Heme oxygenase 1 is an essential enzyme in heme catabolism that cleaves heme to form biliverdin, iron, and carbon monoxide. The human heme-oxygenase-1 gene is transcriptionally activated through the cis-regulatory element (MTE), GTCATATGAC (positions -156 to -147), during 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced differentiation of myelomonocytic cell lines, such as THP-1, to macrophages. MTE is responsible for the myelomonocytic-specific induction of heme-oxygenase-1 gene expression, and is bound by ubiquitous and myelomonocytic cell-line-restricted proteins. In this study, we cloned the cDNA segments coding for a portion of a protein that binds to MTE by a Southwestern procedure from a THP-1 cDNA expression library; we subsequently isolated putative full-length cDNAs by a conventional hybridization procedure. The deduced protein, termed MTB-Zf, consists of 1482 amino acid residues, has a molecular mass of about 162 kDa, and contains the two widely separated zinc-finger domains located near the N- and C-termini. MTB-Zf possesses other structural features characteristic of transcription factors, including a long stretch of acidic amino acids (amino acids 67 - 95), a proline-rich region (positions 733-849), a region rich in basic amino acids (positions 1161-1247), and a leucine repeat-like region (positions 486-514). We show that a portion of MTB-Zf, including an N-terminal zinc-finger domain, binds in vitro to MTE and that the transient coexpression of MTB-Zf cDNA leads to transactivation of the heme-oxygenase-1 gene promoter. Since the 6.5 kb MTB-Zf is expressed in various human cell lines of different lineages, MTB-Zf may represent a ubiquitous MTE-binding protein. Furthermore, the MTB-Zf gene has been mapped to human chromosome 1p35-36.1 by fluorescence in situ hybridization, a region which is frequently deleted in various solid tumors, including neurogenic tumors. We found remarkable differences in the expression patterns of MTB-Zf mRNA and two other hybridizable mRNAs of 5kb and 8.5 kb when human brain and primary brain tumors were compared. Both MTB-Zf and the 8.5-kb mRNAs were abundantly expressed in the five primary brain tumors examined, but only the 5-kb mRNA was detectable in the human brain. These results suggest that MTB-Zf is a transcription factor and may also play an important role in cell growth or differentiation.