Dextran biosynthesis and dextransucrase production by continuous culture of Leuconostoc mesenteroides

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Abstract
Leuconostoc mesenteroides NRRL B‐512(F) was grown in continuous culture under conditions of energy‐limited growth. The extracellular enzyme dextransucrase (sucrose: 1,6‐α‐D‐glucan 6‐α‐glucosyltransferase EC 2.4.1.5), was not detected in glucose‐or maltose‐limited cultures. Under Conditions of sucrose‐limited growth, the enzyme activity of the cell‐free culture supernatant increased with increasing dilution rate only after the critical concentration of enzyme inducer (sucrose) in the chemostat had been achieved. The appearance of fructose in the effluent of the sucrose‐limited chemostat at higher dilution rates indicated that sucrose was being diverted to dextran biosynthesis. The competition between bacteria and extracellular enzyme for the common substrate sucrose represents an inefficiency in the system of enzyme production. Dextransucrase was isolated from the cell‐free culture supernatant by ammonium sulfate precipitation and DEAE‐cellulose chromatography. The enzyme preparation exhibited both dextran biosynthetic activity and an invertase‐like activity. The biosynthetic efficiency was increased by decreasing the temperature from 30 to 10°C. The enzyme was irreversibly denatured by prolonged incubation in the absence of Ca2+.