|Samac, Deborah - Debby|
Submitted to: Proceedings of European Nitrogen Fixation Conference
Publication Type: Book / Chapter
Publication Acceptance Date: 9/2/1995
Publication Date: N/A
Technical Abstract: The primary assimilation of symbiotically fixed N in alfalfa root nodules involves complex intermingling with C metabolism. Understanding how N and C metabolism are controlled requires fundamental knowledge of how plant genes are regulated. While significant progress has been made in understanding the regulation of GS, much less is known about the genes controlling other steps in this process. To that end we have isolated, purified and characterized the enzymes AAT, PEPC, and NADH GOGAT. cDNAs encoding these crucial enzymes were also characterized. While the most prominent forms of GS associated with N assimilation in nodules are located in the cytosol, AAT and NADH GOGAT appear to be organelle associated. The amino acid sequence suggested and immunogold labeling showed that nodule enhanced AAT 2 is located in amyloplasts. Comparison of the amino acid sequence of nodule enhanced NADH GOGAT to the N terminal sequence of the processed protein indicated that NADH GOGAT has a 101 amino acid presequence. However, it is unclear as to which organelle NADH GOGAT is targeted. Cytosolic phosphoenolpyruvate carboxylase (PEPC), which can be expressed in legume root nodules at levels comparable to those detected in leaves of C4 plants. RNA blots showed that GS, AAT, PEPC, and NADH GOGAT mRNAs were enhanced about 15 fold during the development of effective alfalfa nodules. By comparison, the expression of GS, AAT and PEPC mRNAs was reduced by 65 percent in ineffective nodules. NADH GOGAT was different from GS, AAT, and PEPC in that expression had an absolute requirement for a factor(s) related to effective nodules. The data suggest that NADH GOGAT plays a key role in regulating N assimilation. Moreover, plastids in nodules play a major role not only in C metabolism but also in N assimilation.