PATTERN OF PU.1 BINDING TO M-CSF RECEPTOR PROMOTER DURING MONOCYTE TO MACROPHAGE MATURATION 33 Nadia Maria Sposi, Giovanna Marziali, Elvira Pelosi, Angela Battistini, U go Testa, and Cesare Peschle Department of Hematology and Oncology and Department of Virology Istituto Superiore di Sanita Viale Regina Elena 299,00161 Rome Italy ABSTRACT PUI has been reported to control the expression of an increasing number of myeolid-specific genes, including the macrophage CSF receptor (M-CSFR). Macrophage colony stimulating factor (M-CSF) and its receptor are required for pro- liferation, differentiation, and maturation of monocytes/macrophages. To investigate the mechanism of M-CSFR expression, we studied the binding of PUI to M-CSFR promoter during monocyte to macrophage differentiation. We show that induction of a highly phosphorylated form of PUI positively correlates with monocyte to macrophage maturation. INTRODUCTION Hematopoiesis is sustained by a pool of hematopoietic stem cells that can exten- sively self-renew and undergo a commitment to multipotent progenitor cells, which in turn give rise to mature blood cells. Transcription factors playa major role in differ- entiation in a number of cell types, including the various hematopoietic lineages. l -4 Different transcription factors have been identified which orchestrate, at molecular level, hematopoiesis inducing a cascade of events responsible for differentiation/ Molecular Biology of Hemaropoiesis 6. edited by Abraham et al. Kluwer Academic/Plenum Publishers, New York, 1999. 263