Nucleotide sequence and characterization of the yeast PSS gene encoding phosphatidylserine synthase

1. A yeast chromosomal DNA which contains the structural gene for phosphatidylserine synthase (PSS) was isolated by genetic complementation from a wild-type yeast genomic library. The PSS gene was subcloned into a 1.1-kb fragment of the yeast DNA on the YEp13 vector. 2. The PSS gene on the multicopy plasmid caused the fourfold over-production of the enzyme and fully restored the phosphatidylserine content of the transformant. The phospholipid composition of the transformant was similar to that of the wild type. 3. Sequence analysis showed that this DNA fragment contains an open reading frame capable of encoding 276 amino acid residues with a calculated relative molecular mass of 30,804. Northern blot analysis of poly(A)-rich RNA of the wild-type yeast indicated that this DNA segment is transcribed into a single mRNA species. 4. The DNA sequence contained two putative transcriptional initiation signals, each followed by the ATG initiator codon. Deletion experiments indicated that the 5'-proximal ATG codon is essential for the synthesis of the functional phosphatidylserine synthase.