Genomic distribution of three promoters of the bovine gene encoding acetyl-CoA carboxylase α and evidence that the nutritionally regulated promoter I contains a repressive element different from that in rat

Abstract

The enzyme acetyl-CoA carboxylase α (ACC-α) is rate-limiting for the synthesis of long-chain fatty acids de novo. As a first characterization of the bovine gene encoding this enzyme, we established the entire bovine ACC-α cDNA sequence (7041bp) and used experiments with 5′ rapid amplification of cDNA ends to determine the heterogeneous composition of 5′ untranslated regions, as expressed from three different promoters (PI, PII and PIII). The individual locations of these promoters have been defined within an area comprising 35kbp on Bos taurus chromosome 19 (‘BTA19’), together with the segmentation of the first 14 exons. Primer extension analyses reveal that the nutritionally regulated PI initiates transcription from at least four sites. PI transcripts are much more abundant in adipose and mammary-gland tissues than in liver or lung. A 2.6kb promoter fragment drives the expression of reporter genes only weakly in different model cells, irrespective of stimulation with insulin or dexamethasone. Thus bovine PI is basically repressed, like its analogue from rat. Finely graded deletions of PI map two separate elements, which have to be present together in cis to repress bovine PI. The distal component resides within a well-preserved Art2 retroposon element. Thus sequence, structure and evolutionary origin of the main repressor of PI in bovines are entirely different from its functional counterpart in rat, which had been identified as a (CA)28 microsatellite. We show that, in different mammalian species, unrelated genome segments of different origins have been recruited to express as functionally homologous PI the ancient and otherwise highly conserved ACC-α-encoding gene.