PU.1 PIP AND BASIC HELIX-LOOP-HELIX ZIPPER TRANSCRIPTION FACTORS INTERACT WITH BINDING-SITES IN THE CD20 PROMOTER TO HELP CONFER LINEAGE-SPECIFIC 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 footp rinting we identified two sites in the promoter at -45 and -160 that w ere occupied only in CD20(+) B cells. The -45 site is an E box that bi nds basic helix-loop-helix-zipper proteins whereas the -160 site is a composite PU.1 and Pip binding site. Transfection studies with reporte r 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 pr omoter to a distant enhancer.