Details of the primers used in this study are listed in Table 2. The plasmid used for expression of GCY1 and DAK1 was constructed as follows: the PGK1 promoter and terminator were cloned by PCR amplification from S. cerevisiae genomic DNA with primers PF1 and PR2, TF1 and TR2, respectively. The two fragments obtained were digested by SalI, and then inserted into pMD18-T simple vector to create plasmid pMG. The gene which confers resistance to geneticin (G418) in S. cerevisiae was isolated from pPIC9K using primers KF1 and KR2. The primers used to amplify this fragment were designed to introduce the 34-bp loxp site at the 5′- and 3′-ends. The PCR product was cut with NotI and inserted into the corresponding site of pMG. Following this, a partial rDNA fragment of S. cerevisiae used as a homologous integration site was cloned with primers RF1 and RR2, and ligated into the NdeI site of pMG, forming pMGKR. GCY1 and DAK1 were amplified from S. cerevisiae genomic DNA with primers GCY1F and GCY1R, DAK1F and DAK1R, respectively. The amplified fragments were digested by EcoRI and SalI, and inserted into the relevant site of pMGKR, resulting in pMGKR-GCY1 and pMGKR-DAK1, respectively. The PGK1_PT-DAK1 gene cassette was then amplified from pMGKR-DAK1 with primers PF1 and TR2, and then cloned into a unique KpnI site of pMGKR-GCY1 to create pMGKR-GCY1-DAK1 (supplementary Fig. 1). The plasmid pMGKR-GCY1-DAK1 was linearized with SacII. Following purification, the resultant linear DNA fragment was introduced into S. cerevisiae CICIMY0086 via the lithium acetate method. G418 was added to a final concentration of 0.5 mg/ml for yeast selection. The recombinant strain of S. cerevisiae was designated GDS1 (P_PGK-GCY1-DAK1).