Combinatorial regulation of phospholipid biosynthetic gene expression by the UME6, SIN3 and RPD3 genes

The Ume6p–Sin3p–Rpd3p complex negatively regu-lates expression of genes containing a Ume6p binding site. However, these regulatory proteins also function independently to regulate gene expression both negatively and positively. The model system for this combinatorial regulation is the yeast phospho-lipid biosynthetic pathway. Sin3p negatively regu-lates the INO1, CHO1, CHO2 and OPI3 genes while Ume6p negatively regulates the INO1 gene and posi-tively regulates the other genes. We have suggested that the positive regulation results from indirect effects on expression of the INO2 transcriptional activator gene. Here, we demonstrate that the effect of Ume6p on INO2 gene expression is also indirect. We also show that Rpd3p is a negative regulator of phospholipid biosynthetic gene expression. The ability of Ume6p, Sin3p and Rpd3p to differentially regulate expression of the phospholipid biosynthetic genes affects phospholipid composition. A sin3 mutant strain lacks detectable levels of phospha-tidylethanolamine and elevated levels of phospha-tidylcholine (PC) and a rpd3 mutant strain has reduced levels of PC. These alterations in membrane composition suggest that there may exist additional differences in regulation of phospholipid biosyn-thetic gene expression and that membrane composi-tions may be coordinated with other biological processes regulated by Ume6p, Sin3p and Rpd3p