Although frequently used as protein production host, there is only a limited set of promoters available to drive the expression of recombinant proteins in Pichia pastoris. Fine-tuning of gene expression is often needed to maximize product yield and quality. However, for efficient knowledge-based engineering, a better understanding of promoter function is indispensable. Consequently, we created a promoter library by deletion and duplication of putative transcription factor-binding sites within the AOX1 promoter (P_AOX1) sequence. This first library initially spanned an activity range between ∼6% and >160% of the wild-type promoter activity. After characterization of the promoter library employing a green fluorescent protein (GFP) variant, the new regulatory toolbox was successfully utilized in a ‘real case’, i.e. the expression of industrial enzymes. Characterization of the library under repressing, derepressing and inducing conditions displayed at least 12 cis-acting elements involved in P_AOX1-driven high-level expression. Based on this deletion analysis, novel short artificial promoter variants were constructed by combining cis-acting elements with basal promoter. In addition to improving yields and quality of heterologous protein production, the new P_AOX1 synthetic promoter library constitutes a basic toolbox to fine-tune gene expression in metabolic engineering and sequential induction of protein expression in synthetic biology.

The methylotrophic yeast Pichia pastoris (Komagataella phaffii) is one of the most commonly used expression systems for heterologous protein production. However the recombination machinery in P. pastoris is less effective in contrast to Saccharomyces cerevisiae, where efficient homologous recombination naturally facilitates genetic modifications. The lack of simple and efficient methods for gene disruption and specifically integrating cassettes has remained a bottleneck for strain engineering in P. pastoris. Therefore tools and methods for targeted genome modifications are of great interest. Here we report the establishment of CRISPR/Cas9 technologies for P. pastoris and demonstrate targeting efficiencies approaching 100%. However there appeared to be a narrow window of optimal conditions required for efficient CRISPR/Cas9 function for this host. We systematically tested combinations of various codon optimized DNA sequences of CAS9, different gRNA sequences, RNA Polymerase III and RNA Polymerase II promoters in combination with ribozymes for the expression of the gRNAs and RNA Polymerase II promoters for the expression of CAS9. Only 6 out of 95 constructs were functional for efficient genome editing. We used this optimized CRISPR/Cas9 system for gene disruption studies, to introduce multiplexed gene deletions and to test the targeted integration of homologous DNA cassettes. This system allows rapid, marker-less genome engineering in P. pastoris enabling unprecedented strain and metabolic engineering applications.