3820 Nucleic Acids Research, 2005, Vol. 33, No. 12 22. Kohwi,Y. and Kohwi-Shigematsu,T. (1988) Magnesium ion-dependent triple-helix structure formed by homopurine–homopyrimidine sequences in supercoiled plasmid DNA. Proc. Natl Acad. Sci. USA, 85, 3781–3785. 23. Dayn,A., Samadashwily,G.M. and Mirkin,S.M. (1992) Intramolecular DNA triplexes: unusual sequence requirements and influence on DNA polymerization. Proc. Natl Acad. Sci. USA, 89, 11406–11410. 24. Cantatore,P., Roberti,M., Loguercio Polosa,P., Mustich,A. and Gadaleta,M.N. (1990) Mapping and characterization of Paracentrotus lividus mitochondrial transcripts: multiple and overlapping transcription units. Curr. Genet., 17, 235–245.