Structural organization of the human heme oxygenase gene and the function of its promoter.

Human heme oxygenase is induced by its substrate heme, but not induced by heat shock [Yoshida et al. (1988) Eur. J. Biochem. 171, 457-461]. In order to study the molecular mechanisms of heme-mediated induction of human heme oxygenase, we have isolated and characterized the genomic clones for heme oxygenase. The human heme oxygenase gene (HO gene) is about 14 kb long and organized into five exons. The transcription initiation site was identified by S1 nuclease mapping and primer-extension analyses. Using HeLa whole cell extracts, we confirmed that the transcription of the cloned HO gene is initiated accurately at the assigned initiation site. In its 5'-flanking region, a potential heat-shock element (HSE) is present 367 bp upstream from the initiation site, although, in contrast to rat heme oxygenase, human enzyme is not induced by heat shock. We therefore analyzed the effects of heat shock on the transient expression of chimeric fusion genes harboring the promoter of the human HO gene ligated to the Escherichia coli gene gpt in mouse amelanotic melanoma cells. The 5'-flanking region of the human HO gene bearing a potential HSE failed to confer the heat-inducibility of gpt RNA production, suggesting that the HSE of the human HO gene is not functional.