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Title: Methionine and Threonine Synthesis are Limited by Homoserine Availability and not the Activity of Homoserine Kinase in Arabidopsis thalina

Author
item LEE, MINSANG - UNV MN, ST PAUL, MN
item MARTIN, MELINDA - RUTGERS UNV, N BRNSWK, NJ
item HUDSON, ANDRE - RUTGERS UNV, N BRNSWK, NJ
item LEE, JIYEON - RUTGERS UNV, N BRNSWK, NJ
item Muhitch, Michael
item LEUSTEK, THOMAS - RUTGERS UNV, N BRNSWK, NJ

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2004
Publication Date: 5/1/2005
Citation: Lee, M., Martin, M.N., Hudson, A.O., Lee, J., Muhitch, M.J., Leustek, T. 2005. Methionine and Threonine Synthesis are Limited by Homoserine Availability and not the Activity of Homoserine Kinase in Arabidopsis thalina. Plant Journal. 41(5):685-696.

Interpretive Summary: Of the 20 amino acid building blocks of proteins, 10 are not made in animal cells and must be acquired through the diet. Four of these amino acids, lysine, threonine, methionine and isoleucine are all derived from the same plant biochemical pathway. Legumes tend to be deficient in methionine, while grains contain low amounts of lysine. In the present report, we experimentally altered this pathway in the model plant Arabidopsis to determine the effects on the production of these amino acids and provide insight as to how the plant regulates their synthesis. These results will lead to ways to increase these essential amino acids in crops, thus improving their nutritional quality for farm animals as well as for people.

Technical Abstract: Homoserine kinase (HSK) produces O-phospho-L-homoserine (HserP) used by cystathionine gamma-synthase (CGS) for Met synthesis and threonine synthase (TS) for Thr synthesis. The effects of overexpressing Arabidopsis thaliana HSK, CGS, and Escherichia coli TS (eTS), each controlled by the 35S promoter, were compared. The results indicate that in Arabidopsis Hser supply is the major factor limiting the synthesis of HserP, Met and Thr. HSK is not limiting and CGS or TS control the partitioning of HserP. HSK overexpression had no effect on the level of soluble HserP, Met or Thr, however, when treated with Hser these plants produced far more HserP than wild type. Met and Thr also accumulated markedly after Hser treatment but the increase was similar in HSK overexpressing and wild type plants. CGS overexpression was previously shown to increase Met content, but had no affect on Thr. After Hser treatment Met accumulation increased in CGS-overexpressing plants compared to wild type, whereas HserP declined and Thr was unaffected. Arabidopsis responded differentially to eTS expression depending on the level of the enzyme. At the highest eTS level the Thr content was not increased, but phenotype was negatively affected and the T1 plants died before reproducing. Comparatively low eTS did not affect phenotype or Thr/Met level, however after Hser treatment HserP and Met accumulation were reduced compared to wild type and Thr was increased slightly. At intermediate eTS activity seedling growth was retarded unless Met was supplied and CGS expression was induced, indicating that eTS limited HserP availability for Met synthesis.