The rat pyruvate carboxylase gene structure - Alternate promoters generate multiple transcripts with the 5'-end heterogeneity

Pyruvate carboxylase (EC 6.4.1.1) is a biotin-containing enzyme that plays an important role in gluconeogenesis and lipogenesis. Here we report the structural organization of the rat pyruvate carboxylase gene, which spans over 40 kilobases and is composed of 19 coding exons and 4 5'-untranslated region exons, From this data, it is clear that alternative splicing of the primary transcripts from two promoters is responsible for the occurrence of the multiple mRNA species previously reported (Jitrapakdee, S,, Walker, M. E., and Wallace, J. C. (1996) Biochem, Biophys. Res. Commun. 223, 695-700). The proximal promoter, which is active in gluconeogenic and lipogenic tissues, contains no TATA or CAAT boxes but includes a sequence that is typical of a housekeeping initiator protein 1 box while the distal promoter contains three CAAT boxes and multiple Spl binding sites. Several potential transcription factor binding sites are found in both promoters. A series of 5'-nested deletion constructs of both promoters were fused to a firefly luciferase reporter plasmid and transiently expressed in COS-l cells. The results show that the 153 and 187 base pairs, preceding the transcription start sites of the proximal and distal promoters, respectively, are required for basal transcription, Insulin selectively inhibits the expression of the proximal promoter-luciferase reporter gene by 50% but not the distal promoter in COS-1 cells, suggesting the presence of an insulin-responsive element in the proximal promoter. A half-maximal effect was found at similar to 1 nM insulin.