AOX1 promoter-driven expression of yeast-enhanced green fluorescent protein in Pichia pastoris

Green fluorescent protein (GFP) is a widely employed reporter for studying various cellular functions in genetic engineering. Methylotrophic yeast; Pichia pastoris is an excellent host for heterologous protein production. Codon optimized yeast-enhanced green fluorescent protein (yEGFP) has been reported to have higher expression as a yeast reporter which can be used to study many aspects of P. pastoris. The present investigation was focused on studying the yEGFP intracellular expression pattern in the P. pastoris GS115 strain. yEGFP is cloned into the pPICZ A vector under the control of an AOX1 (Alcohol Oxidase) promoter that was induced by adding methanol to the growth medium. However, methanol concentration and time of induction could affect the optimum expression of yEGFP. Hence, those conditions were tested for manifesting optimum yEGFP expression by monitoring the fluorescence intensities in minimal methanol histidine media supplemented with 0.5%, 1%, 2%, and 2.5% methanol. The pH of the media and the composition of the media (minimal or complex) also affect recombinant protein production. The effect of pH and the media composition was investigated in buffered media at pH 6 and unbuffered media of minimal methanol histidine media and methanol complex media. The results showed that the yEGFP expression increased up to the 5th day of induction and then decreased on the 6th day, 2% methanol showed the highest expression, and the pH and the media composition did not have any effect on the yEGFP expression. In conclusion, the optimum yEGFP expression can be obtained in 2% methanol on the 5th day of induction irrespective of minimal or complex media used.