[Screening, identification and characterization of thermotolerant dextranase from a fungus].

Objective: We attempted to obtain a fungus producing thermotolerant dextranase by screening samples from soil. Methods: The fungus producing thermotolerant dextranase was isolated and screened by auxotrophic medium, combined with Pour Plate method and Flat Transparent Circle method. The strain was identified by its colony, cell morphology and cultural characteristics, as well as ITS rDNA sequence analysis. The dextranase produced by the strain was characterized. Results: We obtained the strain DG001 producing thermotolerant dextranase, which was identified as Paecilomyces lilacinus. The optimum catalytic conditions for the dextranase were 55℃, pH 5.0, and the optimum substrate concentration was 5% dextran T70. The dextranase was stable below 60℃ and between pH 4.0 and 7.0. Urea, Mn2+ and Mg2+ could increase enzyme activity, and the low concentration of Mn2+ and Urea could increase enzyme activity to 116.91% and 110.14% respectively, whereas Cu2+ had a strong inhibitory effect on the dextranase. The dextranase, identified as endo-dextranase, hydrolyzed dextran T2000 with main products as isomalt and isomaltotriose. The enzyme-substrate affinity increased with the increasing substrate molecular weight. Conclusion: Strain DG001 producing thermotolerant dextranase was obtained through successful screening, bearing a high activity in a wide temperature range and a good thermal stability. This enzyme shows a promising prospect of application in sugar industry and in the preparation of different molecular weight dextran.