Structural Characterization of i-Motif Structure in the Human Acetyl-CoA Carboxylase 1 Gene Promoters and Their Role in the Regulation of Gene Expression.

The polypurine/polypyrimidine-rich sequences within the promoters (PI and PII) of human acetyl coenzyme A (CoA) carboxylase 1 (ACC1) gene play a vital role in determining hormone- or diet-inducible expression of ACC1. PI and PII contain consecutive runs of three and three to five G/C base pairs, respectively. In a previous study, G-rich DNA sequences of human ACC1 PI and PII were found to fold into G-quadruplex structures; these consequently acted as strong barriers to transcription and DNA replication. Typically, stretches of C-rich sequences that coexist with stretches of guanines have the capacity to form another four-stranded secondary structure known as an i-motif. However, studies on the i-motif structure are limited and its functional significance is unclear. In the current study, through the use of a combination of different techniques, it is demonstrated that C-rich single-stranded DNA derived from ACC1 PI and PII form intramolecular i-motif structures and affect normal DNA metabolic processes. Additionally, the C-rich strands of PI and PII in duplex DNA adopt the i-motif conformation in crowded solution environments at neutral pH. Notably, the i-motif-forming sequences of PI and PII suppressed luciferase gene transcription in HeLa cells. Furthermore, substitution of a nucleotide sequence that has no potential to form the i-motif structure increases luciferase gene expression in HeLa cells. These results support the idea that C-rich sequences within ACC1 PI and PII can form intramolecular i-motif structures, cause suppression of transcription, and thus reveal the functional significance of C-rich sequences in the regulation of ACC1 gene expression.