In E. coli, the rate-limiting enzyme for lysine biosynthesis is aspartate kinase (aspartokinase) (AK) [EC 2.7.2.4], with dihydrodipicolinate synthase (DHPS) [EC 4.2.1.52] playing only a secondary role. However, in plants DHPS is 20 to 100 times more sensitive to lysine inhibition than the E. coli DHPS, and is regarded to be the rate-limiting step (Galili, 1995). Selection for resistance to the lysine analog S-amino-ethylcysteine (AEC) yields mutants of the lysine-sensitive AK-III with reduced sensitivity to lysine in E. coli, but mutants of DHPS with less lysine sensitivity in higher plants. These plants over-produce lysine.

Plants containing feedback-insensitive AK isoenzymes have been identified by selecting for resistance to growth inhibition by high concentrations of lysine + threonine. These two amino acids completely inhibit the activity of all AK isoenzymes, resulting in methionine starvation. The mutants overproduce free threonine, and to a lesser extent, isoleucine and methionine, and in some cases, lysine. This implies that in contrast to bacteria, AK limits mainly threonine but not lysine synthesis. Homoserine dehydrogenase (HSD) [EC 1.1.1.3] activity could play only a secondary role in limiting threonine synthesis in plants, because in contrast to bacteria, plants contain at least one HSD isoenzyme that is insensitive to feedback inhibition (Galili, 1995).

Lysine and threonine overproducer mutants of Nicotiana sylvestris, having altered regulation of DHPS and AK, respectively, have been isolated (Frankard et al, 1992). These two mutants were crossed to assess the effects of having both mutations. The double mutant showed free lysine overproduction reaching up to 50% of the total pool; however, the plants are morphologically and developmentally impaired -- leaf size is markedly reduced, and the plants show no stem elongation or flower formation.

Bryan JK 1980 Aspartate family and branched-chain amino acids. In (BJ Miflin ed) "The Biochemistry of Plants", Vol. 5, Academic Press, New York, pp. 403-452.

Frankard V, Ghislain M, Jacobs M 1992 Two feedback-insensitive enzymes of the aspartate pathway in Nicotiana sylvestris. Plant Physiol. 99: 1285-1293.

Galili G 1995 Regulation of lysine and threonine synthesis. Plant Cell 7: 899-906.

Miflin BJ 1977 Modification controls in time and space. In (H Smith ed) "Regulation of Enzyme Synthesis and Activity in Higher Plants", Academic Press, New York, pp. 23-40.