Phosphatidylethanolamine (PE) in the yeast Saccharomyces cerevisiae is synthesized through decarboxylation of phosphatidylserine (PS), catalysed by PS decarboxylase 1 (Psd1p) and 2 (Psd2p) and the cytidine 5-diphosphate (CDP)-ethanolamine (CDP-Etn) pathway. PSD1 null (psd1δ) and PSD2 null (psd2δ) mutants are viable in a synthetic minimal medium, but a psd1δ psd2δ double mutant exhibits Etn auxotrophy, which is incorporated into PE through the CDP-Etn pathway. We have previously shown that psd1δ is synthetic lethal with deletion of VID22 (vid22δ) [Kuroda et al. (2011) Mol. Microbiol. 80, 248- 265]. In the present study, we found that vid22δ mutant exhibits Etn auxotrophy under PSD1-depressed conditions. Deletion of VID22 in wild-Type and PSD1-depressed cells caused partial defects in PE formation through decarboxylation of PS. The enzyme activity of PS decarboxylase in an extract of vid22δ cells was 70% of that in wild-Type cells and similar to that in psd2δ cells and the PS decarboxylase activity remaining in the PSD1-depressed cells became almost negligible with deletion of VID22. Thus, the vid22δ mutation was suggested to cause a defect in the Psd2p activity. Furthermore, vid22δ cells were shown to be defective in expression of the PSD2 gene tagged with 6×HA, the defect being ameliorated by replacement of the native promoter of the PSD2 gene with a CYC1 promoter. In addition, an α-galactosidase reporter assay revealed that the activity of the promoter of the PSD2 gene in vid22δ cells was 5% of that in wild-Type cells. These results showed that VID22 is required for transcriptional activation of the PSD2 gene.
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