The Major G‑Quadruplex Formed in the Human BCL‑2 Proximal Promoter Adopts a Parallel Structure with a 13-nt Loop in K⁺ Solution

The human BCL-2 gene contains a 39-bp GC-rich region upstream of the P1 promoter that has been shown to be critically involved in the regulation of BCL-2 gene expression. Inhibition of BCL-2 expression can decrease cellular proliferation and enhance the efficacy of chemotherapy. Here we report the major G-quadruplex formed in the Pu39 G-rich strand in this BCL-2 promoter region. The 1245G4 quadruplex adopts a parallel structure with one 13-nt and two 1-nt chain-reversal loops. The 1245G4 quadruplex involves four nonsuccessive G-runs, I, II, IV, V, unlike the previously reported bcl2 MidG4 quadruplex formed on the central four G-runs. The parallel 1245G4 quadruplex with the 13-nt loop, unexpectedly, appears to be more stable than the mixed parallel/antiparallel MidG4. Parallel-stranded structures with two 1-nt loops and one variable-length middle loop are found to be prevalent in the promoter G-quadruplexes; the variable middle loop is suggested to determine the specific overall structure and potential ligand recognition site. A limit of 7 nt in loop length is used in all quadruplex-predicting software. Thus, the formation and high stability of the 1245G4 quadruplex with a 13-nt loop is significant. The presence of two distinct interchangeable G-quadruplexes in the overlapping region of the BCL-2 promoter is intriguing, suggesting a novel mechanism for gene transcriptional regulation and ligand modulation