Polypurine-polypyrimidine tracts

For heterogeneous polypurine polypyrimidine tracts, the effect of various divalent metal ions on the transition to and stability of the H-DNA triplexes can be very different depending on the seqnence of snch For... 

Rusche, J.J., and Hurley, L.H., Facilitation of a structural transition in the polypurine/polypyrimidine tract within the proximal promoter region in the human VEGF gene by the presence of potassium and G-quadruplex-interactive agents, Nucleic Acids Res. 33, 6070-6080, 2005. 

The widespread occurrence of polypurine polypyrimidine tracts in eukaryotic DNA suggests that these sequences may have a biological function. Analysis of eukaryotic sequence databases reveals thousands of polypurine polypyrimidine tracts, many with the potential for triplex formation. These polypurine regions of DNA can potentially influence biology in several ways. They could provide binding sites for regulatory proteins, influ-... 

Similar intramolecular processes are known for homop-urine-homopyrimidine tracts in circular covalently closed DNA. Under the superhelical stress in such circular DNA, half the pyrimidine strand is unpaired and forms Hoogsteen base pairs with the remaining polypurine-polypyrimidine duplex. 

A particularly exotic DNA structure, known as H-DNA, is found in polypyrimidine or polypurine tracts that also incorporate a mirror repeat. A simple example is a long stretch of alternating T and C residues (Fig. 8-23). The H-DNA structure features the triple-stranded form illustrated in Figure 8-22 (a, b). Two of the three strands in the H-DNA triple helix contain pyrimidines and the third contains purines. 

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