Poster: Metabolic Engineering

Abs # 1018: The role of competition between cystathionine gamma-synthase and threonine synthase in controlling the synthesis of methionine and threonine in Arabidopsis thaliana

Presenter:	Lee, Minsang , minsange@hotmail.com
Authors	Lee, Minsang  (A)   Martin, Melinda  (A)   Hudson, Andre  (A)   Leustek, Thomas  (A)
Affiliations:	(A): Rutgers, The State University of New Jersey

Homoserine kinase (HSK) produces O-phospho-L-homoserine (OPH), the substrate for cystathionine gamma-synthase (CGS), the first enzyme for Met synthesis, and threonine synthase (TS), the first enzyme for Thr synthesis. To test the role that competition for OPH has on Met and Thr synthesis, transgenic Arabidopsis thaliana were prepared that overexpress HSK, CGS or Escherichia coli TS (eTS) under transcriptional control of the CaMV 35S promoter. HSK overexpression did not result in OPH, Met or Thr accumulation. However, when either HSK-overexpressing or wild type plants were fed homoserine, OPH, Met, and Thr rapidly accumulated. The HSK-overexpressing plants produced much more OPH than wild type, but Met and Thr accumulated to the same level as wild type. These results suggest that OPH level strongly influences the rate with which both Met and Thr are synthesized and that homoserine is the primary limiting substrate. Overexpression of CGS caused Met to accumulate, but did not affect Thr level. eTS expression caused Thr to accumulate, but did not affect Met level. When the CGS-overexpressing plants were fed homoserine, Met accumulated more rapidly and OPH and Thr accumulation was reduced compared with wild type plants. When eTS-expressing plants were fed homoserine, Thr did not accumulate at a rate significantly greater than wild type, but OPH and Met accumulation were reduced compared with wild type. These results suggest that the partitioning of OPH between Met and Thr depends upon the relative activities of CGS and TS. The fact that under normal growth conditions neither CGS or eTS overexpression reduces the level of the amino acid from the competing pathway suggests that the rate of OPH synthesis may increase in response to increased OPH utilization.