Lysine synthesis and catabolism are coordinately regulated during tobacco seed development

Hagai Karchi, Orit Shaul, Gad Galili

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102 Scopus citations

Abstract

The regulation of synthesis and accumulation of the essential amino acid lysine was studied in seeds of transgenic tobacco plants expressing, in a seed-specific manner, two feedback-insensitive bacterial enzymes: dihydrodipicolinate synthase (EC 4.2.1.52) and aspartate kinase (EC 2.7.2.4). High-level expression of the two bacterial enzymes resulted in only a slight increase in free lysine accumulation at intermediate stages of seed development, while free lysine declined to the low level of control plants toward maturity. To test whether enhanced catabolism may have contributed to the failure of free lysine to accumulate in seeds of transgenic plants, we analyzed the activity of lysine-ketoglutarate reductase (EC 1.5.1.7), an enzyme that catabolizes lysine into saccharopine. In both the control and the transgenic plants, the timing of appearance of lysine-ketoglutarate reductase activity correlated very closely with that of dihydrodipicolinate synthase activity, suggesting that lysine synthesis and catabolism were coordinately regulated during seed development. Notably, the activity of lysine- ketoglutarate reductase was significantly higher in seeds of the transgenic plants than in the controls. Coexpression of both bacterial enzymes in the same plant resulted in a significant increase in the proportions of lysine and threonine in seed albumins. Apparently, the normal low steady-state levels of free lysine and threonine in tobacco seeds may be rate limiting for the synthesis of seed proteins, which are relatively rich in these amino acids.

Original languageEnglish
Pages (from-to)2577-2581
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Volume91
Issue number7
DOIs
StatePublished - 29 Mar 1994
Externally publishedYes

Keywords

  • dihydrodipicoliante synthase
  • essential amino acids
  • lysine
  • lysine ketoglutarate reductase
  • transgenic plants

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