The Serratia LuxR family regulator CarR39006 activates transcription independently of cognate quorum sensing signals

• Published in: Molecular microbiology 2011-05, Vol.80 (4), p.1120-1131
• Main Authors: Poulter, Simon, Carlton, Timothy M., Spring, David R., Salmond, George P.C.
• Format: Article
• Language: English
• Subjects: Bacteriology; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; Microbiology; Miscellaneous
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/j.1365-2958.2011.07634.x

Summary 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 CarR39006 from Serratia 39006 has no detectable affinity for cognate AHL molecules. Furthermore, CarR39006 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 CarR39006 may be responsible for conferring AHL independence. In contrast, we show that the homologous CarREcc 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, CarREcc is not directly protected from cellular proteolysis by AHL binding, but via AHL‐induced DNA binding. At physiological protein concentrations, AHL binding induces CarREcc to bind to its target promoter with higher affinity and activate transcription.