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Brameyer, Sophie (2015): Cell-cell communication via LuxR solos in Photorhabdus species. Dissertation, LMU München: Faculty of Biology



Bacteria constantly need to monitor their environment and adapt the bacterial group-coordinated behaviour to changing habitats like nutrition alterations or host variations. Commonly cell-cell communication via acyl homoserine lactones (AHLs)is used to synchronise the behaviour of a bacterial population dependent on cell size. This process is referred to as quorum sensing (QS) and predominantly occurs in Gram-negative bacteria. The typical QS system consists of a LuxI-synthase that synthesises AHLs, and a LuxR-type receptor, which then responds to these AHLs. Upon AHL-binding, the LuxR-type receptor regulates the expression of different target genes and thus influences several processes, like biofilm formation, virulence, antibiotic production or cell-cell interaction. Interestingly, many proteobacteria possess additional LuxR homologs, but lack a cognate LuxI-type synthase. Those LuxR-type receptors are referred to as LuxR orphans or LuxR solos and can expand the regulatory QS network. Photorhabdus species are insect pathogenic bacteria, living in symbiosis with entomopathogenic nematodes. They all possess an exceptionally high number of LuxR solos, but lack LuxI homologs and therefore do not produce AHLs. The function of these LuxR solos, their role in cell-cell communication and the identification of their cognate signalling molecules in Photorhabdus species is the main focus of this work. In this thesis a novel signalling molecule used for QS could be identified for the first time in P. luminescens. This novel QS molecule is an α-pyrone named photopyrone (PPY) and produced endogenously by the photopyrone synthase (PpyS). The PPYs are specifically recognized by the LuxR solo regulator PluR, which then activates expression of the pcf (Photorhabdus clumping factor) operon leading to cell clumping of P. luminescens cells. Moreover, the PpyS/PluR quorum sensing system and its induced cell clumping contribute to the overall toxicity of P. luminescens. Furthermore, a second novel signalling molecule sensed by a LuxR solo of Photorhabdus species could be identified besides PPYs. The insect and human pathogenic bacteria P. asymbiotica lacks a PpyS homolog as well as a LuxI homolog, but harbours a pcf operon and a homologue to PluR, which is named PauR. The signalling molecule sensed by the LuxR-type receptor PauR could be identified, which is neither an AHL nor a PPY. PauR recognises a 2,5-dialkylresorcinol (DAR) produced by the DarABC pathway. Upon binding of the cognate signalling molecule, Summary XII PauR activates expression of the pcf operon. This also leads to cell clumping in P. asymbiotica. Furthermore, the DarABC/PauR QS system also contributes to the overall pathogenicity of P. asymbiotica against Galleria mellonella insect larvae. A bioinformatics approach revealed a high number of LuxR solos present in P. temperata and P. asymbiotica like in P. luminescens. Thereby, several conserved motives of amino acids could be identified, which are potentially important for signalbinding and -specificity. Variations in these amino acid motifs are assumed to reflect the overall variety of signals that can be sensed by LuxR solos. Furthermore, the specificity of the two LuxR solos PluR and PauR towards their cognate signalling molecules, PPYs and DARs, respectively, was analysed. Thereby, it could be shown that the previously identified conserved amino acid motives in the signal-binding domain (SBD), the TYDQCS-motif of PluR and the TYDQYI-motif of PauR, are essential but not sufficient for ligand-binding. Similar as for AHLs, it was unclear how the signalling molecules PPYs and DARs can cross the bacterial cell membrane. In the last part of this thesis the import mechanism for the Photorhabdus-specific signalling compounds PPYs and DARs were identified. Initial evidence could be provided that the membrane-integrated transporter FadL is mainly involved in the import of these hydrophobic compounds, and that they are not transported via simple diffusion across the cell membrane, which is assumed for AHLs. In conclusion, the data that is compiled presents two LuxR solos of Photorhabdus species adapted to sense and respond to novel non-AHL signalling molecules used for QS. Therefore, this thesis reveals that cell-cell communication via LuxR-type receptors goes far beyond AHL-signalling in nature.