Regulation of aspartokinase, aspartate semialdehyde dehydrogenase, dihydrodipicolinate synthase and dihydrodipicolinate reductase in Lactobacillus plantarum

Abstract

The use of a lysine-overproducing strain ofLactobacillus plantarumin food or feed fermentations may lead to the production of lysine-rich products. The availability of functional genes and information on the regulation of lysine biosynthesis are required to develop a lysine-overproducing strain. The genome sequence ofL. plantarumrevealed putative lysine biosynthetic genes, some of which may produce isozymes. This study examined the functionality of the genes and the regulation of the first four enzymes of lysine biosynthesis, together with homoserine dehydrogenase, inL. plantarum. The genes were expressed inEscherichia coli, and the regulation of the enzymes was studied in cell extracts of both recombinantE. coliandL. plantarum. Among seven lysine biosynthetic genes studied (aspartokinase genes,thrA1andthrA2; aspartate semialdehyde dehydrogenase genes,asd1andasd2; dihydrodipicolinate synthase genes,dapA1anddapA2; and the dihydrodipicolinate reductase gene,dapB) plus two homoserine dehydrogenase genes (hom1andhom2), the products of six genes, i.e.thrA2,asd2,dapA1,dapB,hom1andhom2, showed obvious enzyme activitiesin vitro. The product of one of the homoserine dehydrogenase genes,hom1, exhibited both homoserine dehydrogenase and aspartokinase activities. However, the aspartokinase activity was mainly due to ThrA2 and was inhibited byl-lysine and repressed byl-threonine, and the homoserine dehydrogenase activity was mainly due to Hom2 and was inhibited byl-threonine. The aspartate semialdehyde dehydrogenase, dihydrodipicolinate synthase and dihydrodipicolinate reductase were not regulated by the end-products of the pathway.