Isolation of mutants of Aspergillus awamori with enhanced production of extracellular xylanase and β-xylosidase.
Plate screening tests were designed for the selection and isolation of mutant strains of the fungus Aspergillus awamori CMI 142717 showing over-production and constitutive synthesis of xylanase and β-xylosidase. Following mutation by N-methyl-N-nitro-N-nitrosoguanidine, nitrous acid and UV (254 nm), two generations of mutants were isolated and cultured in shake fiasks containing glucose, ball-milled oat straw or oat speit xylan as carbon source. Growth of a number of selected mutants in shake flask culture on medium containing oat spelt xylan produced the highest titres of xylanase and β-xylosidase. Thus, xylanase producton by mutant AANTG43 was 132 U/ml when the Somogyl-Nelson (alkaline copper) method of measuring reducing sugar released was used, or 1160 U/ml using the dinitrosalicylic acid method of reducing sugar analysis. These values were 8-fold higher than those produced by the wild type. A 20-fold improvement in β-xylosidase production was produced by mutant AANO19 (3.51 U/ml). The titres for these two enzyme activities are the highest recorded so far in the literature. Mutant AANTG43 also produced high levels of xylanase (49.8 U/ml) in submerged culture in a fermenter and showed a substantial improvement in the overall productivity of enzyme compared to the wild type strain.