3.2. Construction of an invertase P. pastoris overproducer strain
DNA sequencing of the pUC57 plasmid confirmed the correct nucleotide sequences of the invGS gene. The invertase synthetic gene had a full length of 1755 bp. We engineered the DNA sequencetoproduce theA. niger GH1 invertase as amaturepolypeptide of 589amino acids. PCR analysis of the recombinant plasmid, using AOX1
primers, showed a 2188 bp product that confirmed the pPIC9invGS construct.Transformation of P. pastoris KM71 with the SalI-digested pPIC9invGS plasmid gave about 24 His+ transformants. PCR analysis of the genomic DNAs isolated from P. pastoris KM71invGS (His+transformants) showed a 2188-bp band that corresponds to the alpha-factor prepro-secretion signal (255 bp) and synthetic invertase (1725 bp) coding sequences, and fragments of the multiple cloning site from the pPIC9 vector (15 bp), AOX1 promoter (94 bp),and transcription terminator (99 bp).Cell-free culture media from 24 h methanol-induced recombinant strain micro-cultures showed an extracellular protein per cell ranging from 0.31 to 1.87 pg/cell for the 24 recombinant stainstested. The selected P. pastoris KM71invGS strain yielded an extracellular
proteinper cellthat was 6-foldhigher thanthe extracellularprotein per cell determined for the lowest producer strain.