The bacterium AG-2 originally isolated from soil sample in Nontaburi province was capable of producing dextranase at 2.446 units/ml. The strain was classified by morphological and biochemical characteristics including 16S rRNA gene sequencing as Arthrobacter sp. After 3 rounds of UV exposure for 80-120 sec, a mutant strain designated CU15 was isolated that capable of producing dextranase at 3.201 unit/ml. Further treatment of cell suspension thereof with 50 mug/ml of NTG (N-methyl-N'-nitro-N-nitrosoguanidine) for 5-60 min at 37 ํC, yielded in a number of mutants with high dextranase activities after four rounds of treatment. The highest producing strain, CUN4-10, gave the dextranase avtivity of 4.530 units/ml. However, this organism failed to showed its stability toward dextranase production meanwhile strain CUN26, obtained 3 rounds of UV exposure and 1 round of NTG treatment respectively, was quite stable by giving approximately the same dextranase activity after 30 rounds of subculture. Under the optimalconditions for dextranase production, strain CUN26 was able to produce 6.276 units of dextranase per ml, or a 2.50-fold increase after appropiate mutation and medium optimization. Optimal conditions for dextranase activity were at 50 ํC, pH 6.5 in 0.05 M phosphate buffer. The enzyme produced exhibit stability to pH from 5.0 to 9.0 and to temperature up to 45 ํC. The enzyme had an apparent Km value for dextran T-2000 of 2.796 muM.
