Functional characterization of the INO2 gene of Saccharomyces cerevisiae. A positive regulator of phospholipid biosynthesis.

The INO2 locus encodes a novel product showing structural similarity to the basic helix-loop-helix (b-HLH) family of regulatory proteins (Nikoloff, D.M., McGraw, P., and Henry, S.A. (1992) Nucleic Acids Res. 20, 3253). The ino2 mutants exhibit pleiotropic defects in phospholipid metabolism including inability to derepress the biosynthetic enzyme inositol-1-phosphate synthase. Localization of mutations in ino2 strains has demonstrated that the b-HLH domain is required for biological activity and is sensitive to perturbation, thereby establishing a correlation between the structure and function of Ino2p. Defects in the b-HLH domain of Ino2p resulted in reduced DNA binding activity. In addition, the absence of a specific DNA-protein complex correlated with a reduction or loss of INO1 transcription. Studies using Ino2p-specific antibody revealed that Ino2p participates in the formation of specific DNA-protein complexes. Ino2p-dependent binding activity overlapped with a region of the INO1 promoter that contains two potential HLH consensus binding sites. Furthermore, Ino2p showed single base pair discrimination in a putative binding site, establishing a relationship between Ino2p and its target binding site.