Exoproteome Remodeling by the BraI/R Quorum Sensing Circuit and Interspecies Competition in Paraburkholderia.

Characterization of a quorum-sensing communication based on gamma-butyrolactones in Rhodococcus erythropolis

This study provides novel and unexpected insights into the involvement of a LuxR homolog in regulating a QS system in Gram-positive bacteria and demonstrates that functional GBL-based QS systems are conserved and active in R. erythropolis.

A switch–rheostat circuit governs quorum sensing homeostasis in a phytopathogen

Structure-Based Insights into the Molecular Mechanisms of LuxR-Type Quorum Sensing Receptors.

Quorum sensing (QS) enables bacteria to coordinate collective behaviors in response to population density. LuxI/R QS systems, common among gram-negative bacteria, consist of a LuxI-type synthase that produces an N-acyl L-homoserine lactone (AHL) signaling molecule and a LuxR-type receptor that senses the AHL. At threshold AHL concentrations, LuxR-type receptors undergo ligand-induced conformational changes that affect DNA binding and target gene transcription. Because of their role in regulating myriad collective behaviors, LuxI/R systems are targets for many applications, including antivirulence strategies and the engineering of beneficial microbiomes. Recent structural studies have led to substantial progress in understanding molecular mechanisms of LuxI/R systems. However, LuxR-type receptors fall into functionally diverse subfamilies for which structural bases remain incompletely understood. In this article, we summarize recent structural and mechanistic insights into LuxR-type receptor function, including interactions with small molecules, protein partners, and DNA. We identify critical knowledge gaps that highlight the need for additional structural and mechanistic information regarding LuxI/R QS.

Rsm-mediated post-translational control of the Pseudomonas putida Type VI Secretion System

The Type VI secretion system (T6SS) is a bacterial nanoweapon that injects toxic effectors into prokaryotic and eukaryotic cells. It is widely found among gram-negative bacteria and provides a significant fitness advantage in interbacterial competition. Pseudomonas putida KT2440 possesses three T6SS clusters (K1-, K2- and K3-T6SS) that combat phytopathogens. This makes this strain a potent biocontrol agent that protects plants from pathogens and can be further enhanced by a better understanding of its T6SS regulation. Although the core components of T6SS are conserved, the elements controlling its regulation differ among bacterial species. T6SS activity is regulated by various factors acting at different levels, from transcription to post-translational modification, to ensure precise control of its activity. Here, we demonstrate the critical importance that the three Rsm proteins, RsmIEA, have in controlling the K1-T6SS structural components and related orphan elements at the post-transcriptional level in Pseudomonas putida. We identified multiple Rsm-binding sites responsible for directly repressing the translation of T6SS proteins (Hcp1 and Hcp5) and their associated effectors (Tke2 and Tke7). Derepression of K1-T6SS mRNA in the rsmIEA mutant led to enhanced translation and expression of the K1-T6SS components and effectors, and critically increased the number of cells in the population with assembled T6SS. This results in a greater capacity to secrete toxins and kill prey cells via the T6SS-dependent mechanism. Finally, we demonstrate the K1-T6SS ability to kill environmental pathogens, including Salmonella enterica and Erwinia amylovora.

Interplay between different SHP/Rgg quorum sensing systems that control production of post-translationally modified peptides in Streptococcus thermophilus.

In Bacillota, the Rap-Rgg-NprR-PlcR-PrgX-AimR (RRNPPA) superfamily of regulators mediates quorum sensing via autoinducing peptides (AIPs). AIPs are secreted, processed and reimported before interacting with their cognate regulators, modulating the latter's activity. Within RRNPPA families, short hydrophobic peptides (SHPs) interact with Rgg transcriptional regulators. A widely used bacterium in the dairy industry, Streptococcus thermophilus, possesses an unusually high number of Rgg regulators (mean of 5 and max of 10 per genome). These regulators mainly control the expression of operons involved in the production of ribosomally synthesized and post-translationally modified peptides (RiPPs). Because SHP/Rgg systems display similarities at the amino acid level, we investigated potential crosstalk (i.e. cross-activation) among systems. We focused on five SHP/Rgg systems located upstream of operons involved in producing RiPPs that are modified by radical SAM enzymes as well as a putative RiPP that is modified by a ThiF-like adenylyltransferase/cyclase enzyme. Using genetic approaches (reporter fusions into different genetic backgrounds) and analytical chemistry (liquid chromatography tandem mass spectrometry), we found three key results: (1) each Rgg regulator activates the transcription of its proximal RiPP operon, (2) cross-activation also occurs, where one operon is co-regulated by two distinct Rgg regulators and (3) a single Rgg regulator can be activated by multiple SHPs. A particularly novel finding is that one of the five operons is expressed under the control of two independent quorum-sensing pathways: an SHP/Rgg system and the ComR/S competence system. The result is the production of a newly identified RiPP, named ThiF-like RiPP6405, that undergoes post-translational modifications catalysed by a ThiF-like adenylyltransferase/cyclase. Overall, our study has revealed that individual SHP/Rgg systems function autonomously but that their activity is further shaped by crosstalk involving both SHP- and DNA-binding. These findings underscore that these systems should be thoroughly characterized at the strain level to allow precise, targeted modifications.

Disrupting Bacterial Quorum Sensing: A Review of Protein Families and Natural Inhibitors for Combating Resistance.

The Quorum Sensing (QS) system plays a central role in regulating several processes that influence bacterial physiology, communication, and pathogenesis. It governs the production of virulence factors, facilitates biofilm formation, and contributes to the development of antibiotic resistance, all of which are critical to bacterial survival and pathogenicity. QS enables both intraspecies and inter-species bacterial communication, operating through distinct pathways in Gram-positive and Gram-negative bacteria. This review provides a comprehensive overview of the major families of quorum-sensing proteins involved in the working of the signalling molecules. It examines the structural features, functional roles, and mechanisms of action of these proteins, highlighting their significance in bacterial behaviour. Furthermore, the study explores the potential of plant-derived secondary metabolites as natural inhibitors targeting QS systems to disrupt bacterial pathogenicity. Finally, it discusses the implications of exploiting these inhibitors as promising strategies to address the growing challenge of antibiotic resistance in bacterial infections.