Study On Quorum Sensing System Involved In Resistance Of Deinococcus Radiodurans In Response To Oxidative Stress

Deinococcus radiodurans belongs to the Deinococcus, a genus of bacteria characterized by an unparalleled ability to survive lethal effects of extreme stresses, including ionizing radiation, ultraviolet (UV) radiation, desiccation, and oxidants. For years, D. radiodurans has been used as an ideal model organism for studying microbial resistance to extreme stresses. However, it remains unclear how does D. radiodurans response to the stresses on the level of bacterial population. In this work, we characterized quorum sensing circuits and investigated the function and regulation mechanism of the quorum sensing circuits involved in response of D. radiodurans to oxidative stress, using biochemical and molecular biological techiques.First, we show that AHLs can be employed by D. radiodurans. An AHL-mediated quorum-sensing system (DqsI/DqsR), which plays important roles in cell resistance to oxidative stress, was identified in D. radiodurans. D. radiodurans has special AHL synthesases (DqsⅠ-1 and DqsI-2), different from Luxl-type and LuxM-type AHL synthesases. We found that under non-stress conditions, the AHL level was "shielded" by quorum quenching enzymes, whereas AHLs accumulated by induction of DqsⅠ-1 and DqsI-2 when D. radiodurans was exposed to oxidative stress, indicating an oxidative stress-dependent quorum sensing. The dynamic expression of AHL synthesases and quorum quenching enzymes might be used by the bacterium to control AHL level under different conditions. The DqsR, acting as a regulator of quorum sensing, controls gene expression along with AHLs. Both dqsl mutant (DMDqsI) and dqsR mutant (MDqsR) were more sensitive to oxidative stress compared with the wild-type strain. Exogenous AHLs could completely restore the survival fraction of DMDqsI under oxidative stress. RNA-seq analysis showed that a number of genes involved in stress-response, cellular cleansing and DNA repair had altered transcriptional levels in the mutant of DqsR. Hence, the DqsIR-mediated quorum sensing is an adaptive strategy for D. radiodurans in response to oxidative stress and might be conserved in the extremophilic Deinococcus bacteria.Second, Autoinducer-2 (AI-2) serves as a quorum-sensing signaling molecule that mediates both intraspecies and interspecies communication among bacteria, and plays roles in regulating various bacterial behaviors. In the present study, we investigated the functions of AI-2 signaling in resistance of D. radiodurans to oxidative stress by construction of the AI-2 synthase (LuxS homolog) gene disruption mutant, survival phenotype assay, and gene transcription assay. The gene mutant (DRΔLuxS), which was unable to produce AI-2, was significantly more sensitive to both gamma radiation and H2O2 compared with the wild-type strain. Complementation of the wild-type-derived spent medium into the cell culture of DRΔLuxS completely restored the resistance phenotype of DRΔLuxS. A higher level of reactive oxygen species was accumulated in the mutant compared with the wild type under normal or oxidative stress. Quantitative real-time PCR assays showed that transcriptional levels of some resistant proteins, including catalase, extracellular nuclease, Dps-1 and ABC transporters were decreased in DRΔLuxS, indicating that AI-2 is involved in regulation of the resistant genes of D. radiodurans. Hence, AI-2 signaling may contribute to the extreme resistance of D. radiodurans to oxidative stresses.In sum, the present study investigated the quorum-sensing circuits and their regulation on resistant genes in D. radiodurans under oxidative stress.The results provide fundament for elucidation regulation mechanism of D. radiodurans in response to oxidative stresse from the point of view of quorum sensing and for understanding the resistance of D. radiodurans to extreme stresses.