ELUCIDATION OF A PROMOTER ACTIVITY THAT DIRECTS THE EXPRESSION OF ACETYL-COA CARBOXYLASE-ALPHA WITH AN ALTERNATIVE N-TERMINUS IN A TISSUE-RESTRICTED FASHION

Previous studies in rats and humans have demonstrated that acetyl-CoA carboxylase a (ACC-alpha), the principal ACC isoenzyme in lipogenic ti ssues, is transcribed from two promoters, PI and PII, that operate in a tissue-specific fashion. Each promoter gives rise to ACC-alpha mRNA isoforms that differ in their 5' untranslated regions but essentially encode the same protein product. In the present study we demonstrate t hat such a pattern of promoter usage is evident in sheep tissues but i n addition we have detected the expression of a novel ACC-alpha mRNA i soform that is expressed in a variety of tissues including kidney, lun g, liver and mammary gland, where it is markedly induced during lactat ion. This novel transcript differs from the previously described ACC-a lpha mRNA in that exon 5, the primary coding exon in both PI and PII t ranscripts, is replaced by a 424-nt sequence that seems to represent t he 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-alpha 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 exo n 5A demonstrates promoter activity when transiently transfected into HepG2 cells and HCll mouse mammary cells and this is markedly enhanced when insulin is present in the culture medium. Promoter activity is a lso evident in primary sheep mammary epithelial cells. These results d emonstrate the presence of a third promoter, PIII, in the ACC-alpha ge ne that results in the tissue-restricted expression of an ACC isoenzym e.