THE MOUSE GENE FOR HYPOXIA-INDUCIBLE FACTOR-1-ALPHA - GENOMIC ORGANIZATION, EXPRESSION AND CHARACTERIZATION OF AN ALTERNATIVE FIRST EXON AND 5'-FLANKING SEQUENCE

The ubiquitously expressed hypoxia-inducible factor-1 (HIF-1) is invol ved in expression of a large number of oxygen-regulated genes. HIF-1 i s a heterodimer consisting of an alpha and a beta subunit, both belong ing to the basic-helix-loop-helix Per-aryl hydrocarbon receptor nuclea r translocator-Sim (PAS) family of transcription factors. Whereas HIF- 1 alpha is a novel member of this family, HIF-1 beta is identical to t he aryl hydrocarbon receptor nuclear translocator, previously recogniz ed to be involved in xenobiotic metabolism. cDNA cloning revealed that mouse HIF-1 alpha can be expressed as two mRNA isoforms containing al ternative 5' untranslated regions and two different predicted translat ional start sites. We cloned and characterized 20.5 kb of the mouse HI F-1 alpha gene (Hifla) containing exon II-XV. The two alternative firs t exons, I.1 and I.2, are separated from exon II by approximately 24 k b and 17 kb, respectively. We also sequenced Hifla exon I.1 and flanki ng regions, and mapped a single exon I.1 transcription initiation site . Reverse transcription PCR analysis of total RNA derived from normoxi c and hypoxic mouse hepatoma and fibroblast cell lines suggested that the two alternative mRNA isoforms are constitutively coexpressed in th ese cells, and that two different promoters drive transcription of HIF -1 alpha. A minimal exon I.1 promoter was identified which moderately activated heterologous gene expression, indicating that additional cis -elements are required for efficient HIF-1 alpha transcription in vivo .