Structure and expression of the rat apolipoprotein E gene.

The rat apolipoprotein E gene was isolated from a genomic library by screening with a cDNA probe. The nucleotide sequence of the gene plus a 5' flanking region of 623 nucleotides and a 3' flanking region of 234 nucleotides was determined. The gene contained three introns, which were located in the 5' nontranslated region and in the regions coding for the signal peptide and the mature protein. A "TATA box" sequence TATAATT was found beginning at nucleotide -32. Two copies of the sequences GGGCGG or CCGCCC, potential binding sites for the transcription protein factor Sp 1, were found adjacent to the TATA box, beginning at nucleotides -52 and -72. The 5' proximal flanking region, up to about 140 nucleotides, was found to be highly conserved (82% homology) in the gene for rat and human. This region was GC-rich (68% G + C) and contained self-complementary sequences with the potential to form hairpin loop structures, suggesting a regulatory role of this region in gene transcription. Two open reading frames with promoter sequences were found, one located in the first and the other in the second intron. Expression of rat apolipoprotein E mRNA in murine L cells transfected with the cloned gene was detected at levels comparable to that in rat liver. Rat apolipoprotein E expressed in transfected cells was secreted from the cells into the medium.