present on parD. The data clearly show that only when the Kis concentration equals or exceeds the Kid concentration a strong cooperative effect exists between strong DNA binding and Kid(2)-Kis(2)-Kid(2)-Kis(2) complex formation. We propose a model in which transcriptional repression of the parD operon is tuned by the relative molar ratio of the antitoxin and toxin proteins in solution. When the concentration of the toxin exceeds that of the antitoxin tight Kid(2)-Kis(2)-Kid(2) complexes are formed, which only neutralize the lethal activity of Kid. Upon increasing the Kis concentration, (Kid(2)-Kis(2))(n) complexes repress the kid-kis operon.
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