Characterization of a rat insulin-like growth factor I gene promoter

Rat IGF-I mRNAs contain one of two alternative 5'-untranslated regions which are encoded by alternative exons (exons 1 and 2) and whose expression is controlled by alternative promoter elements. We investigated the ability of fragments of DNA which contain exon 1 and its 5'-flanking region to regulate transcription of a luciferase reporter gene in transient transfection assays. Maximal promoter activity was obtained with a construct which contained 412 bp of 5'-flanking region, while constructs which contained 1120 and 1690 bp of 5'-flanking region induced approximately 50% less enzymatic activity. Mapping of transcription start sites by RNase protection assay demonstrated that native start sites were used by these constructs, although the relative use of different start sites was different from start site usage by the endogenous gene. These data demonstrate that the 5'-flanking region of exon 1 is capable of regulating transcription of IGF-I mRNAs.