CRYSTAL-STRUCTURE OF THE HIGHLY DISTORTED CHIMERIC DECAMER R(C)D(CGGCGCCG)R(G) SPERMINE COMPLEX - SPERMINE BINDING TO PHOSPHATE ONLY AND MINOR-GROOVE TERTIARY BASE-PAIRING

The crystal structure of the self-complementary chimeric decamer duple x r(C)d(CGGCGCCG)r(G), with RNA base pairs at both termini, has been s olved at 1.9 Angstrom resolution by the molecular replacement method a nd refined to an R value of 0.145 for 2,314 reflections. The C3'-endo sugar puckers of the terminal riboses apparently drive the entire chim eric duplex into an A-DNA conformation, in contrast to the B-DNA confo rmation adopted by the all-deoxy decamer of the same sequence. Five sy mmetry related duplexes encapsulate a spermine molecule which interact s with ten phosphate groups, both directly and through water molecules to form multiple ionic and hydrogen bonding interactions. The spermin e interaction severely bends the duplexes by 31 degrees into the major groove at the fourth base pair G(4).C(17), jolts it and slides the 'b ase plate' into the minor groove. This base pair, together with the ad jacent base pair in the top half and the corresponding pseudo two-fold related base pairs in the bottom half, form four minor groove base-pa ired multiples with the terminal base pairs of two neighboring duplexe s.