A protease and a lipoprotein jointly modulate the conserved ExoR-ExoS-ChvI signaling pathway critical inSinorhizobium melilotifor symbiosis with legume hosts
Authors: Bustamante Julian A. , Ceron Josue S. , Gao Ivan Thomas , Ramirez Hector A. , Aviles Milo V. , Bet Adam Demsin , Brice Jason R. , Cuellar Rodrigo , Dockery Eva , Jabagat Miguel Karlo , Karp Donna Grace , Lau Joseph Kin-On , Li Suling , Lopez-Magaña Raymondo , Moore Rebecca R. , Morin Bethany Kristi R. , Nzongo Juliana , Rezaeihaghighi Yasha , Sapienza-Martinez Joseph , Tran Tuyet Thi Kim , Huang Zhenzhong , Duthoy Aaron J. , Barnett Melanie J. , Long Sharon , Chen Joseph C. 
Published: May 2023

Sinorhizobium melilotiis a model alpha-proteobacterium for investigating microbe-host interactions, in particular nitrogen-fixing rhizobium-legume symbioses. Successful infection requires complex coordination between compatible host and endosymbiont, including bacterial production of succinoglycan, also known as exopolysaccharide-I (EPS-I). InS. melilotiEPS-I production is controlled by the conserved ExoS-ChvI two-component system. Periplasmic ExoR associates with the ExoS histidine kinase and negatively regulates ChvI-dependent expression ofexogenes, necessary for EPS-I synthesis. We show that two extracytoplasmic proteins, LppA (a lipoprotein) and JspA (a metalloprotease), jointly influence EPS-I synthesis by modulating the ExoR-ExoS-ChvI pathway and expression of genes in the ChvI regulon. Deletions ofjspAandlppAled to lower EPS-I production and competitive disadvantage during host colonization, for bothS. melilotiwithMedicago sativaandS. medicaewithM. truncatula. Overexpression ofjspAreduced steady-state levels of ExoR, suggesting that the JspA protease participates in ExoR degradation. This reduction in ExoR levels is dependent on LppA and can be replicated with ExoR, JspA, and LppA expressed exogenously inCaulobacter crescentusandEscherichia coli. Akin to signaling pathways that sense extracytoplasmic stress in other bacteria, JspA and LppA may monitor periplasmic conditions during interaction with the plant host to adjust accordingly expression of genes that contribute to efficient symbiosis. The molecular mechanisms underlying host colonization in our model system may have parallels in related alpha-proteobacteria.

Author summary

Symbiotic bacteria that live in the roots of legume plants produce biologically accessible nitrogen compounds, offering a more sustainable and environmentally sound alternative to industrial fertilizers generated from fossil fuels. Understanding the multitude of factors that contribute to successful interaction between such bacteria and their plant hosts can help refine strategies for improving agricultural output. In addition, because disease-causing microbes share many genes with these beneficial bacteria, unraveling the cellular mechanisms that facilitate host invasion can reveal ways to prevent and treat infectious diseases. In this report we show that two genes in the model bacteriumSinorhizobium meliloticontribute to effective symbiosis by helping the cells adapt to living in host plants. This finding furthers knowledge about genetics factors that regulate interactions between microbes and their hosts.