Graduate Thesis Or Dissertation

 

Homospermidine, spermidine, and putrescine : the biosynthesis and metabolism of polyamines in Rhizobium meliloti Public Deposited

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  • Rhizobium, in symbiotic association with leguminous plants, is able to fix atmospheric nitrogen after first forming root nodules. Since polyamines are associated with and found in high concentration in rapidly growing cells and are thought to be important for optimal cell growth, it is possible that these polycations are involved in the extensive cell proliferation characteristic of the nodulation process. As a first step towards elucidating the role(s) of polyamines in the rhizobial-legume interaction, I have characterized polyamine biosynthesis and metabolism in free-living Rhizobium meliloti. In addition to the detection of putrescine and spermidine, the presence of a less common polyamine, homospermidine, was confirmed in Rhizobium meliloti, with homospermidine comprising 79 percent of the free polyamines in this procaryote. The presence of an exogenous polyamine both affected the intracellular levels of each polyamine pool and inhibited the accumulation of a second amine. DL-∞-Difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, was found to: (1) inhibit the rhizobial enzyme both in vitro and in vivo; (2) increase the final optical density; and (3) create a shift in the dominant polyamine from homospermidine to spermidine. A series of radiolabeled amino acids and polyamines were studied as polyamine precursors. Ornithine, arginine, aspartic acid, putrescine, and spermidine, but not methionine, resulted in the isolation of labeled putrescine, spermidine, and homospermidine. Evidence is presented for the existence of an alternate pathway of spermidine synthesis in R. meliloti, one which is derived from aspartic acid and is unrelated to methionine and S-adenosylmethionine. Homospermidine synthase, the enzyme which synthesizes homospermidine from putrescine, was partially purified and characterized. A Kin with respect to putrescine and pH and temperature optima were determined. Spermidine was the most effective inhibitor of the analogs tested and exhibited competitive inhibition kinetics.
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