Genomic organization of human arylamine N-acetyltransferase Type I reveals alternative promoters that generate different 5′-UTR splice variants with altered translational activities

摘要

pIn humans, a polymorphic gene encodes the drug-metabolizing enzyme NAT1 (arylamine iN/i-acetyltransferase Type 1), which is widely expressed throughout the body. While the protein-coding region of iNAT1/i is contained within a single exon, examination of the human EST (expressed sequence tag) database at the NCBI revealed the presence of nine separate exons, eight of which were located in the 5′ non-coding region of iNAT1/i. Differential splicing produced at least eight unique mRNA isoforms that could be grouped according to the location of the first exon, which suggested that iNAT1/i expression occurs from three alternative promoters. Using RT (reverse transcriptase)-PCR, we identified one major transcript in various epithelial cells derived from different tissues. In contrast, multiple transcripts were observed in blood-derived cell lines (CEM, THP-1 and Jurkat), with a novel variant, not identified in the EST database, found in CEM cells only. The major splice variant increased gene expression 9–11-fold in a luciferase reporter assay, while the other isoforms were similar or slightly greater than the control. We examined the upstream region of the most active splice variant in a promoter-reporter assay, and isolated a 257 bp sequence that produced maximal promoter activity. This sequence lacked a TATA box, but contained a consensus Sp1 site and a CAAT box, as well as several other putative transcription-factor-binding sites. Cell-specific expression of the different iNAT1/i transcripts may contribute to the variation in NAT1 activity iin vivo/i./p