Hormonal discrimination among transcription start sites of aspartate aminotransferase

The promoter of the gene coding for the rat cytosolic aspartate aminotransferase was cloned from a Charon 4A genomic library. We have sequenced a 1.1-kilobase PstI-PstI fragment which contains the first exon of the gene, the beginning of the first intron and 682 base pairs of the 5' regulatory region (+1 being the A of the first ATG codon), which exhibits promoter activity. The promoter region is G + C rich, does not include any TATA-like element, but has 4 putative Sp1-binding sites and 6 regularly spaced CCAAT boxes. The promoter activity of the 5' regulatory region, as well as its sensitivity to glucocorticoids, were assessed by transient gene expression assays after fusion to the chloramphenicol acetyltransferase gene in the hepatoma cell lines HepG2 and Fao. Multiple transcription start sites were found on the gene over a short distance (55 base pairs), but they were differentially regulated by glucocorticoids as determined by both primer extension analysis and S1 mapping. In particular, transcription from 2 start sites was increased 15- to 18-fold, whereas transcription from the 3 other ones was increased 3-fold. In addition, three new start sites, below the detection limit in control cells, were highly induced. Therefore, a hormonal regulatory element can discriminate among closely related transcription start sites.