A CYSTEINE RESIDUE IN HELIX(II) OF THE BHLH DOMAIN IS ESSENTIAL FOR HOMODIMERIZATION OF THE YEAST TRANSCRIPTION FACTOR PHO4P

The yeast transcription factor Pho4p is required for expression of the phosphate-repressible acid phosphatase encoded by the PHO5 gene. Func tional studies have shown that the molecule is composed of an N-termin al acidic activation domain, a central region which is necessary for i nteraction with a negative regulatory factor (the cyclin Pho80) and a C-terminal basic helix-loop-helix domain, which mediates DNA binding a nd homodimerization. In this study the homodimerization domain maps sp ecifically to helix(II) of this region and a cysteine residue within t his region is essential for this function. Experiments support the rol e of an intermolecular disulfide bond in stabilization of homodimeriza tion, which is critical for DNA binding.