CLONING AND CHARACTERIZATION OF THE RAT APOBEC-1 GENE - A COMPARATIVE-ANALYSIS OF GENE STRUCTURE AND PROMOTER USAGE IN RAT AND MOUSE

ApolipoproteinB (apoB) mRNA editing involves a C to U deamination of t he nuclear apoB mRNA and occurs in mammalian small intestine and in th e liver of certain species. This reaction is mediated by a multicompon ent enzyme complex that includes a catalytic subunit, apobec-1. Apobec -1 mRNX is widely expressed in the rat and mouse and is subject to tis sue-specific regulation. In order to under stand the basis for the spe cies- and tissue-specific pattern of apobec-1 gene expression we have cloned and characterized the rat chromosomal alobec-1 gene. We demonst rate its structural organization and regulation in comparison to that of the mouse apobec-1 gene. The rat apobec-1 gene spans 16 kb and incl udes one untranslated (exon A) and five translated exons (exons 1-5). The mouse apobec-1 gene contains eight exons, of which the first three (exons A, B, C) are untranslated. Independent approaches demonstrated three distinct clusters of transcription initiation sites in both spe cies, including exon A, the distal region of exon 1, and a separate gr oup in the proximal region of exon 1. These transcription start sites generate three distinct mRNA species whose proportions differ in a tis sue-specific fashion. Promoter-luciferase reporter constructions using regions flanking exon A and exon 1 of the rat apobec-1 gene identifie d two functional regions upstream of exon 1 that independently promote luciferase expression in transfected hepatoma and colon cancer cells. These data serve as a basis for an understanding of the regulation of apobec-1 gene expression, in particular the mechanisms that serve to restrict its expression to the gastrointestinal tract in higher mammal s.