Primary Structure and Product Characterization of the Saccharomyces cerevisiae CHO1 Gene That Encodes Phosphatidylserine Synthase

An open reading frame of 828 base pairs was found in the CHOI gene region of Saccharomyces cerevisiae by nucleotide sequencing analysis. Its enhanced expression with the aid of the PH05 regulatory sequence resulted in an overproduction of a protein with a molecular weight of approximately 30,000, which in turn was converted by proteolysis to active phosphatidylserine synthase, whose molecular weight was approximately 23,000. The larger protein was concluded to be the primary product of the CHOI gene, since its amino-terminal sequence was identical to that deduced from the nucleotide sequence of the above open reading frame, except for the terminal methionine residue. A partial homology in primary structures was noticed between this yeast enzyme and phosphatidylglycerophosphate synthase of Escherichia coli which also uses CDP-diacylglycerol as a substrate. The overproduced phosphatidylserine synthase in both microsomal and extensively purified fractions displayed two different Km values for L-serine, i.e., 0.14 mM at low L-serine concentrations and 9.5 mM at high L-serine concentrations. This may indicate a negatively cooperative regulation of this enzyme activity or the presence of two active components with different affinities for L-serine.