A cloned gene encoding phosphatidylserine decarboxylase complements the phosphatidylserine biosynthetic defect of a Chinese hamster ovary cell mutant.

A phosphatidylserine-auxotrophic mutant of cultured Chinese hamster ovary cells, PSA-3, manifests a defect in phosphatidylserine synthase I activity (Kuge, O., Nishijima, M., and Akamatsu, Y. (1986) J. Biol. Chem. 261, 5790-5794). We cloned a Chinese hamster gene, designated pssC, which was able to transform the PSA-3 cell line to a phosphatidylserine prototroph. The resultant transformant contained phosphatidylserine in normal amounts but remained defective in phosphatidylserine synthase I activity, indicating that pssC is a suppressor gene. Using the genomic fragment of pssC as a probe, a cDNA clone of pssC was isolated, and its nucleotide sequence was determined. A computer search through a protein data bank revealed that pssC had homology with the Escherichia coli psd gene encoding the proenzyme of phosphatidylserine decarboxylase at the amino acid level. Introduction of the cloned pssC gene into PSA-3 resulted in a 2-fold increase in phosphatidylserine decarboxylase activity. When the pssC cDNA was placed downstream of the yeast GAL1 promoter and introduced into yeast Saccharomyces cerevisiae cells, the phosphatidylserine decarboxylase activity increased in a galactose-dependent manner. These results indicate that pssC encodes phosphatidylserine decarboxylase. The mechanism by which pssC complements the defect of PSA-3 in phosphatidylserine biosynthesis is discussed.