PHOSPHATIDYLCHOLINE BIOSYNTHESIS VIA THE CDP-CHOLINE PATHWAY IN SACCHAROMYCES-CEREVISIAE - MULTIPLE MECHANISMS OF REGULATION

Multiple mechanisms of regulation in the CDP-choline pathway for phosphatidylcholine (PC) synthesis were revealed by exploring the effects of choline and inositol on this pathway in Saccharomyces cerevisiae. At exogenous choline concentrations below 100 mu M, phosphocholine cytidylyltransferase was rate-limiting; at higher choline concentrations the conversion of choline to phosphocholine by choline kinase became rate-limiting. Choline and inositol were found to regulate choline uptake; this established another regulatory mechanism by which PC synthesis is regulated in yeast. Inositol addition did not immediately affect labeled choline uptake or its incorporation into PC in actively dividing cells; however, preculturing the cells in the presence of choline decreased the rate of choline uptake, and this effect was amplified by the concomitant addition of inositol and choline. Additionally, a growth phase dependent effect of inositol supplementation was observed. Inositol addition to stationary phase cells resulted in an increase in choline uptake and subsequent PC production in these cells. This increase was shown to be due to an increase in the rate of choline transport into the cell. In the presence of inositol, choline transport is the main regulatory mechanism controlling flux through the CDP-choline pathway in S. cerevisiae. Inositol supplementation resulted in changes in the levels of enzyme activity detected in vitro. However, the effects observed in vivo correlated exclusively with changes in choline uptake. Choline transporter assays were consistent with these results. Since both the CPT1 and EPT1 gene products catalyze the cholinephosphotransferase reaction in vitro (Hjelmstad, R. H., and Bell, R. M. (1991) J. Biol. Chem. 266, 4357-4365), the effect of inositol on these two separate routes for PC biosynthesis was investigated. The data revealed that only cells harboring a functional CPT1 gene synthesized PC in vivo. These cells (ept1-Delta 1::URA3) also displayed an identical mode of regulation in response to inositol as did cells containing an intact EPT1 gene (wild type) indicating there is no requirement for an alternate functional CDP-aminoalcohol pathway for inositol to regulate PC synthesis via the CDP-choline pathway.