Molecular genetic dissection and potential manipulation of lysine metabolism in seeds

Lysine is among the most important essential amino acids in the diet of hum an and livestock because it is presented in severely limiting amounts in ce reals and other important crops. Attempts to increase lysine levels in plan ts were made by reducing the sensitivity of the key enzyme in lysine biosyn thesis, namely dihydrodipicolinate synthase, to feedback inhibition by lysi ne. However, these studies showed that in plant, seeds, lysine accumulation is determined not only by the rate of its synthesis, but also by the rate of its catabolism via the a-amino adipic acid pathway. Our laboratory is cu rrently studying the regulation of lysine catabolism in plants in order to explore potentials of reducing lysine catabolic fluxes in transgenic plants . In plants, like animals, lysine is catabolized via saccharopine by two co nsecutive enzymes, lysine-ketoglutarate reductase (LKR) and saccharopine de hydrogenase (SDH), which are linked on a single bifunctional polypeptide. Y et SDH activity of the LKR/SDH polypeptide may be limiting in vivo because of its non-physiological pH optimum of activity. in some plants, like Arabi dopsis and canola, this is overcome by the presence of an additional monofu nctional SDH enzyme, which is encoded by the same locus that encodes the bi functional LKR/SDH enzyme. Results from our. and other laboratories imply t hat attempts to generate high-lysine crop plants should take into account a seed-specific reduction of lysine catabolism.