PU.1/Pip and Basic Helix Loop Helix Zipper Transcription Factors Interact With Binding Sites in the CD20 Promoter to Help Confer Lineage- and Stage-Specific Expression of CD20 in B Lymphocytes

CD20 is a B-lineage–specific gene expressed at the pre–B-cell stage of B-cell development that disappears on differentiation to plasma cells. As such, it serves as an excellent paradigm for the study of lineage and developmental stage-specific gene expression. Using in vivo footprinting we identified two sites in the promoter at −45 and −160 that were occupied only in CD20+ B cells. The −45 site is an E box that binds basic helix-loop-helix-zipper proteins whereas the −160 site is a composite PU.1 and Pip binding site. Transfection studies with reporter constructs and various expression vectors verified the importance of these sites. The composite PU.1 and Pip site likely accounts for both lineage and stage-specific expression of CD20 whereas the CD20 E box binding proteins enhance overall promoter activity and may link the promoter to a distant enhancer.