3.1. pho13-negative phenotype was found in a few xylose-fermenting engineered strains
As reported previously, pho13 improves the xylose fermentation capability of engineered strains of S. cerevisiae(Ni et al., 2007; Van Vleet et al., 2008), and other studies have confirmed it with different strain backgrounds (Fujitomi et al., 2012; Jeong et al., 2020; Kim et al., 2013d). Although the detailed molecular mechanism is still unknown, pho13 results in the transcriptional activation of the genes in non-oxidative PPP (Kim et al., 2015; Ye et al., 2019) and reduction of the dephosphorylation product of sedoheptulose-7-phosphate, possibly suggesting the phosphatase activity of Pho13 (Xu et al., 2016). To further explorepho13 -mediated metabolic regulation, the pho13 effect was tested with a broader range of strains that we have constructed, as listed in Fig. 1a and Table 1. When a plasmid expressing heterologous xylose pathway (XYL1 , XYL2 , and XYL3 derived fromS. stipitis ) was introduced to four different strain backgrounds, all of the resulting strains (DX123, JX123, CX123, and LX123) and their derivatives (SR6, SR7, and DGX23) showed an improved xylose consumption rate by pho13 . However, two strains, YSX3 and its derivative (DA24), which were constructed independently from LX123 strain, did not show any improvement by pho13 . Because the YSX3 strain underwent multiple transformations and vigorous screening processes for strain optimization previously (Jin et al., 2003), we suspected that some unknown mutations were acquired by chance.