Human acyl-CoA:cholesterol acyltransferase-1 (ACAT-1) gene organization and evidence that the 4.3-kilobase ACAT-1 mRNA is produced from two different chromosomes

Acyl-CoA:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis. Four human ACAT-1 mRNAs (7.0, 4.3, 3.6, and 2.8 kilobases (kb)) share the same short 5'-untranslated region (exon 1) and coding sequence (exons 2-15). The 4.3-kb mRNA contains an additional 5'-untranslated region (1289 nucleotides in length; exons X-a and X-b) immediately upstream from the exon 1 sequence. One ACAT-1 genomic DNA insert covers exons 1-16 and a promoter (the P1 promoter). A separate insert covers exon X-a (1277 base pairs) and a different promoter (the P7 promoter). Gene mapping shows that exons 1-16 and the P1 promoter sequences are located in chromosome 1, while exon X-a and the P7 promoter sequence are located in chromosome 7. RNase protection assays demonstrate three different protected fragments, corresponding to the 4.3 kb mRNA and the two other mRNAs transcribed from the two promoters. These results are consistent with the interpretation that the 4.3-kb mRNA is produced from two different chromosomes, by a novel RNA recombination mechanism involving trans-splicing of two discontinuous precursor RNAs.