Structural and functional characterizations of the 5 '-flanking region of the mouse glucagon receptor gene: Comparison with the rat gene

A putative proximal promoter was defined previously for the mouse glucagon receptor (GR) gene. In the present study, a distal promoter was characteriz ed upstream from a novel non-coding exon revealed by the 5'-rapid amplifica tion of cDNA ends from mouse liver tissue. The 5'-flanking region of the mo use GR gene was cloned up to 6 kb and the structural organization was compa red to the 5' untranslated region of the rat gene cloned up to 7 kb. The no vel exon, separated by an intron of 3.8 kb from the first coding exon, disp layed a high homology (80%) with the most distal of the two untranslated ex ons found in the 5' region of the rat GR gene. The mouse distal promoter re gion, extending up to -1 kb from the novel exon, displayed 85% identity wit h the rat promoter. Both contain a highly GC-rich sequence with five putati ve binding sites for Sp1, but no consensus TATA or CAAT elements. To evalua te basal promoter activities, 5'-flanking sequences of mouse or rat GR gene s were fused to a luciferase reporter gene and transiently expressed in a m ouse and in a rat cell line, respectively or in rat hepatocytes. Both mouse and rat distal promoter regions directed a high level of reporter gene act ivity. Deletion of the Spl binding sites region or mutation of the second p roximal Spl sequence markedly reduced the distal promoter activity of the r eporter gene. The mouse proximal promoter activity was 2- to 3-fold less th an the distal promoter, for which no functional counterpart was observed in the similar region of the rat gem. (C) 2000 Academic Press.