Formate from THF-C1 metabolism induces the AOX1 promoter in formate dehydrogenase-deficient <i>Pichia pastoris</i>

Pichia pastoris(Komagataella phaffii), formate is a recognized alternative inducer to methanol for expression systems based on the AOX1 promoter (pAOX1). By disrupting the formate dehydrogenase encoding FDH1 gene, we converted such a system into a self-induced one, as adding any inducer in the culture medium is no longer requested for pAOX1 induction. In cells, formate is generated from serine through the THF-C1 metabolism, and it cannot be converted into carbon dioxide in an fdh1Δ strain. Under non-repressive culture conditions, such as on sorbitol, the intracellular formate generated from the THF-C1 metabolism is sufficient to induce pAOX1 and initiate protein synthesis. This was evidenced for two model proteins, namely intracellular eGFP and secreted CalB lipase from C. antarctica. Similar protein productivities were obtained for anfdh1Δ strain on sorbitol and a non-disrupted strain on sorbitol-methanol. Considering a P. pastoris fdh1Δstrain as a workhorse for recombinant protein synthesis paves the way for the further development of methanol-free processes in P. pastoris.