ROLES OF CCAAT ENHANCER-BINDING PROTEIN AND ITS BINDING-SITE ON REPRESSION AND DEREPRESSION OF ACETYL-COA CARBOXYLASE GENE/

The gene for acetyl-CoA carboxylase, the rate-limiting enzyme in the b iosynthesis of long-chain fatty acids, contains two distinct promoter regions, denoted PI and PII, which control the generation of different forms of mRNA. Multiple forms of acetyl-CoA carboxylase (ACC) mRNA wi th 5'-end heterogeneity are generated as a result of differential spli cing of two primary transcripts formed under the control of these two promoters. PI is responsible for the generation of class I mRNAs of AC C, which are induced in a tissue-specific manner under lipogenic condi tions. PII generates class II mRNAs of ACC, which are expressed consti tutively. Possible mechanisms for the regulation of PI under normal ph ysiological conditions and agents that activate the promoter have been investigated. PI contains a TATA and a CCAAT box. In addition to thes e sequences, this promoter contains a 28-CA repeat sequence 220 bases upstream from the transcription initiation site; the presence of this sequence leads to about 70% repression of the basal promoter activity. Repression by the 28-CA repeat sequence requires the GCAAT sequence i n the CCAAT box. The negative effect of the 28-CA repeat sequence is r elieved by a CCAAT/enhancer-binding protein (C/EBP), which binds to th e GCAAT sequence. Insertion of the 28-CA repeat sequence into the thym idine kinase promoter results in repression that can also be relieved by the C/EBP gene product. However, the same sequence exerts no effect on ACC promoter II, which has no CCAAT box. During the differentiatio n of 30A5 preadipocytes into adipocytes, the expression of class I ACC mRNA and C/EBP mRNA is coordinately increased. Therefore, the presenc e of the CA repeat in the promoter may be responsible for the inactivi ty of PI, and C/EBP may be one of the factors that is responsible for the activation of PI under lipogenic conditions. Interaction of the CA repeat and the CCAAT box in the repression and derepression of the AC C gene provides a novel function for the CCAAT box and C/EBP in gene r egulation.