Classically, quorum sensing (QS) is defined as a form of bacterial communication that regulates gene expression in response to population density and other environmental conditions. While a variety of QS systems exist, N -acyl homoserine lactone-based quorum sensing (AHL-QS) is the best characterized in Gram-negative Pseudomonadota (formerly Proteobacteria), often in medically relevant taxa. However, it remains unclear what AHL-QS systems look like in underexplored environments, and how often or far the canonical model extends beyond the Pseudomonadota. Here, we investigated AHL production in environmental Bacteria from underexplored environments, including two newly described Pseudomonadota species, Brenneria uluponensis K61 T , from a taro lo‘i in Hawai‘i, and Bradyrhizobium prioriisuperbiae BL16A T , from a lava cave on the Island of Hawai‘i, and two Gram-positive Actinomycetota (formerly Actinobacteria), Pseudonocardia alni GV4, from a culture of Gloeobacter violaceus, and Rhodococcus kroppenstedtii Y88A, from a dehydrated hypersaline mat on San Salvador Island, Bahamas. Custom hidden Markov models (HMMs) were used to identify putative luxI/R homologs, followed by analyses of genomic context, luxI/R-family protein phylogeny, and LuxR domain architecture. AHL production was assessed using liquid chromatography-multiple reaction monitoring-mass spectrometry (LC-MRM-MS), and targeted gene disruption was performed in R. kroppenstedtii Y88A. Putative luxI/R homologs were identified in all strains. In R. kroppenstedtii Y88A, disruption of the sole luxI homolog resulted in loss of all detectable AHLs, indicating that this gene (designated rhdI) is required for AHL production under the tested conditions. Across Actinomycetota genomes, luxI genes occurred as solitary elements rather than canonical luxI/R pairs and were associated with genes linked to metabolism and redox processes. Phylogenetic analyses revealed that Actinomycetota luxI/R -family proteins diverge from canonical systems, with LuxR-family proteins lacking canonical autoinducer-binding domains and instead comprising only helix-turn-helix regulatory architectures. Together, these findings expand the known diversity of AHL-QS systems into underexplored microbial lineages and environments and suggest broader ecological roles for AHL signaling. The canonical luxI/R model derived largely from Pseudomonadota may represent only one of several evolutionary architectures for AHL signaling, raising the possibility that AHL production in Actinomycetota operates through regulatory frameworks distinct from classical AHL-QS systems.
Handel et al. (Mon,) studied this question.