Regulatory mechanisms after short- and long-term perturbed lysine biosynthesis in the aspartate pathway: the need for isogenes in Arabidopsis thaliana.

The aspartate-derived amino acid pathway in plants is an intensively studied metabolic pathway, because of the biosynthesis of the four essential amino acids lysine, threonine, isoleucine and methionine. The pathway is mainly controlled by the key regulatory enzymes aspartate kinase (AK; EC 2.7.2.4), homoserine dehydrogenase (HSDH; EC 1.1.1.3) and 4-hydroxy-tetrahydrodipicolinate synthase (EC 4.3.3.7), formerly referred to as dihydrodipicolinate synthase (DHDPS). They are encoded by isoenzyme families and it is not known why such families are evolutionarily maintained. To gain more insight into the specific roles and regulation of the isoenzymes, we inhibited DHDPS in Arabidopsis thaliana with the chemical compound (N,N-dimethylglycinatoboranyloxycarbonylmethyl)-dimethylamine-borane (DDAB) and compared the short-term effects on the biochemical and biomolecular level to the long-term adaptations in dhdps knockout mutants. We found that DHDPS2 plays a crucial role in controlling lysine biosynthesis, thereby stabilizing flux through the whole aspartate pathway. Moreover, DHDPS2 was also shown to influence the threonine level to a large extent. In addition, the lysine-sensitive AKs, AKLYS1 and AKLYS3 control the short- and long-term responses to perturbed lysine biosynthesis in Arabidopsis thaliana.