Tetrahymena quadruplex

Telomeric repeats from other organisms are also prone to quadruplex polymorphism. The Tetrahymena TG4T2G4T telomeric sequence, for example, may adopt several conformations. This sequence forms two novel G-quadruplex structures in Na -containing solution. In the first structure (head-to-head), the two loops are at one end of the G-tetrad core in the second structure (head-to-tail), the two loops are located on opposite ends of the G-tetrad core. In contrast to the human telomere sequence, the proportions of the two forms are similar for a wide range of temperatures their unfolding rates are also similar, with an activation enthalpy of 37 kcal mol . The (G4T4) sequence may also interconvert between parallel and antiparallel structures. 

The four-repeat Tetrahymena telomeric d(T2G4)4 sequence was shown to form an intramolecular G-quadruplex (Figure 11a) with one double-chain-reversal and two edgewise loops. This is the first observation of a double-chain-reversal loop sequence T2 bridges three G-tetrad layers. Furthermore this... 

Figure 11 Structures of the (3+1) core-containing G-quadruplexes formed by (a) the four-repeat Tetrahymena telomeric d[ (TTGGGG)4] sequence in Na solu-tion, (b) the three-repeat human telomeric d[GGG(TTAGGG)2T] sequence in Na solution,and (c) the four-repeat human telomeric d[TAG-GG(TTAGGG)s] sequence in solution ...

A.T. Phan, Y.S. Modi and D.J. Patel, Two-repeat Tetrahymena telomeric d(TGGGGTTGGGGT) sequence interconverts between asymmetric dimeric G-quadruplexes in solution, J. Mol. Biol., 2004, 338, 93-102. 

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