The PU.1 protein is an ets-related transcription factor that is expressed in macrophages and B lymphocytes. We present evidence that PU.1 binds to the promoter of the I-A beta gene, i.e. a PU box located next to the Y box. Transfection of PU.1 in B lymphocytes or in interferon-gamma-treated macrophages represses I-A beta gene expression. The inhibitory effect of PU.1 was obtained with the DNA binding domain of the protein, but not with the activation domain. Using the gel shift retardation assay we found that in vitro transcribed/translated NF-YA and NF-YB bind to the Y box of the I-A beta promoter. When PU.1 was added to the assay, a supershifted DNA band was found, indicating that PU.1 and NFY proteins bind to the same DNA molecule. We conclude that I-A beta gene expression is repressed by PU.1 binding to the PU box domain.
