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

In humans, five distinct mRNAs code for gamma -glutamyltransferase (GGT). T heir coding regions are identical and their 5' untranslated regions exhibit both common and type-specific sequences. To elucidate the mecanisms that g enerate these different mRNAs, we cloned and determined the structure of th e 5' region of the human GGT gene. The common regions of the 5' UTR are enc oded by five exons, localized within a 2.4-kb region of the genomic DNA. Th ree of them are separated only by intron-donor or intron-acceptor sites at their boundaries. Alternative splicing of these exons may determine the uni que 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 analys is revealed one major transcription initiation site while 5' RACE indicated that one more distal initiation site could be present. In the putative pro moter sequence neither classical TATA or CAAT boxes were found. However, si tes for AP1, AP2, CREB, GRE and SP1 transcription factors were identified. Chimeric plasmids, containing this genomic region fused to the luciferase g ene, were transiently expressed in three cell lines of different origin: He La cells, ovarian carcinoma A2780 cells and V79 lung fibroblasts. The signi ficant promoter activities obtained indicate a transcription start within t his region. However, differences in the level of expression were found betw een the different cell lines used. These data suggest that the human GGT ge ne employs regulatory sequences and alternative splicing, and gene expressi on may therefore be regulated in tissue specific and cell-type-specific man ners.