Schematic representation of the construction of the expression vectors in this work. (A) The inducible expression vectors, pAOαMH-Kla and pAOαMH-Aor, carrying β-galactosidase gene from Aspergillus oryzae and Kluyveromyces lactis, respectively. (B) The constitutive expression vectors, pGAPα-Aor, pSDHZαH-Aor and pTef1ZαH-Aor with PGAP, PSDH, and PTEF1 as promoters. pGAPα-Aor contains no HIS4 gene. (C) The chaperone co-expression vectors, pGAPZ-PDI, pGAPZ-KAR and pGAPZ-SSO carrying PD1, KAR2 and SSO1, respectively.