Abstract
The biocontrol agent Rhodococcus erythropolis disrupts virulence of plant and human Gram-negative pathogens by catabolizing their N-acyl-homoserine lactones. This quorum-quenching activity requires the expression of the qsd (quorum-sensing signal degradation) operon, which encodes the lactonase QsdA and the fatty acyl-CoA ligase QsdC, involved in the catabolism of lactone ring and acyl chain moieties of signaling molecules, respectively. Here, we demonstrate the regulation of qsd operon expression by a TetR-like family repressor, QsdR. This repression was lifted by adding the pathogen quorum signal or by deleting the qsdR gene, resulting in enhanced lactone degrading activity. Using interactomic approaches and transcriptional fusion strategy, the qsd operon derepression was elucidated: it is operated by the binding of the common part of signaling molecules, the homoserine lactone ring, to the effector-receiving domain of QsdR, preventing a physical binding of QsdR to the qsd promoter region. To our knowledge, this is the first evidence revealing quorum signals as inducers of the suitable quorum-quenching pathway, confirming this TetR-like protein as a lactone sensor. This regulatory mechanism designates the qsd operon as encoding a global disrupting pathway for degrading a wide range of signal substrates, allowing a broad spectrum anti-virulence activity mediated by the rhodococcal biocontrol agent. Understanding the regulation mechanisms of qsd operon expression led also to the development of biosensors useful to monitor in situ the presence of exogenous signals and quorum-quenching activity.
Highlights
Bacteria use cell-to-cell communication systems based on both synthesis and perception of signaling molecules to synchronize their social behavior
P. aeruginosa is well known to use two acyl-L-homoserine lactones (AHLs)-based QS systems regulating the production of many virulence factors: the las and rhl systems produce and respond to the N-3-oxo-dodecanoyl-L-homoserine lactone (3-oxo-C12-HSL) and N-butanoyl-L-homoserine lactone (C4-HSL), respectively (Jimenez et al, 2012; Mund et al, 2017)
To investigate the role of the qsd operon in biocontrol, anti-virulence activity of R. erythropolis R138 was compared with a R. erythropolis mutant strain defective in the production of the lactonase QsdA
Summary
Bacteria use cell-to-cell communication systems based on both synthesis and perception of signaling molecules to synchronize their social behavior. These quorum-sensing (QS) systems control diverse functions, which require the concerted actions of numerous cells in order to be productive, such as antibiotic synthesis, motility, symbiosis, biofilm maturation, sporulation, and virulence (Papenfort and Bassler, 2016). The most-studied system relies on the production of molecules belonging to the family of N-acyl-L-homoserine lactones (AHLs). Many AHL-controlled functions have already been reported in the Pectobacterium atrosepticum species They comprise up to a quarter of its genome, in particular a huge arsenal of plant cellwall degrading enzymes as well as their secretion systems (Liu et al, 2008). A single mutation of the AHL synthase gene, AHL receptor/transcription factor, or the degradation of signal molecules before their release in the microenvironment is sufficient to remove all symptoms on host plants (Smadja et al, 2004b; Latour et al, 2007; Põllumaa et al, 2012)
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