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

Overlapping phage lambda clones were utilized to determine the complete nucleotide (nt) sequence of the rat gene encoding HO-2, the major heme oxygenase isozyme in the brain. This isozyme is the constitutive cognate of HSP32 (HO-1). The 12 563-bp gene consists of five exons and four introns, the first two exons are separated by a large intron of 8429 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-histone chromosomal protein, HMG-17. In addition to the coding region, the similarity includes 40 bp upstream from the putative start codon and 800 bp of 3' untranslated sequence. The HO-2 gene lacks a conventional TATA box, but a TATA-like sequence (TAACTA) is found 26 nt upstream from the major transcription start point (tsp), as determined by primer extension. Upstream of the tsp, only a glucocorticoid-response element is found. The structure of the regulatory region is consistent with the previously demonstrated refractory nature of this isozyme to common inducers of gene expression and its apparent response to developmental changes in the adrenal steroid hormone profile. HO-2 is encoded by two transcripts (approx. 1.3 and approx. 1.9 kb), the larger of which is translated less efficiently than the smaller. Presently, we show that the transcripts are the products of a single gene and differ in the use of the polyadenylation signal. Two polyadenylation signals appear to be differentially utilized in different organs; in kidney they are used to approximately the same extent, while in liver and testes the shorter transcript is almost exclusively produced. Although in intact brain both transcripts are equally represented, transformed brain astrocytes and glia lack the ability to produce the larger transcript. HO-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.