1987 Volume 102 Issue 5 Pages 1089-1100
An open reading frame of 828 base pairs was found in the CH01 gene region of Saccharomyces cerevisiae by nucleotide sequencing analysis. Its enhanced expression with the aid of the PHO5 regulatory sequence resulted in an overproduction of a protein with a molecular weight of approximately 30, 000, which in turn was con-verted 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 CH01 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 overpro-duced 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.
