Structure of poly (I)center dot poly (A)center dot poly (I)

The molecular structure of poly (I) poly (A) poly (I) has been determined a nd refined using the continuous intensity data on layer lines in the x-ray diffraction pattern obtained from an oriented fiber of this polymorphic RNA complex. The polymer forms a 12-fold right-handed triple-helix of pitch 39 .7 Angstrom and each base-triplet is stabilized by quasi Crick-Watson-Hoogs teen hydrogen bonds. The ribose rings in all the three strands have C3'-end o conformations. The final R-value for this best structure is 0.24 and the x-ray fit is significantly superior to all the alternative structures where the different chains might have different furanose conformations. This all -purine triple-helix, counter-intuitively, has a diameter roughly 3 Angstro m shorter than that of DNA and RNA triple-helices containing a homopurine a nd two complementary homopyrimidine strands. Its compact, grooveless cylind rical shape is consistent with the lack of lateral organization.