A cysteine residue in helixII 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. Functional studies have shown that the molecule is composed of an N-terminal acidic activation domain, a central region which is necessary for interaction with a negative regulatory factor (the cyclin Pho80) and a C-terminal basic helix-loop-helix domain, which mediates DNA binding and homodimerization. In this study the homodimerization domain maps specifically to helixII of this region and a cysteine residue within this region is essential for this function. Experiments support the role of an intermolecular disulfide bond in stabilization of homodimerization, which is critical for DNA binding.