Expression of the glucose oxidase gene from Aspergillus niger in Hansenula polymorpha and its use as a reporter gene to isolate regulatory mutations

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Abstract
The glucose oxidase gene (god) from Aspergillus niger was expressed in Hansenula polymorpha using the methanol oxidase promoter and transcription termination region and the MF‐α leader sequence from Saccharomyces cerevisiae to direct secretion. The expression cassette was cloned into the S. cerevisiae vector YEp13 and used to transform H. polymorpha strain A16. In the initial transformants plasmid replication was unstable, but was stabilized by a growth regime consisting of alternating cycles of selective and non‐selective growth. The stabilized strain was grown to high cell density by fed‐batch fermentation. Upon induction of the MOX promoter, glucose oxidase synthesis was initiated. At the end of the fermentation, the culture density was 76 g dry weight/1 and 108 IU/ml (0·5 g/l or 0·65% dry weight) glucose oxidase was found in the culture medium; a further 86 IU/ml (0·43 g/l or 0·56% dry weight) was recovered from the cell lysate. A plate assay was used to monitor glucose oxidase levels in individual colonies. This was then used to isolate mutants which showed abnormal regulation of god expression or which showed an altered pattern of secretion. One mutant, which showed increased production of glucose oxidase, was grown to high cell density by fed‐batch fermentation (100·6 g/l) and produced 445 IU/ml (2·25 g/l or 2·2% dry weight) extracellularly and 76 IU/ml (0·38 g/l or 0·4% dry weight) intracellularly. The mutant thus not only increased total production but exported 83% of the total enzyme made compared to 55% in the parent strain.