Abstract
Quorum sensing is a typical communication system among Gram-negative bacteria used to control group-coordinated behavior via small diffusible molecules dependent on cell number. The key components of a quorum sensing system are a LuxI-type synthase, producing acyl-homoserine lactones (AHLs) as signaling molecules, and a LuxR-type receptor that detects AHLs to control expression of specific target genes. Six conserved amino acids are present in the signal-binding domain of AHL-sensing LuxR-type proteins, which are important for ligand-binding and -specificity as well as shaping the ligand-binding pocket. However, many proteobacteria possess LuxR-type regulators without a cognate LuxI synthase, referred to as LuxR solos. The two LuxR solos PluR and PauR from Photorhabdus luminescens and Photorhabdus asymbiotica, respectively, do not sense AHLs. Instead PluR and PauR sense alpha-pyrones and dialkylresorcinols, respectively, and are part of cell-cell communication systems contributing to the overall virulence of these Photorhabdus species. However, PluR and PauR both harbor substitutions in the conserved amino acid motif compared to that in AHL sensors, which appeared to be important for binding the corresponding signaling molecules. Here we analyze the role of the conserved amino acids in the signal-binding domain of these two non-AHL LuxR-type receptors for their role in signal perception. Our studies reveal that the conserved amino acid motif alone is essential but not solely responsible for ligand-binding.
Highlights
Bacteria constantly need to monitor changing environments and hosts to adapt the bacterial group-behavior
PluR and PauR were each tested on their ability to sense photopyrone D (PPYD) from P. luminescens, the P. asymbiotica-specific signaling molecules dialkylresorcinol (DAR) and its precursors, dialkylcyclohexane-1,3-diones (CHDA and CHDB) and isopropylstilbene (IPS), and the acyl-homoserine lactones (AHLs) N-3-oxooctanoyl-L-homoserinelactone (C8-HSL)
To prove suitability of the reporter system, we proved that no luminescence induction occurred either in the presence of one regulator, PluR or PauR, with a promoter-less reporter plasmid or on the other hand in the absence of PluR or PauR with a reporter plasmid containing the pcfA promoter of P. luminescens or of P. asymbiotica, respectively (Fig 2)
Summary
Bacteria constantly need to monitor changing environments and hosts to adapt the bacterial group-behavior. This process of cell-cell communication is mediated via quorum sensing (QS) systems among proteobacteria. Thereby, the bacterial behavior is controlled dependent on the population size by communication via small diffusible molecules. The basic molecular QS system of Gram-negative bacteria consists of a LuxI-like autoinducer synthase and a LuxR-type receptor that detects the signaling molecule to control expression of specific target genes [1]. Gram-negative bacteria use acyl-homoserine lactones (AHLs) for communication, which are constantly synthesized by LuxI at a basal level, and PLOS ONE | DOI:10.1371/journal.pone.0124093. Gram-negative bacteria use acyl-homoserine lactones (AHLs) for communication, which are constantly synthesized by LuxI at a basal level, and PLOS ONE | DOI:10.1371/journal.pone.0124093 April 29, 2015
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