2.1. The overexpression of heterologous xylose reductase, xylitol de hydrogenase, and endogenous xylulokinase are essential to permit Y lipolytica to grow on xylose Recently, Stephanopoulos and Tai (2013) patented an en gineered strain of Y lipolytica which overexpresses ssxR and ssXDH from S. stipitis under the control of the promoters TEFin and hp4 respectively. In this work, we constructed a strain that over expressed these genes under the control of the TEF promoter. The expression of both genes was checked by qPCR (Supplementary 1). Like the previously patented strain, our modified strain was unable to grow in xylose (Fig. 2). However, the patented strain was able to slowly utilize xylose following an adaptation of the strain using serial dilutions, as previously reported for S. cerevisiae Stephanopoulos and Tai, 2013). Unfortunately. the adapted strain had a time of 25 h, which strongly limits its industrial doubling application. In our hands, and probably due to the different strengths of the promoters, the strain that overexpressed both genes was unable to grow even after we attempted a similar adaptation. Likewise, strains Polt (prototrophic strain of Polg) and W29 (the parent of both Pold and Polg) were unable to grow with xylose as the sole carbon source (Fig. 2) of the first genes re- In order to study the transcription level ylxDH (YA quired for assimilating xylose, ylXR (YALIODO7634), analysis of LI0E12463) and yixK (YALIF10923), we performed qPCR RNA extracted from wild type Y lipolytica grown either in glucose or in xylose containing media. The highest transcription level was found for ylXR, followed by ylXDH (Fig. 3), which seems to agree