, 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...

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We have used 75% to 90% pure murine erythroid colony-forming units (CFU- E) to delineate the processes and factors underlying their maturation. These CFU-E form 32 cell colonies and are drawn from what we term generation I of a six-generation long maturation sequence (Landschulz et al, Blood 79:2749, 1992). Applying assays of 59Fe-heme biosynthesis and colony numbers as measures of maturation and analyses of DNA degradation as an index of programmed cell death, we find that (1) erythropoietin (Epo) enhances maturation throughout most of its course; (2) Epo first seems able to forestall DNA degradation when CFU-E reach generation II; (3) the processes that Epo elicits thereafter start to persist when Epo is withdrawn; (4) insulin-like growth factor I (IGF-I) also forestalls DNA breakdown, but later loses effectiveness; (5) IGF-I adds little to maturation when Epo levels are high, but when Epo levels are low, enhances it substantially; and (6) for maturation to be entirely optimal, an unidentified serum factor(s) is probably required when Epo levels are high and is certainly needed when Epo levels are like those in normal animals. Quantitatively, about 40% of optimal in vitro erythropoiesis at normal Epo levels depends on Epo alone, another 30% or less on the addition of IGF-I, and the remaining 30% or more on the addition of unidentified serum factor(s). Applied together, these three or more factors lead to two-thirds of the maximum maturation realized with saturating Epo levels. Because we also find that heme accumulated in CFU-E culture can closely approach levels in red blood cells, we suppose that our conclusions apply as well to CFU-E maturation in vivo.