A two-component regulatory system mediates quorum sensing-dependent morphology and motility transitions in the archaeon Haloferax volcanii

Abstract

ABSTRACT Quorum sensing (QS) enables microorganisms—including bacteria, eukaryotes, and viruses—to coordinate collective behaviors in response to population density. Despite their ecological and evolutionary significance, QS mechanisms in Archaea remain poorly characterized. The halophilic archaeon Haloferax volcanii provides a model for archaeal QS, transitioning from motile rods to non-motile disks in a density-dependent response to a secreted disk-forming signal (DFS). To identify components of the DFS regulatory network, we screened for spontaneous mutants that retained motility in DFS-containing soft-agar medium. One candidate, HVO_1357, encodes a predicted response regulator located adjacent to a histidine kinase (HVO_1356) and a second response regulator (HVO_1358), consistent with an extended two-component regulatory system (TCS). Based on our results, these genes encode quorum-sensing-associated regulators (Qar); therefore, we propose renaming them to qarA (HVO_1357), qarB (HVO_1356), and qarC (HVO_1358). Deletion of qarA enabled cells to swim on DFS-containing soft-agar plates and conferred hypermotility on standard soft-agar media; however, these phenotypes were not due to changes in motility-related parameters, but a reduced sensitivity to DFS for induction of the non-motile, disk-shaped state. In contrast, ΔqarB and ΔqarC strains were non-motile and exhibited premature disk formation during normal growth. Suppressor mutations restoring motility to ΔqarB and ΔqarC mapped exclusively to qarA, and epistasis analysis indicated QarA as the central regulator of this system. Phosphoablative variants of QarA, QarB, and QarC failed to complement their respective deletion strains, supporting QarABC as a TCS. Transcriptomic analyses revealed that qarA deletion leads to upregulation of genes involved in motility and rod-shape formation. Together, these findings reveal qarABC as a DFS-responsive regulatory module and represent the first TCS in archaea shown to control QS-dependent behavior. IMPORTANCE Archaea are ubiquitous and play key roles across diverse ecosystems—including human microbiomes—yet little is known about how they communicate with one another and with other organisms, or how these interactions shape their ecological impact. Such intercellular communication, including quorum sensing (QS), allows microorganisms to coordinate behaviors critical for survival, adaptation, and community organization. In this study, we identify the first archaeal two-component regulatory system that is involved in QS-dependent regulation, providing a foundation for understanding how organisms in this domain sense and respond to population cues. By revealing a previously unknown aspect of archaeal biology, this work represents an important step toward understanding how archaeal communication shapes both their physiology and their interactions within complex microbial communities. Archaea are ubiquitous and play key roles across diverse ecosystems—including human microbiomes—yet little is known about how they communicate with one another and with other organisms, or how these interactions shape their ecological impact. Such intercellular communication, including quorum sensing (QS), allows microorganisms to coordinate behaviors critical for survival, adaptation, and community organization. In this study, we identify the first archaeal two-component regulatory system that is involved in QS-dependent regulation, providing a foundation for understanding how organisms in this domain sense and respond to population cues. By revealing a previously unknown aspect of archaeal biology, this work represents an important step toward understanding how archaeal communication shapes both their physiology and their interactions within complex microbial communities.