ThecinandraiQuorum-Sensing Regulatory Systems inRhizobium leguminosarumAre Coordinated by ExpR and CinS, a Small Regulatory Protein Coexpressed with CinI

Abstract

To understand how the Rhizobium leguminosarum raiI-raiR quorum-sensing system is regulated, we identified mutants with decreased levels of RaiI-made N-acyl homoserine lactones (AHLs). A LuxR-type regulator, ExpR, is required for raiR expression, and RaiR is required to induce raiI. Since raiR (and raiI) expression is also reduced in cinI and cinR quorum-sensing mutants, we thought CinI-made AHLs may activate ExpR to induce raiR. However, added CinI-made AHLs did not induce raiR expression in a cinI mutant. The reduced raiR expression in cinI and cinR mutants was due to lack of expression of cinS immediately downstream of cinI. cinS encodes a 67-residue protein, translationally coupled to CinI, and cinS acts downstream of expR for raiR induction. Cloned cinS in R. leguminosarum caused an unusual collapse of colony structure, and this was delayed by mutation of expR. The phenotype looked like a loss of exopolysaccharide (EPS) integrity; mutations in cinI, cinR, cinS, and expR all reduced expression of plyB, encoding an EPS glycanase, and mutation of plyB abolished the effect of cloned cinS on colony morphology. We conclude that CinS and ExpR act to increase PlyB levels, thereby influencing the bacterial surface. CinS is conserved in other rhizobia, including Rhizobium etli; the previously observed effect of cinI and cinR mutations decreasing swarming in that strain is primarily due to a lack of CinS rather than a lack of CinI-made AHL. We conclude that CinS mediates quorum-sensing regulation because it is coregulated with an AHL synthase and demonstrate that its regulatory effects can occur in the absence of AHLs.

LuxR- and LuxI-Type Quorum-Sensing Circuits Are Prevalent in Members of the Populus deltoides Microbiome

“…There were four subfamilies of solo LuxRs that were present among multiple isolates of Rhizobium species, including homologs of ExpR (Fig. 2, turquoise lines) from Rhizobium leguminosarum and E. meliloti (41), indicating that each of these LuxR homologs is common among the Rhizobiaceae. There was also a Pseudomonas subfamily of solo LuxRs, which includes PpoR from Pseudomonas putida (42), which was conserved in 20 of 21 Pseudomonas isolates (Fig.…”

Lon Protease of Azorhizobium caulinodans ORS571 Is Required for Suppression of reb Gene Expression

“…PraR represses the expression of the rhiR and raiR genes, which encode transcription regulators involved in quorum-sensing regulation, and PraR is displaced from these promoters when PraR is bound to a small regulatory protein, CinS (21). Although cinS is present in only a few rhizobia (18) and not in A. caulinodans, it is possible that the activity of PraR in A. caulinodans might be controlled by binding to an unknown regulator other than CinS and that the properties of such a protein might be influenced by Lon protease activity. As another option, there still remains the possibility that Lon and PraR independently regulate the expression of reb genes.…”

Gut and Root Microbiota Commonalities

“…Those that occur in the mouth (159) may guide research in gut and root symbioses. In Rhizobium, mutants in quorum sensing are affected in rhizosphere colonization (160). Acylhomoserine lactones may be degraded by rhizospheric bacteria causing interference with quorum signals that regulate gene expression in other bacteria (161).…”