Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
G-Quadruplex, Telomere and Telomerase
Telomeres serve as protective caps at the ends of linear eukaryotic chromosomes, playing a crucial role in cell survival and proliferation. Tandem repeats of the sequence TTAGGG con-stitute the human telomeres, with pendent G-rich single strands of 100–200 nt at the 3’ ends. The propensity of these G-rich overhangs to form G-quadruplexes, and the inhibitory effects of such structures on the catalytic activity of the enzyme telomerase, have led to a growing interest in the study of telomeric G-quadruplexes and the development of specific telomeric quadruplex-stabilizing ligands as anticancer drugs. Previously, it has been reported that human telomeric DNA sequences could adopt in different experimental conditions four different intramolecular G-quadruplexes each involving three G-tetrad layers, namely, Na+ solution antiparallel-stranded basket form (1), K+ crystal parallel-stranded propeller form (2), K+ solution (3 + 1) Form 1 (3-5), and K+ solution (3 + 1) Form 2 (6). Here we report novel intramolecular G-quadruplex struc-tures adopted by canonical (TTAGGG) (7) and variant (CTAGGG, TAGGG) (8,9) four-repeat human telomeric sequences in K+ solution, which surprisingly utilize only two of the three contiguous guanines from successive G-tracts for G-tetrad core forma-tion. Structural elucidation of these oligonucleotides revealed extensive base pairing and stacking interactions in the loops, indicating that the overall G-quadruplex topology of a G-rich sequence is defined not only by maximizing the number of G-tetrads but also by maxi-mizing all possible interactions in the loops. On the other hand, promoter G-quadruplex for-mation represents an alternative approach of selective gene regulation at the transcriptional level. The promoter for the catalytic subunit of human telomerase, hTERT, contains many guanine-rich stretches on the same DNA strand suitable for targeting (10-12). We also show here that one particular G-rich sequence in this region coexists in two G-quadruplex conforma-tions (11), each of which comprises several robust structural elements. Recurrence of struc-tural motifs in the structures presented suggests a “cut-and-paste” principle for the design and prediction of G-quadruplex topologies, for which different elements could be extracted from one G-quadruplex and inserted into another.
This research was supported by grants from Singapore Biomedical Research Council, Singa-pore Ministry of Education, and Nanyang Technological University.