Isolation and prominent characteristics of an L-lysine hyperproducing strain of Corynebacterium glutamicum

Abstract

A strain of Corynebacterium glutamicum was isolated that accumulated up to 44 g/l of L-lysine-HCl from 100 g/l of glucose × H2O in a simple mineral salts medium. This strain was obtained from the wildtype by two mutagenesis steps. In the first step the aminoethyl-cysteine-resistant strain MH20 was obtained and in the second step the Leu — derivative MH20-22B. Enzymatic analysis of the hyperproducer MH20-22B revealed that this strain has feedback-resistant aspartate kinase and is devoid of isopropylmalate dehydratase. In addition, this strain has an extraordinarily high secretion rate of lysine (0.57 mmol/g dry weight and h), whereas strain MH20 has a low secretion rate (0.19 mmol/g per hour), and both strains have comparable cytosolic lysine concentrations. This suggests that the secretory step is influenced n the hyperproducer. Applying gene-directed mutagenesis, the aspartate kinase gene of the isolated strain (coding for feedback-resistant enzyme) was replaced by the gene coding of feedback-sensitive wild-type enzyme. The resulting strain still secretes lysine, although in low amounts (≤2 g/l). This is proof of the superior role of kinase regulation in metabolite flow and is indicative of unknown mutations, one of which is probably in the secretory system.