A novel myeloid-restricted zebrafish CCAAT/enhancer-binding protein with apotent transcriptional activation domain

The CCAAT/enhancer-binding protein (C/ EBP) family consists of transcriptio n factors essential for hematopoiesis. The defining feature of the C/EBPs i s a highly conserved carboxy-terminal bZIP domain that is necessary and suf ficient for dimerization and DNA binding, whereas their amino-terminal doma ins are unique. This study reports a novel c/ebp gene (c/ebp1) from zebrafi sh that encodes a protein homologous to mammalian C/EBPs within the bZIP do main, but with an amino terminus lacking homology to any C/EBP or to any kn own sequence. In zebrafish embryos, c/ebp1 expression was initially observe d in cells within the yolk sac circulation valley at approximately the 16-t o 18-somite stage, and at 24 hours postfertilization (hpf), also in circula ting cells. Most c/ebp1(+) cells also expressed a known early macrophage ma rker, leukocyte-specific plastin (l-plastin). Expression of both markers wa s lost in cloche, a mutant affecting hematopoiesis at the level of the hema ngioblast. Expression of both markers was retained in m683 and spadetail, m utants affecting erythropoiesis, but not myelopoiesis. Further, c/ebp1 expr ession was lost in a mutant with defective myelopoiesis, but intact erythro poiesis. These data suggest that c/ebp1 is expressed exclusively in myeloid cells. In electrophoretic mobility shift assays, c/ebp1 was able to bind a C/EBP consensus DNA site. Further, a chimeric protein containing the amino -terminal domain of c/ebp1 fused to the DNA-binding domain of GAL4 induced a GAL4 reporter 4000-fold in NIH3T3 cells. These results suggest that c/ebp 1 is a novel member of the C/EBP family that may function as a potent trans criptional activator in myeloid cells. (Blood.2001;97:2611-2617) (C) 2001 b y The American Society of Hematology.