James Michael Cregg

Pichia pastoris as a host system for transformations.

We developed a methylotrophic yeast, Pichia pastoris, as a host for DNA transformations. The system is based on an auxotrophic mutant host of P. pastoris which is defective in histidinol dehydrogenase. As a selectable marker, we isolated and characterized the P. pastoris HIS4 gene. Plasmid vectors which contained either the P. pastoris or the Saccharomyces cerevisiae HIS4 gene transformed the P. pastoris mutant host. DNA transfer was accomplished by a modified version of the spheroplast generation (CaCl2-polyethylene glycol)-fusion procedure developed for S. cerevisiae. In addition, we report the isolation and characterization of P. pastoris DNA fragments with autonomous replication sequence activity. Two fragments, PARS1 and PARS2, when present on plasmids increased transformation frequencies to 10(5)/micrograms and maintained the plasmids as autonomous elements in P. pastoris cells.

Use of site-specific recombination to regenerate selectable markers.

SummaryA method which allows the repeated use of a single selectable marker in DNA transformations was demonstrated. This marker regeneration method employed portions of the Saccharomyces cerevisiae 2 μm circle plasmid: the inverted repeat sequences (FRTs), and the FLP gene whose product, a site-specific recombinase, catalyzes recombination events between FRTs. When FRTs were oriented as direct repeats and integrated into the genome of the yeast Pichia pastoris, FLP-mediated recombination resulted in the efficient and precise deletion of DNA located between the repeats. In the example described, the S. cerevisiae ARG4 gene, placed between a set of FRTs and integrated into Pichia in a prior transformation, was deleted by FLP, thereby regenerating an arginine-requiring phenotype in the P. pastoris strain.