Oral Presentation Australian Society for Microbiology Annual Scientific Meeting 2023

A novel quorum sensing system regulates a network of non-coding RNAs (94013)

Tahlia Bastholm 1 , Elena Colombi 1 , Timothy Haskett 2 , Hayley Knights 2 , Nigel Halliday 3 , Alex Truman 3 , Paul Williams 3 , Clive Ronson 4 , Phillip Poole 2 , Joshua Ramsay 1
  1. Curtin Medical School and Curtin Health Innovation Research Institute, Perth, Western Australia
  2. Department of Biology, University of Oxford, Oxford, United Kingdom
  3. School of Life Sciences, University of Nottingham, Nottingham, United Kingdom
  4. Department of Microbiology and Immunology , University of Otago, Dunedin, New Zealand

Bacteria use intercellular chemical signaling systems called quorum sensing (QS) to control various phenotypes including plant-symbiosis and horizontal gene transfer. For LuxRI-family QS systems, cells produce membrane-diffusible molecules called N-acyl-homoserine lactones (AHLs) which increase in concentration with cell density. AHLs bind cytoplasmic receptor proteins in neighboring cells, which then activate or derepress phenotypes under QS control. The TraR/TraI QS system has been characterized in Mesorhizobium japonicum R7A and controls transfer of genes responsible for nitrogen-fixation and symbiosis. We have identified a separate, novel QS locus conserved across the genus Mesorhizobium, named the Mesorhizobium quorum-sensing locus (MQS).

 

Distinct from all LuxRI-family QS systems, MQS encodes a second AHL synthase gene mqsC in addition to mqsI, both of which are required for activation of the MQS receptor MqsR. This second AHL synthase protein – MqsC – is related to the crotonase family of enzymes, which in other bacteria produce a distinct family of unsaturated fatty-acid QS molecules called diffusible signal factors. This unique MQS system appears to represent an evolutionary amalgamation of these two enzymes for production of AHLs containing unsaturated fatty acids.

 

Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to detect AHLs produced by the MQS locus of R7A, which identified the long-chain unsaturated AHL molecule – 5-cis-C12-HSL. Chemically synthesized 5-cis-C12-HSL activated the MqsR system in the absence of mqsI and mqsC.

 

We were unable to identify roles for the MQS system in gene transfer or symbiosis, however, RNA sequencing of two strains identified six MQS-regulated non-coding RNAs (ncRNAs). Each of the ncRNA promoter regions contained a conserved inverted repeat sequence; the likely the binding site for MqsR. Promoter-lacZ fusions confirmed expression of each ncRNA was dependent on MqsR and 5-cis-C12-HSL. Work is underway to determine the physiological role of this ncRNA network.

 

In summary, we have identified a QS system conserved throughout the genus Mesorhizobium that produces an unsaturated 5-cis-C12-HSL molecule. The system uniquely requires two synthesis genes and regulates a number of ncRNAs, so far of unknown function.