Role of the Bacillus methanolicus Citrate Synthase II Gene, citY , in Regulating the Secretion of Glutamate in l -Lysine-Secreting Mutants

Abstract

The thermotolerant, restrictive methylotroph Bacillus methanolicus MGA3 (ATCC 53907) can secrete 55 g of glutamate per liter (maximum yield, 0.36 g/g) at 50°C with methanol as a carbon source and a source of ammonia in fed-batch bioreactors. A homoserine dehydrogenase mutant, 13A52-8A66, secreting up to 35 g of l -lysine per liter in fed-batch fermentations had minimal 2-oxoglutarate dehydrogenase activity [7.3 nmol min ⁻¹ (mg of protein) ⁻¹ ], threefold-increased pyruvate carboxylase activity [535 nmol min ⁻¹ (mg of protein) ⁻¹ ], and elevated citrate synthase (CS) activity [292 nmol min ⁻¹ (mg of protein) ⁻¹ ] and simultaneously secreted glutamate (20 to 30 g per liter) and l -lysine. The flow of carbon from oxaloacetate is split between transamination to aspartate and formation of citrate. To investigate the regulation of this branch point, the B. methanolicus gene citY encoding a CSII protein with activity at 50°C was cloned from 13A52-8A66 into a CS-deficient Escherichia coli K2-1-4 strain. A citY -deficient B. methanolicus mutant, NCS-L-7, was also isolated from the parent strain of 13A52-8A66 by N -methyl- N ′-nitro- N -nitrosoguanidine mutagenesis, followed by selection with monofluoroacetate disks on glutamate plates. Characterization of these strains confirmed that citY in strain 13A52-8A66 was not altered and that B. methanolicus possessed several forms of CS. Analysis of citY cloned from NCS-L-7 showed that the reduced CS activity resulted from a frameshift mutation. The level of glutamate secreted by NCS-L-7 was reduced sevenfold and the ratio of l -lysine to glutamate secreted was increased 4.5-fold compared to the wild type in fed-batch cultures with glutamate feeding. This indicates that glutamate secretion in l -lysine-overproducing mutants can be altered in favor of increased l -lysine secretion by regulating in vivo CS activity.