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Phosphorylation and the Transcription Factor YY1

Yin‐Yang 1 (YY1) is a multifunctional transcription factor that is involved in the regulation of a large array of genes controlling cellular proliferation, differentiation and even apoptosis. Although our knowledge of the events regulated by YY1 has grown significantly over the past decade, we still know little of what regulates YY1 and its functions. YY1 has been previously described as a phosphoprotein. However, only indirect evidence for this has been provided, and the role of phosphorylation in regulating YY1's functions has not been elucidated. Here we provide direct evidence for YY1's phosphorylation by in vivo labeling studies. Additionally we have studied the effect of phosphorylation on purified YY1's affinity to several of its DNA binding sites. Since YY1 is involved in the regulation of replication‐dependent histone genes at the G1/S transition of the cell cycle, we examined YY1 as a possible substrate for cell cycle kinases. We found that YY1 is a substrate for CyclinE/Cdk2 in an in vitro kinase assay. Through site directed mutagenesis, we have identified the phosphorylated amino acid. Phosphorylation of this site was independently confirmed in vivo by Mass Spec analysis of the purified protein. The function of phosphorylation of YY1 at this site is the subject of current research in our lab.