REGULATION OF LYSINE CATABOLISM THROUGH LYSINE-KETOGLUTARATE REDUCTASE AND SACCHAROPINE DEHYDROGENASE IN ARABIDOPSIS

In plant and mammalian cells, excess lysine is catabolized by a pathwa y that is initiated by two enzymes, namely, lysine-ketoglutarate reduc tase and saccharopine dehydrogenase. In this study, we report the clon ing of an Arabidopsis cDNA encoding a bifunctional polypeptide that co ntains both of these enzyme activities linked to each other. RNA gel b lot analysis identified two mRNA bands-a large mRNA containing both ly sine-ketoglutarate reductase and saccharopine dehydrogenase sequences and a smaller mRNA containing only the saccharopine dehydrogenase sequ ence. However, DNA gel blot hybridization using either the lysine-keto glutarate reductase or the saccharopine dehydrogenase cDNA sequence as a probe suggested that the two mRNA populations apparently are encode d by the same gene. To test whether these two mRNAs are functional, pr otein extracts from Arabidopsis cells were fractionated by anion excha nge chromatography. This fractionation revealed two separate peaks-one containing both coeluted lysine-ketoglutarate reductase and saccharop ine dehydrogenase activities and the second containing only saccharopi ne dehydrogenase activity. RNA gel blot analysis and in situ hybridiza tion showed that the gene encoding lysine-ketoglutarate reductase and saccharopine dehydrogenase is significantly upregulated in floral orga ns and in embryonic tissues of developing seeds. Our results suggest t hat lysine catabolism is subject to complex developmental and physiolo gical regulation, which may operate at gene expression as well as post -translational levels.