CDP-diacylglycerol (
CDP-DG) is an important branchpoint intermediate in eucaryotic
phospholipid biosynthesis and could be a key regulatory site in
phospholipid metabolism. Therefore, we examined the effects of growth phase,
phospholipid precursors, and the disruption of
phosphatidylcholine (PC) synthesis on the membrane-associated
phospholipid biosynthetic
enzymes CDP-DG synthase, phosphatidylglycerolphosphate (
PGP) synthase,
phosphatidylinositol (PI) synthase, and
phosphatidylserine (PS) synthase in
cell extracts of the fission yeast Schizosaccharomyces pombe. In complete synthetic medium containing
inositol, maximal expression of
CDP-DG synthase,
PGP synthase, PI synthase, and PS synthase in wild-type cells occurred in the exponential phase of growth and decreased two- to fourfold in the stationary phase of growth. In cells starved for
inositol, this decrease in
PGP synthase, PI synthase, and PS synthase expression was not observed.
Starvation for
inositol resulted in a twofold derepression of
PGP synthase and PS synthase expression, while PI synthase expression decreased initially and then remained constant. Upon the addition of
inositol to
inositol-starved cells, there was a rapid and continued increase in PI synthase expression. We examined expression of these
enzymes in cho2 and cho1 mutants, which are blocked in the methylation pathway for synthesis of PC.
Choline starvation resulted in a decrease in PS synthase and
CDP-DG synthase expression in cho1 but not cho2 cells. Expression of
PGP synthase and PI synthase was not affected by
choline starvation.
Inositol starvation resulted in a 1.7-fold derepression of
PGP synthase expression in cho2 but not cho1 cells when PC was synthesized. PS synthase expression was not depressed, while
CDP-DG synthase and PI synthase expression decreased in cho2 and cho1 cells in the absence of
inositol. These results demonstrate that (i)
CDP-DG synthase,
PGP synthase, PI synthase, and PS synthase are similarly regulated by growth phase; (ii)
inositol affects the expression of
PGP synthase, PI synthase, and PS synthase; (iii) disruption of the methylation pathway results in aberrant patterns of regulation of growth phase and
phospholipid precursors. Important differences between S. pombe and Saccharomyces cerevisiae with regard to regulation of these
enzymes are discussed.