Structure of the 5' sequences of the human gamma-glutamyltransferase gene

In humans, five distinct mRNAs code for gamma-glutamyltransferase (GGT). Their coding regions are identical and their 5' untranslated regions exhibit both common and type-specific sequences. To elucidate the mecanisms that generate these different mRNAs, we cloned and determined the structure of the 5' region of the human GGT gene. The common regions of the 5' UTR are encoded by five exons, localized within a 2.4-kb region of the genomic DNA. Three of them are separated only by intron-donor or intron-acceptor sites at their boundaries. Alternative splicing of these exons may determine the unique pattern of the different GGT mRNA 5' UTRs in a tissue-specific manner. In addition, we have isolated a genomic fragment containing the most distal 5' sequences of the major GGT mRNA in HepG2 cells. Primer extension analysis revealed one major transcription initiation site while 5' RACE indicated that one more distal initiation site could be present. In the putative promoter sequence neither classical TATA or CAAT boxes were found. However, sites for AP1, AP2, CREB, GRE and SP1 transcription factors were identified. Chimeric plasmids, containing this genomic region fused to the luciferase gene, were transiently expressed in three cell lines of different origin: HeLa cells, ovarian carcinoma A2780 cells and V79 lung fibroblasts. The significant promoter activities obtained indicate a transcription start within this region. However, differences in the level of expression were found between the different cell lines used. These data suggest that the human GGT gene employs regulatory sequences and alternative splicing, and gene expression may therefore be regulated in tissue specific and cell-type-specific manners.