Oral Presentation Australian Society for Microbiology Annual Scientific Meeting 2021

Epigenetic control of bacterial quorum sensing and horizontal DNA transfer (#24)

Tahlia R Bastholm 1 , Joshua P Ramsay 1 , John T Sullivan 2 , Dinah D Tambalo 3 , Liam K Harold 2 , Callum J Verdonk 1 4 , Elena Colombi 1 , Benjamin J Perry 2 , William Jowsey 2 , Calum Morris 2 , Michael F Hynes 5 , Charles S Bond 4 , Andrew D.S Cameron 3 , Christopher K Yost 3 , Clive W Ronson 2
  1. Curtin Medical School and Curtin Health Innovation Research Institute, Curtin University , Perth, Western Australia
  2. Department of Microbiology and Immunology , University of Otago, Dunedin, New Zealand
  3. Biology Department, University of Regina, Regina, Canada
  4. School of Molecular Sciences, University of Western Australia , Perth, Western Australia
  5. Department of Biological Sciences, University of Calgary, Calgary, Canada

Bistable gene regulation can enable genetically identical bacteria to differentiate into phenotypically distinct populations. Here we describe how the Mesorhizobium japonicum R7A mobile element ICEMlSymR7A epigenetically controls its transfer rate through a complex network of auto-regulating DNA-binding proteins and bacterial cell-cell communication called quorum sensing (QS). We discover that a subpopulation of R7A cells is induced to epigenetically differentiate into high-frequency ICEMlSymR7A donors that produce QS signalling molecules called N-acyl-homoserine lactones (AHLs). These cells, termed R7A*, emerged from ~2% of colonies derived from R7A populations and remained in this state even following plant symbiosis. Extensive genome sequencing failed to identify genetic changes responsible; moreover, the R7A* state was switched off following ICEMlSymR7A transfer to an isogenic non-symbiotic recipient.

ICEMlSymR7A transfer is normally repressed by the QS antiactivator gene qseM, and transcription of qseM is controlled by a DNA-binding protein QseC. Here, transcriptome sequencing revealed qseM transcription was abolished in R7A*; furthermore, an RNA transcript antisense to qseC was present in R7A but not R7A*. Deletion of the antisense promoter converted R7A cells into an R7A*-like state. A second adjacently-encoded DNA-binding protein QseC2 was found to repress anti-qseC transcription and QseC2 overexpression also stimulated R7A* establishment. The switch components were further dissected through mutagenesis, complementation, transcriptional fusions and DNA-binding assays. To summarise, R7A* establishment likely requires the sequential accumulation of QseC2 and QseC in cells and then qseM repression, ultimately resulting in the activation of quorum-sensing and horizontal transfer in stationary-phase cells.

Like other bistable systems, the described R7A > R7A* switch likely represents a bet-hedging strategy in which ICEMlSymR7A stochastically apportions small subpopulations to prepare for horizontal transfer while repressing transfer in the vast majority of cells. Uniquely however, the ICEMlSymR7A switch enables persistent vertical inheritance of horizontal transfer competence. These discoveries provide novel insights into strategies employed by mobile elements to optimize horizontal transfer rates in the face of potential host impacts.