In the yeast Saccharomyces cerevisiae, biochemical and genetic evidence have established that a number of phospholipid biosynthetic enzymes are coordinately regulated in response to the soluble precursors inositol and choline and a common set of regulatory factors (1). The ino2 and ino4 mutants show pleiotropic defects in phospholipid metabolism. Recessive mutations at the IN02 locus lead to reduced phosphatidylcholine synthesis and inositol auxotrophy due to an inability to derepress expression of the INO] structural gene (which encodes inositol-1-phosphate synthase) (2), (3). Ino2 mutant extracts also lack a specific DNAprotein complex that is present in wildtype extracts (4). Thus, the IN02 locus encodes a positive regulatory factor required for derepression of the coregulated phospholipid biosynthetic enzymes.The wildtype IN02 gene was isolated by functional complementation of the inositol auxotrophy in an ino2 mutant. Upon transformation with a partial Sau3A genomic library, one plasmid harboring a 1.8 kilobase SmaI-Xbal insert restored inositol prototrophy. Integrative transformation established linkage to the ino2-21 mutation. In a cross between an integrant and wildtype strain 48 tetrads showed 4:0 segregation for the Ino+ phenotype, confirming that the cloned DNA represents the authentic IN02 locus. The nucleotide sequence of the IN02 gene was determined on both strands by the Sanger dideoxy chain termination method (5). Computer-assisted sequence analysis revealed 912 base pair open reading frame, capable of encoding a 304 amino acid protein with a predicted molecular mass of 34,234 daltons. The Ino2 protein (Ino2p) is largely hydrophilic and acidic (pl = 5.76). Proline residues comprise 8.5% of the protein. A potential structural similarity between the carboxy-terminus of Ino2p and the helix-loop-helix (HLH) domain of the proto-oncogene c-myc mapped to residues 253-291 of Ino2p (see Figure 1) (6). Basic residues precede the HLH domain of Ino2p. Interestingly, the Ino4 protein, which encodes a known transcriptional activator of phospholipid synthesis shares sequence similarity to Ino2p in a region that is restricted the the 68 amino acid HLH structural domain (9). Extracts prepared from ino4 mutants lack the same DNA-protein complex that is missing in extracts prepared from ino2 mutants (4). Thus, it is tempting to speculate that Ino2p and Ino4p may be dimerization partners that associate in a DNA-protein complex to regulate the expression of phospholipid structural genes. ACKNOWLEDGEMENTSWe thank Dr John E.Hill for expertise in the computer sequence analysis.

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