COORDINATE REGULATION OF PHOSPHOLIPID BIOSYNTHESIS IN SACCHAROMYCES CEREVISIAE (INOSITOL, YEAST, MUTANTS, METHYLATION, GENETICS)
LOEWY, BRENDA S.
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Coordinate regulation between a membrane bound process and a cytoplasmic enzyme has been discovered in the phospholipid biosynthetic pathway in Saccharomyces cerevisiae. This striking observation was made through analysis of ino2 and ino4 mutants, originally isolated on the basis of inositol auxotrophy. These mutants are unable to derepress the cytoplasmic enzyme inositol-1-phosphate synthase (I-1-P S) and were thus identified as positive regulatory genes whose wild type products are required for expression of the INO1 gene product I-1-P S. We have shown that these mutations also exert pleiotropic effects on the methylation of phosphatidylethanolamine (PE) to form phosphatidylcholine (PC) a process catalyzed by membrane associated methyltransferases. All existing alleles of the INO2 and INO4 loci exhibit a deficiency in the synthesis of PC and an elevation in PE and the two methylated derivatives. We set out to study this coordinate regulation by isolating mutants relaxed in this regulation. Spontaneous inositol prototrophs were selected against an ino2 ino4 double mutant background. One such mutant is an overproducer of inositol and retains the methylation defect in PC biosynthesis. The mutation confers constitutive synthesis of I-1-P S and is linked to the INO1 locus, structural gene for I-1-P S. Southern blots performed on genomic DNA of the mutant using cloned INO1 DNA as a probe revealed that the mutant is the result of a major rearrangement at the INO1 locus consistent with the insertion of a transposable element. In addition to insertion or rearrangement mutations at INO1 the selection for ino('+) "revertants" also resulted in the identification of a new locus, OPI5. The new class of mutants, also confer constitutive synthesis of I-1-P S while retaining the methylation defect but are unlinked to INO1. They differ from all other previously isolated constitutive mutants (Greenberg et al., 1982) in that they are both dominant, and epistatic to ino2 and ino4. This new locus may identify a positive regulator of INO1 that communicates more directly on INO1 than the products of the INO2 and INO4 genes.