Characterization of a salmon insulin-like growth factor I promoter

We have identified four transcription initiation sites in the salmon insulin-like growth factor I (IGF-I) gene. Use of the most upstream transcription start site generates a minor mRNA species with a 5' untranslated region (UTR) of approximately 450-550 nucleotides, whereas transcription starting at the downstream initiation sites results in more abundant IGF-I mRNAs with 5'-UTRs approximately 250, 245, and 165 nucleotides in length. No consensus TATA box-like elements are present immediately upstream of the most upstream start site identified, nor is this region particularly GC-rich. Transient expression assays, however, demonstrated orientation-dependent promoter activity in a 386-nucleotide-long fragment containing the major downstream transcription start sites. Additionally, reverse transcriptase polymerase chain reaction (RT-PCR) analyses demonstrated tissue and developmental stage-specific use of the various transcription start sites identified.