A Two-Component Regulatory System Mediates Quorum Sensing-Dependent Morphology and Motility Transitions in the Archaeon Haloferax volcanii.

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 rename them 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 that restored motility to ΔqarB and ΔqarC mapped exclusively to qarA, suggesting QarA is the central regulator of this system. 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.