Development of a β-glucosidase hyperproducing mutant by combined chemical and UV mutagenesis

The extracellular β-glucosidase from microorganisms is generally produced in low levels. Therefore, in this study, a β-glucosidase hyperproducing mutant was developed by multiple exposures of ethyl methyl sulfonate (EMS) and ultraviolet (UV) radiation (both individually and jointly) to Bacillus subtilis strain (PS). The developed mutants were screened, selected and characterized. The mutant, PS-UM1 developed after UV exposure alone, indicated a small increase in β-glucosidase production (718 U/l) in comparison to the wild-type strain, PS (675 U/l). The mutant, PS-CM5 developed after EMS exposure alone, displayed a slightly better production (762 U/l) than both the above strains. However, after exposure of the wild-type strain to both UV and EMS mutagens jointly, a better mutant (PS-CM5-UM3) was developed with 1.2-fold increase in production (806 U/l). Further, optimization of culture conditions by classical "one-variable-at-a-time" approach was done to determine the optimum, pH, temperature and nitrogen sources. The selected mutant (PS-CM5-UM3) produced up to 1,797 U/l enzyme and was found to be stable for ten generations. The β-glucosidase from the selected mutant (PS-CM5-UM3) was concentrated and purified using ammonium sulfate, dialysis and size-exclusion chromatography. The enzyme displayed maximal activity at 60 °C and it was found to be fairly stable at temperatures up to 70 °C for 30 min. Its molecular weight was determined to be around 60 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).