sinI- and expR-dependent quorum sensing in Sinorhizobium meliloti

Area of Science:

• Microbiology
• Bacterial Physiology
• Plant-Microbe Interactions

Background:

• Quorum sensing (QS) in Sinorhizobium meliloti involves multiple N-acyl homoserine lactone (AHL) signals and receptors.
• Sinorhizobium meliloti is a nitrogen-fixing bacterial symbiont of Medicago host plants.

Purpose of the Study:

• To investigate the role of SinI (AHL synthase) and ExpR (AHL receptor) in regulating protein accumulation in S. meliloti.
• To identify QS-regulated proteins and their functions, particularly in symbiotic interactions and motility.
• To assess the impact of QS on nodule initiation in Medicago truncatula.

Main Methods:

• Proteomic analysis to identify proteins dependent on SinI and/or ExpR.
• Gas chromatography-mass spectrometry and electrospray ionization tandem mass spectrometry to identify AHL signals.
• Beta-glucuronidase (GUS) transcriptional reporters to assess gene activity in mutant backgrounds and with added AHLs.
• Assays for nodule initiation and surface swarming motility.

Main Results:

• Fifty-five proteins were found to be regulated by SinI and/or ExpR.
• Identified AHL signals include 3-oxo-C(14)-HSL, C(16)-HSL, 3-oxo-C(16)-HSL, C(16:1)-HSL, and 3-oxo-C(16:1)-HSL.
• QS-regulated proteins have plausible roles in symbiotic interactions (e.g., ExpE6, IdhA) and motility.
• SinI-dependent QS is crucial for efficient nodule initiation on Medicago truncatula.
• SinI and ExpR mutants exhibit defects in swarming motility, with SinI mutant swarming restored by C(16:1)-HSL.

Conclusions:

• SinI-dependent quorum sensing regulates a significant number of proteins in S. meliloti.
• QS plays a critical role in symbiotic nodule initiation and surface motility.
• Understanding QS in S. meliloti provides insights into bacterial communication and host-microbe interactions.