Stability and structure of long intramolecular G-quadruplexes.

G-quadruplexes are formed from guanine-rich sequences of DNA and RNA. They consist of stacks of square arrangements of guanines called G-quartets. Increasing evidence suggests that these structures are involved in cellular processes such as transcription or translation. Knowing their structure and their stability in vitro should help to predict their formation in vivo and to understand their biological functions. Many studies have been performed on isolated G-quadruplexes, but little attention has been given to their interactions. Here, we present non-denaturing gel electrophoresis, UV melting, and circular dichroism data obtained for long sequences of DNA which are capable of forming two simultaneous G-quadruplexes, namely, d(TG(3)T(3)G(3)T(3)G(3)T(3)G(3)T(n)G(3)T(3)G(3)T(3)G(3)T(3)G(3)T), with n varying from one to seven. These sequences can form up to two separate G-quadruplexes. We also study mutated versions of these sequences designed to form one G-quadruplex at specific positions on the strand. Comparing results from the original sequences and their mutated versions, we show that for the former different folded states coexist: either with six stacked G-quartets or only three, in various combinations. Which ones are favored depends on n. Moreover, for n greater than three, the thermodynamic stability stays constant, contrary to an expected decrease in stability if the six G-quartets were stacked together in a single structure. This result agrees with a beads-on-a-string folding model for long sequences of G-quadruplexes, where two adjacent G-quadruplexes fold independently.