Complex Regulation of Symbiotic Functions Is Coordinated by MucR and Quorum Sensing in Sinorhizobium meliloti

Abstract

In Sinorhizobium meliloti, the production of exopolysaccharides such as succinoglycan and exopolysaccharide II (EPS II) enables the bacterium to invade root nodules on Medicago sativa and establish a nitrogen-fixing symbiosis. While extensive research has focused on succinoglycan, less is known concerning the regulation of EPS II or the mechanism by which it mediates entrance into the host plant. Previously, we reported that the ExpR/Sin quorum-sensing system is required to produce the symbiotically active low-molecular-weight fraction of this exopolysaccharide. Here, we show that this system induces EPS II production by increasing expression of the expG-expC operon, encoding both a transcriptional regulator (ExpG) and a glycosyl transferase (ExpC). ExpG derepresses EPS II production at the transcriptional level from MucR, a RosR homolog, while concurrently elevating expression of expC, resulting in the synthesis of the low-molecular-weight form. While the ExpR/Sin system abolishes the role of MucR on EPS II production, it preserves a multitude of other quorumsensing-independent regulatory functions which promote the establishment of symbiosis. In planktonic S. meliloti, MucR properly coordinates a diverse set of bacterial behaviors by repressing a variety of genes intended for expression during symbiosis and enhancing the bacterial ability to induce root nodule formation. Quorum sensing precisely modulates the functions of MucR to take advantage of both the production of symbiotically active EPS II as well as the proper coordination of bacterial behavior required to promote symbiosis.

Protective Efficacy and Safety of Brucella melitensis 16MΔmucRagainst Intraperitoneal and Aerosol Challenge in BALB/c Mice

“…Among these, a mucR (BMEI 1364) mutant was attenuated for survival in the mouse and macrophage model (40). The role of MucR protein in Brucella is unknown, but the function of the mucR gene has recently been identified in soil and plant bacteria such as Agrobacterium tumefaciencis and Sinorhizobium meliloti (19,28). MucR is a transcriptional regulator that coordinates a diverse set of bacterial behaviors, including the control of exopolysaccharide production, which is important not only in bacteriumplant symbiosis but also in biofilm formation (4,5).…”

Diverse Genetic Regulon of the Virulence-Associated Transcriptional Regulator MucR in Brucella abortus 2308

“…While early studies genetically linked ros and mucR mutations to growth defects and differences in colony mucoidy, the mechanism of action of the Ros/MucR proteins was not known at the time; however, it has since been determined that Ros/MucR proteins are transcriptional repressors that regulate numerous genes, including those involved in polysaccharide synthesis, motility, and quorum sensing (11,12,13,14,15,16,17,18). Ros/MucRtype regulators are unusual in that they contain a Zn finger motif that is uncommon in prokaryotes, whereas transcriptional regulators with this type of motif are commonly found in eukaryotes (19).…”

The Sinorhizobium meliloti SyrM Regulon: Effects on Global Gene Expression Are Mediated by syrA and nodD3

“…Mutations in chvI, exoS, and exoR cause severe and pleiotropic phenotypes, suggesting that, in addition to regulating EPS-I biosynthesis, this circuit affects symbiosis, motility, biofilm formation, cell envelope composition, EPS-II production, and central metabolism (39, 42-44, 46, 49-53). Other proteins affecting EPS-I production are MucR, which acts positively (54,55), and ExoX, which acts negatively (56,57).…”