Asymmetric expression of transcripts derived from the shared promoter between the divergently oriented ACACA and TADA2L genes.

The mammalian gene (ACACA) encoding acetyl-CoA carboxylase-alpha, a key regulatory enzyme of fatty acid synthesis, is transcribed from multiple promoters. We have delineated the 5' boundary of ACACA in four species (human, mouse, rat, and ovine). The 5' end of ACACA is located within a 600- to 700-bp CpG island encompassing a bidirectional promoter shared with the divergently oriented TADA2L, which encodes a component of chromatin-modifying complexes. In mouse and rat, this promoter, now referred to as Acaca PI, is located 43 kb upstream of the previously known regulatory regions. The shared promoter coregulates transcripts for TADA2L and ACACA in an asymmetric fashion in human and mouse tissues. A higher concentration of RNA polymerase II (Pol II) within the intergenic region in brain compared to liver of mouse reflects the greater abundance of the two transcripts in brain. The concentration of Pol II tracking downstream, which is lower than at the promoter, is not significantly different in either gene in the two tissues and does not reflect the 10- and >200-fold greater abundance of Tada2l and Acaca PI transcripts, respectively, in brain. Thus, regulation of clearance of Pol II from the promoter and the rate of elongation may therefore be determinants of the asymmetric expression of these transcripts.

Induction of transcripts derived from promoter III of the acetyl-CoA carboxylase-alpha gene in mammary gland is associated with recruitment of SREBP-1 to a region of the proximal promoter defined by a DNase I hypersensitive site.

ACC-alpha (acetyl-CoA carboxylase-alpha), a key regulator of fatty-acid metabolism, is encoded by mRNAs transcribed from three promoters, PI, PII and PIII, in the ovine genome. Enhanced expression of transcripts encoded by PIII in mammary gland during lactation is associated with alterations in chromatin structure that result in the detection of two DNase I hypersensitive sites, upstream of the start site. The most proximal site, located between -190 and -10, is characterized by the presence of an inverted-CCAAT box, C2 at -167, and E-boxes, E1 and E2, at -151 and -46. Deletion of these motifs, which bind nuclear factor-Y and upstream stimulatory factors respectively in gel-shift assays, attenuates the activity of luciferase reporter constructs in transfected cells. Chromatin immunoprecipitation demonstrated that these transcription factors were associated with PIII in vivo in both lactating and non-lactating mammary tissues. The basic helix-loop-helix-leucine zipper transcription factor, SREBP-1 (sterol-regulated-element-binding protein-1), transactivated PIII reporter constructs in transfected HC11 mammary cells, and this was dependent on the presence of E1, but not on C2 or E2. SREBP-1 was only associated with PIII in chromatin from lactating animals, which was coincident with a 4-fold increase in the precursor (125 kDa) form of SREBP-1 in microsomes and the appearance of the mature form (68 kDa) in the nucleus. SREBP-1 motifs are also present in the proximal region of PII, which is also induced in lactation. This indicates that SREBP-1 is a major developmental regulator of the programme of lipid synthesis de novo in the lactating mammary gland.