THE STRUCTURE - ORGANIZATION AND DIFFERENTIAL EXPRESSION OF THE GENE ENCODING RAT HEME OXYGENASE-2

Overlapping phage lambda clones were utilized to determine the complet e nucleotide (nt) sequence of the rat gene encoding HO-2, the major he me oxygenase isozyme in the brain. This isozyme is the constitutive co gnate of HSP32 (HO-1). The 12 563-bp gene consists of five exons and f our introns, the first two exons are separated by a large intron of 84 29 nt. The minus strand of intron 1 contains a nested sequence of 1046 nt with 87% identity to the cDNAs encoding the mouse and human non-hi stone chromosomal protein, HMG-17. In addition to the coding region, t he similarity includes 40 bp upstream from the putative start codon an d 800 bp of 3' untranslated sequence. The HO-2 gene lacks a convention al TATA box, but a TATA-like sequence (TAACTA) is found 26 nt upstream from the major transcription start point (tsp), as determined by prim er extension. Upstream of the tsp, only a glucocorticoid-response elem ent is found. The structure of the regulatory region is consistent wit h the previously demonstrated refractory nature of this isozyme to com mon inducers of gene expression and its apparent response to developme ntal changes in the adrenal steroid hormone profile. HO-2 is encoded b y two transcripts (approx. 1.3 and approx. 1.9 kb), the larger of whic h is translated less efficiently than the smaller. Presently, we show that the transcripts are the products of a single gene and differ in t he use of the polyadenylation signal. Two polyadenylation signals appe ar to be differentially utilized in different organs; in kidney they a re used to approximately the same extent, while in liver and testes th e shorter transcript is almost exclusively produced. Although in intac t brain both transcripts are equally represented, transformed brain as trocytes and glia lack the ability to produce the larger transcript. H O-2 shares only a moderate similarity (50-57%) in two exons with HO-1 (HSP32), the inducible isozyme. One of these, exon 4 of HO-2, contains the suspected heme-binding domain.