LYSINE SYNTHESIS AND CATABOLISM ARE COORDINATELY REGULATED DURING TOBACCO SEED DEVELOPMENT

The regulation of synthesis and accumulation of the essential amino ac id lysine was studied in seeds of transgenic tobacco plants expressing , in a seed-specific manner, two feedback-insensitive bacterial enzyme s: 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 lev el of control plants toward maturity. To test whether enhanced catabol ism may have contributed to the failure of free lysine to accumulate i n seeds of transgenic plants, we analyzed the activity of lysine-ketog lutarate reductase (EC 1.5.1.7), an enzyme that catabolizes lysine int o saccharopine. In both the control and the transgenic plants, the tim ing of appearance of lysine-ketoglutarate reductase activity correlate d very closely with that of dihydrodipicolinate synthase activity, sug gesting that lysine synthesis and catabolism were coordinately regulat ed during seed development. Notably, the activity of lysine-ketoglutar ate reductase was significantly higher in seeds of the transgenic plan ts than in the controls. Coexpression of both bacterial enzymes in the same plant resulted in a significant increase in the proportions of l ysine and threonine in seed albumins. Apparently, the normal low stead y-state levels of free lysine and threonine in tobacco seeds may be ra te limiting for the synthesis of seed proteins, which are relatively r ich in these amino acids.