Quantitative characterization of the regulation of the synthesis of alcohol oxidase and of the expression of recombinant avidin in a Pichia pastoris Mut+ strain

Pichia pastoris is increasingly used as a host for the expression of heterologous proteins both in academic research and at industrial scale. It is therefore important to have a good understanding of the regulation of its expression system. Usually, heterologous proteins are expressed under the control of the AOX1 promoter, which drives the expression of alcohol oxidase 1 in the wild-type strain. The regulation of the synthesis of alcohol oxidase and the recombinant protein productivity were investigated in chemostat cultures on glycerol and on methanol as sole carbon sources for a P. pastoris Mut+ strain expressing and secreting a recombinant avidin. Under glycerol limitation, alcohol oxidase was derepressed, and both on glycerol and on methanol, the specific AOX activities increased with decreasing dilution rate. However, specific AOX activities in cultures on glycerol were only 1–5% of the activities measured in cultures on methanol, and recombinant avidin could only be detected in cultures on methanol. The recombinant avidin production rate increased linearly with dilution rate in cultures on methanol. A growth-associated linear model was applied in order to predict the specific recombinant avidin productivity as a function of dilution rate. The black box stoichiometry was also characterized quantitatively as a function of dilution rate. After validation of the steady-state data with a statistical hypothesis testing method, analysis revealed that because of the low maintenance demand of P. pastoris, the stoichiometry did not change significantly in the range of dilution rate studied.