, Mol. Cell. Biol. 8:381-392, 1988). Bi was cloned by expression of a murine erythroleukemia (MEL) cell cDNA library in transfected COS cells and screening by electrophoretic mobility shift analysis. Bi is identical to the proto-oncogene Spi-1IPU.1 (Spi-l), an ets family member. Protein-DNA contacts are shown to resemble those of the helix-turn-helix homeodomain proteins. By Northern (RNA) analysis, we found that Spi-1 mRNA is present at low levels during murine CFU-E maturation and is at least 20-fold higher in uninduced MEL, a transformed proerythroblast-like cell line which contains an activating/transforming insertion of spleen focus-forming virus at the Spi-I locus. Dimethyl sulfoxide-induced MEL cell differentiation decreases Spi-) mRNA to approximately 20%o of the uninduced level before commitment occurs. In addition to erythroid cells, Spi-I mRNA is present in B cells, myelomonocytes, and mast cells but not in T cells and nonhematopoietic cell types. In situ hybridization demonstrated Spi-l mRNA expression in bone marrow, spleen, interstitial nonhepatocytes of the liver, and interstitial nontubular cells of the testis. The Spi-) locus was mapped on human chromosome 11 to the same interval as ACP2 (lysosomal acid phosphatase), between the anonymous DNA markers D11S33 and D11S14. This region has not yet been found to be associated with a human malignancy.We have previously identified by electrophoretic mobility shift analysis (EMSA) several factors in murine erythroleukemia (MEL) cells that bind to specific sequences within the mouse n-major (3M)-globin intervening sequence 2 (IVS2) (27). The region encompassing these sites has been observed by others to be a tissue-specific DNase I-hypersensitive site and the border of nucleosome phasing on the globin gene in erythroid cells (7,39,78). This region includes two homologous sites for factor B1 (B1-A and B1-B) and one site each for factors Oct-1 (40) and B2 (now designated We report here the cloning of the cDNA for MEL factor B1 by using a modification of the approach taken by Tsai et al. (83), involving a mammalian expression system combined with EMSA analysis of extracts from the transfected COS cells as the detection method. The cDNA sequence revealed that factor B1 is identical to the proto-oncogene/DNAbinding factor Spi-1/Pu.1ISfpi-1 (Spi-1 stands for spleen focus-forming virus [SFFV] proviral integration 1) (33,47,55,65), an ets gene family member which had been previously cloned by different methodologies. On the basis of analysis of the protein sequence and protein-DNA contacts, we propose that there are structural similarities between the DNA-protein interactions of the ETS domain, in particular that of Spi-1, and of the helix-turn-helix (HTH) motif of homeodomain proteins.MEL cells are generated by transformation with the Friend virus which is a complex of a replication-defective SFFV and a replication-competent Friend murine leukemia virus (F-MuLV) helper (for a review, see reference 43) and are thought to be blocked from further differentiation a...