A Rhodococcal Transcriptional Regulatory Mechanism Detects the Common Lactone Ring of AHL Quorum-Sensing Signals and Triggers the Quorum-Quenching Response

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.

Quorum sensing mediated response of Achromobacter denitrificans SP1 towards prodigiosin production under phthalate stress

“…Colorimetric detection of QS molecules by Yang et al [18] is the most simple and easy to use the method, which analyses the number of lactone compounds in the sample. This detection strategy, which has already been demonstrated to be operational [28,29] and here ester group of HSLs react with hydroxylamine leads to the formation of hydroxamic acid in alkaline solution, then it reacts with the ferric ions and forms a dark brown colored complex. Initially (6,12, and 18 h), the HSL activity has seemed to be low, compared with the increased activity showed up to the third day.…”

A Flavor Lactone Mimicking AHL Quorum-Sensing Signals Exploits the Broad Affinity of the QsdR Regulator to Stimulate Transcription of the Rhodococcal qsd Operon Involved in Quorum-Quenching and Biocontrol Activities

“…This protection is principally based on a catabolic pathway, which degrades various AHLs. This pathway requires expression of the q uorum-sensing s ignal d egradation ( qsd ) operon encoding the intracellular lactonase QsdA, for lactone bond hydrolysis, and the fatty acyl-CoA ligase QsdC for the activation of resulting acyl chains before their oxidation or recycling (Barbey et al, 2018). Using this QQ pathway, R. erythropolis decreased disease levels in hydroponic conditions and initial field trials (Cirou et al, 2011, 2012).…”

“…This lack of reproducibility can be attenuated by stimulating in a sustainable way the antagonistic activity of the biocontrol agent. The regulatory mechanism of the qsd pathway has recently been elucidated with a molecular docking approach coupled to electrophoretic mobility shift assays and a transcriptional fusion strategy (Barbey et al, 2018; Chane et al, 2019). The expression of the genes of the qsd operon is controlled by QsdR, a TetR-like transcriptional repressor.…”

“…In the absence of inducer, the QsdR regulator binds to the DNA promoter region, switching off the pathway. When AHLs are present in the cellular environment, their γ-butyrolactone moiety [i.e., homoserine lactone (HSL)] binds to QsdR, preventing its binding to the qsd operon promoter region, thereby allowing the expression of the qsd catabolic genes (Barbey et al, 2018; Chane et al, 2019). Interestingly, the ligands of the TetR family regulators involved in catabolic activities are small molecules, often serving as substrates or catabolic intermediates of the target gene product (Cuthbertson and Nodwell, 2013).…”

“…This molecule induced an increase in the proportion of AHL-degrading bacteria in rhizosphere soil samples, producing microbial consortia with a higher capacity for AHL degradation (Cirou et al, 2007). GCL assimilation by R. erythropolis was subsequently elucidated and shown to involve the principal enzymes of the qsd catabolic pathway (Barbey et al, 2012, 2018). In this study, we applied the same interactomic and transcriptional fusion approaches that revealed the regulatory mechanism of the qsd operon (Barbey et al, 2018), to assess the ability of GCL to bind to the QsdR repressor and to activate expression of the qsd operon.…”