Specialized adaptations in the model alpha-proteobacterium Sinorhizobium meliloti - Project Summary Alphaproteobacteria are a diverse taxon and many are capable of motility using flagella. They comprise several genera, including model organisms such as the plant symbiont Sinorhizobium meliloti and the plant pathogen Agrobacterium tumefaciens. Flagellar motility is an invaluable colonization factor required by soil bacteria to navigate the rhizosphere and accelerate associations with their plant hosts. Research within my laboratory is centered on flagellar-driven motility. We discovered that rhizobia evolved complex regulatory networks that adjust their cellular physiology and control the assembly of a specially-adapted flagellar machinery to permit efficient navigation of the soil environment in search for nutrients and host plants. Specifically, S. meliloti employs a unidirectionally rotating, variable-speed flagellar motor that is compositionally distinguished from the well- characterized switch-type motors of enteric bacteria, enabling it to swim in highly viscous environments. Our studies also revealed that the diverse and rapidly varying soil environment and the metabolic diversity and specific adaptations of soil bacteria to host signals led to the evolution of complex chemosensory systems compared to those of enteric bacteria. Recently, we uncovered several unique components and features that control S. meliloti chemotaxis to direct the bacterium toward nutrient sources and hosts. Our significant progress in deciphering these complex pathways establishes S. meliloti as model for studying the architecture and evolution of bacterial chemosensory and motility systems. Bacteria use variations of the chemotaxis signaling cascade to control cellular development, but knowledge of their specific roles remains scarce. The functions of alternate chemosensory pathways are understudied because their activating signals are mostly unknown and difficult to identify, but this knowledge is crucial for studying their regulatory functions. As such, the role of the Che2 chemosensory system in S. meliloti is unknown. My research goals for the next five years are to decipher the underlying molecular mechanisms of the unidirectional, variable-speed flagellar motor that enables bacteria to move efficiently in the soil. The receptor modification system of S. meliloti plays a pivotal role in its chemotaxis- directed movement, yet, remarkably, little is known about the regulation of pathway sensitivity and stimuli adaptation, which are essential for an effective chemotactic response. We plan to investigate how S. meliloti retains its chemotactic memory and how it effectively senses attractants over a wide range of attractant concentrations. We also plan to identify environmental and/or host signals that activate the Che2 system and characterize its regulatory pathway, which will contribute to the knowledge of these alternate systems and their roles in cellular functions. The overall vision of my research program is to advance a fundamental understanding of rhizobial motility and cell physiology, which are important pre-requisites for survival under various challenges and, ultimately, host engagement. This research will reveal new processes that support optimal growth and inform about new concepts in signal transduction, which will be transformative to other bacterial systems.
