Previous studies in rats and humans have demonstrated that acetyl-CoA carboxylase α (ACC-α), the principal ACC isoenzyme in lipogenic tissues, is transcribed from two promoters, PI and PII, that operate in a tissue-specific fashion. Each promoter gives rise to ACC-α mRNA isoforms that differ in their 5´ untranslated regions but essentially encode the same protein product. In the present study we demonstrate that such a pattern of promoter usage is evident in sheep tissues but in addition we have detected the expression of a novel ACC-α mRNA isoform that is expressed in a variety of tissues including kidney, lung, liver and mammary gland, where it is markedly induced during lactation. This novel transcript differs from the previously described ACC-α mRNA in that exon 5, the primary coding exon in both PI and PII transcripts, is replaced by a 424-nt sequence that seems to represent the 5´ terminus of the mRNA. The 424-nt sequence encodes a 17-residue N-terminal region as the N-terminal residue in the deduced sequence is a methionine flanked by several in-frame stop codons. The 5´ terminal 424 nt are present as a single exon, which we have termed exon 5A, in the sheep ACC-α gene and this is located approx. 15 kb downstream of exon 5 and 5 kb upstream of exon 6. A 1.5 kb HindIII–BglII fragment encompassing the 5´ terminus and sequence immediately upstream of exon 5A demonstrates promoter activity when transiently transfected into HepG2 cells and HC11 mouse mammary cells and this is markedly enhanced when insulin is present in the culture medium. Promoter activity is also evident in primary sheep mammary epithelial cells. These results demonstrate the presence of a third promoter, PIII, in the ACC-α gene that results in the tissue-restricted expression of an ACC isoenzyme.
The nucleotide sequence data reported will appear in DDBJ, EMBL and GenBank Nucleotide Sequence Databases under the accession numbers X80045 and AJ001056.