Pichia kudriavzevii WB17-1, a phytase-producing yeast isolated from duck excrement,
was found to produce both cell-bound and extracellular phytases. To enhance extracellular
phytase, P. kudriavzevii WB17-1 was subjected to induced mutation. Ethylmethane sulfonate
(EMS) induced mutation resulted in 2,400 mutants. The mutant P. kudriavzevii WB17-1 EMS3
showed the highest extracellular phytase activity. This mutant possessed a 6.2-fold increase in
enzyme activity compared to the wild type level. The wild type and mutant were subjected to
characterization of PHYPk, a gene encoding P. kudriavzevii phytase. An open reading frame of
1,071 bp encoding 357 amino acids with a predicted protein molecular mass of 40.056 kDa
was identified. To optimize the extracellular phytase activity of P. kudriavzevii WB17-1 EMS3, a
response surface methodology (RSM) was employed. The highest extracellular phytase activity
was obtained in the medium containing 2.95% dextrose and 0.58% peptone with an initial pH
of 5.8. The optimized phytase activity was 2.3 times the level obtained under unoptimized
conditions and was 14.4 times the wild type level. The results obtained here show successful
yeast strain improvement and optimization of extracellular phytase by yeast using RSM.