The Serratia LuxR family regulator CarR 39006 activates transcription independently of cognate quorum sensing signals

Mol Microbiol. 2011 May;80(4):1120-31. doi: 10.1111/j.1365-2958.2011.07634.x. Epub 2011 Mar 28.


In Gram-negative bacteria, quorum sensing control of gene expression is mediated by transcription factors of the LuxR family, whose DNA-binding affinity is modulated by diffusible N-acyl homoserine lactone (AHL) signalling molecules. In Serratia sp. ATCC 39006 and the plant pathogen Erwinia carotovora ssp. carotovora (Ecc), the biosynthesis of the β-lactam antibiotic 1-carbapen-2-em-3-carboxylic acid (Car) is under quorum sensing control. This study has revealed that, uniquely, the LuxR family transcriptional activator CarR(39006) from Serratia 39006 has no detectable affinity for cognate AHL molecules. Furthermore, CarR(39006) was shown to be naturally competent to bind to its target promoter with high affinity, activate transcription and resist cellular proteolysis, and was unaffected by AHL signals. Experiments with chimeric proteins suggest that the C-terminal DNA-binding domain of CarR(39006) may be responsible for conferring AHL independence. In contrast, we show that the homologous CarR(Ecc) protein binds to its 3O-C6-HSL ligand with high affinity, and that the highly conserved Trp-44 residue is critical for this interaction. Unlike TraR from Agrobacterium tumefaciens, CarR(Ecc) is not directly protected from cellular proteolysis by AHL binding, but via AHL-induced DNA binding. At physiological protein concentrations, AHL binding induces CarR(Ecc) to bind to its target promoter with higher affinity and activate transcription.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • DNA-Binding Proteins
  • Escherichia coli
  • Gene Expression Regulation, Bacterial
  • Pectobacterium carotovorum / genetics
  • Pectobacterium carotovorum / metabolism
  • Promoter Regions, Genetic
  • Protein Binding
  • Quorum Sensing*
  • Repressor Proteins / genetics
  • Repressor Proteins / metabolism*
  • Serratia / genetics*
  • Serratia / metabolism
  • Signal Transduction / genetics
  • Trans-Activators / genetics
  • Trans-Activators / metabolism*
  • Transcription, Genetic*


  • Bacterial Proteins
  • DNA-Binding Proteins
  • Repressor Proteins
  • Trans-Activators
  • LuxR autoinducer binding proteins