Lipopolysaccharide, K antigen, and other components of the bacterial cell surface important for S. meliloti-alfalfa symbiosis

Abstract

Sinorhizobium meliloti can form a symbiotic relationship with alfalfa plants in which the bacteria fix atmospheric nitrogen into ammonia for the plant in exchange for nutrients. During this symbiosis, the bacteria live in organs called nodules that develop in the roots of the plant. This thesis explores a range of bacterial components necessary for an effective symbiosis. One overarching theme to these components is that most of them are involved with the bacterial cell surface. Chapter 2 is a study of the lpsB389 mutant, which codes for a predicted glycosyl transferase involved with LPS core synthesis. Bacteria carrying the lpsB389 mutation are capable of carrying out the first steps of nodule invasion on alfalfa plants. However, they display dramatic abnormalities upon entry into plant cells. In Chapter 3, I identified four new classes of mutants involved in K antigen synthesis. Finally, in Chapters 4 and 5, I made use of the newly sequenced S. meliloti genome to determine the locations of the insertions in 43 mutants identified by having an effect on the cell surf ace. Most of these mutants also have symbiotic defects. Chapter 4 examines a series of mutations that affect either LPS or other factors involved in membrane integrity. In contrast to a previous report, I found·that most of these mutants have symbiotic defects. In addition, this study has identified genes coding for proteins not previously recognized as being involved with the integrity of the cell surface or as being required for symbiosis including: GreA, and a protein with a Sua5/Yci0/YrdC family signature domain. Chapter 5 implicates many new genes and processes in symbiosis including: the vitamin B12 , or cobalamin; QxtA, a predicted terminal oxidase of respiration; a predicted magnesium and cobalt transporter; and the process of carbon fixation. Undoubtedly the genes, factors, and metabolic processes identified in these studies will lead to a greater understanding of the complex interaction between S. meliloti and alfalfa during symbiosis. Furthermore, since S. meliloti is closely related to a number of mammaljan and plant pathogens, these discoveries may also contribute to our understanding of how pathogens infect their hosts.