How do bacteria and multicellular host organisms recognize and respond to each other? This is a fundamental question in studies of health and disease. Our project investigates the control of gene expression in a symbiosis between the alpha-proteobacterial species Sinorhizobium meliloti and its host, genus Medicago (such as alfalfa/M. sativa and barrel-medic/M. truncatula). Our lab has previously pioneered discoveries about host-bacterial signaling during the early steps of this complex symbiotic and developmental process. We have discovered that bacterial gene expression in symbiosis can follow one of six major patterns during symbiosis (that is, when and where the genes are turned on and off). In the next project period, we will identify promoters for these stage-specific genes and will determine the regulatory genes and proteins that control expression during development. One of the major players in gene expression control is the enzyme RNA polymerase, which recognizes DNA and reads out (expresses) only certain specific genes. In bacteria, a key component that determines which genes to express is the sigma (?) factor of this RNA polymerase. We found that ECF (extra-cytoplasmic function) ? factors are required for symbiosis: we now plan to analyze the mechanisms by which ECF ? factors are controlled in the context of a multi-stage developmental process. The bacterial stringent response usually occurs in reaction to nutrient deprivation;in Sinorhizobium, it is required for correct gene expression in symbiosis. We hypothesize this is programmed, and not merely a response to nutrient shift. We will directly test several hypotheses about how the stringent response is involved in the mechanism of differentiation. In parallel we will also look at how Sinorhizobium cells respond to alterations at the cell surface: This is important to understand because bacterial surface interactions with its eukaryotic host inevitably become an arena in which the two organisms signal and respond to each other.

Public Health Relevance

Bacteria are abundant and diverse, and they can be either friend or foe. How do organisms as different as a bacterium and a eukaryote recognize and signal to each other? We work on symbiotic nitrogen-fixing bacteria that associate with legumes. Through molecular and genetic analyses we have elucidated the ways in which bacteria and a higher-organism host can signal each other and control each other's activities. This model system has numerous experimental assets, including the ability to genetically manipulate the host organism. The rules for symbiosis that we discover in this system of study are helpful clues to focus work on how beneficial bacteria interact with animals including humans.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Special Emphasis Panel (ZRG1-GGG-L (90))
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Sledjeski, Darren D
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Stanford University
Schools of Arts and Sciences
United States
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Lang, Claus; Barnett, Melanie J; Fisher, Robert F et al. (2018) Most Sinorhizobium meliloti Extracytoplasmic Function Sigma Factors Control Accessory Functions. mSphere 3:
Lang, Claus; Smith, Lucinda S; Long, Sharon R (2018) Characterization of Novel Plant Symbiosis Mutants Using a New Multiple Gene-Expression Reporter Sinorhizobium meliloti Strain. Front Plant Sci 9:76
Lehman, Alisa P; Long, Sharon R (2018) OxyR-Dependent Transcription Response of Sinorhizobium meliloti to Oxidative Stress. J Bacteriol 200:
Barnett, Melanie J; Long, Sharon R (2018) Novel Genes and Regulators That Influence Production of Cell Surface Exopolysaccharides in Sinorhizobium meliloti. J Bacteriol 200:
Long, Sharon R (2016) SnapShot: Signaling in Symbiosis. Cell 167:582-582.e1
Ichida, Hiroyuki; Long, Sharon R (2016) LDSS-P: an advanced algorithm to extract functional short motifs associated with coordinated gene expression. Nucleic Acids Res 44:5045-53
Wippel, Kathrin; Long, Sharon R (2016) Contributions of Sinorhizobium meliloti Transcriptional Regulator DksA to Bacterial Growth and Efficient Symbiosis with Medicago sativa. J Bacteriol 198:1374-83
Barnett, Melanie J; Long, Sharon R (2015) The Sinorhizobium meliloti SyrM regulon: effects on global gene expression are mediated by syrA and nodD3. J Bacteriol 197:1792-806
Lang, Claus; Long, Sharon R (2015) Transcriptomic Analysis of Sinorhizobium meliloti and Medicago truncatula Symbiosis Using Nitrogen Fixation-Deficient Nodules. Mol Plant Microbe Interact 28:856-68
Peck, Melicent C; Fisher, Robert F; Bliss, Robert et al. (2013) Isolation and characterization of mutant Sinorhizobium meliloti NodD1 proteins with altered responses to luteolin. J Bacteriol 195:3714-23

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