GENOMIC ORGANIZATION OF THE HUMAN CATECHOL O-METHYLTRANSFERASE GENE AND ITS EXPRESSION FROM 2 DISTINCT PROMOTERS

Human genomic DNA fragments containing catechol O-methyltransferase (C OMT) sequences were isolated and the exon-intron structure analysed by sequencing, PCR and comparing to the human COMT cDNA sequences. The g ene contains six exons, of which exons 1 and 2 are noncoding. MB-ATG a nd S-ATG codons, responsible for the initiation of translation of the membrane-bound (MB) and soluble (S) forms of the enzyme, are located i n exon 3. Two distinct COMT-specific transcripts, 1.3 kb and 1.5 kb, w ere detected in various human tissues and cell lines. Different quanti ties of the shorter COMT-specific mRNA in the tissues studied suggest a tissue-specific regulation of the COMT gene at transcriptional level . Mapping of the 5' ends of the COMT mRNAs showed that transcription i nitiates at multiple sites in two separate DNA regions, which are prec eded by functional promoter sequences. The proximal promoter (P1), loc ated between the two translation initiation codons and extending appro ximately 200 bp upstream of the MB-ATG initiation codon, apparently gi ves rise to the 1.3-kb S-COMT mRNA (S-mRNA). The distal promoter (P2) is located in a DNA fragment in front of and partly overlapping the tr anscription-start region of the 1.5-kb transcript, suggesting that it controls the expression of this MB-mRNA. Similarities between the rat and human COMT gene promoters are analyzed.