Most P. pastoris host strains grow on methanol at the wild-type rate (Mut+, methanol utilization plus phenotype). However, two other types of host strains are available that vary with regard to their ability to utilize methanol because of deletions in one or both AOX genes. Strains with AOX mutations are sometimes better producers of foreign proteins than wild-type strains (Cregg et al. 1987; Tschopp et al. 1987; Chiruvolu et al. 1997). Additionally, these strains require only small amounts of methanol for induction and, therefore, do not have the potential methanol feeding problems that large-scale fermentations of Mut+ strains can experience. KM71 (his4 arg4 aox1Δ::SARG4) is a strain in which AOX1 has been partially deleted and replaced with the S. cerevisiae ARG4 gene (Cregg and Madden 1987). Since the strain must rely on the weaker AOX2 for methanol metabolism, it grows slowly on this carbon source (Muts, methanol utilization slow phenotype). Another strain, MC100–3 (his4 arg4 aox1Δ::SARG4 aox2Δ::Phis4), is deleted for both AOX genes and is totally unable to grow on methanol (Mut–, methanol utilization minus phenotype) (Cregg et al. 1989; Chiruvolu et al. 1997). All of these strains, including the Mut– strain, retain the ability to grow at wild-type rates on other carbon sources and induce expression at high levels from the AOX1 promoter.
We do not recommend that new P. pastoris users utilize these AOX-mutated strains as a first choice for their expression projects but try them only after they have performed initial and reasonably successful expression experiments with a wild-type host to see if these mutant host are better producers or in more convenient for their specific usage.