POTENTIALLY RIGHT HANDED SEQUENCE CRYSTALLIZES AS LEFT HANDED DNA: THE CRYSTAL STRUCTURE OF d(CCCGGG). P. Karthe¹, S. Krishnaswamy² & N. Gautham¹, ¹Department of Crystallography & Biophysics, University of Madras, Guindy Campus, Madras 600 025, India; ²Bioinformatics Centre, School of Biotechnology, Madurai Kamaraj University, Madurai - 625 021, India

The DNA duplex d(CCCGGG).d(CCCGGG) has only one alternating pyrimidine-purine base step. Despite this, it crystallizes as a left handed helix and packs into a orthorhombic unit cell with a=17.76, b=30.92, c=43.92 Å, similar to the one observed previously for Z DNA hexamers. Moreover, the structure exhibits several remarkable features that are not hitherto observed in left handed Z DNA. The most stricking of these is that the successive base pairs with in the central tetranucleotide show uniform values of twist and rise, resulting in a novel uniform left handed DNA double helix. This structure thus demonstrates that DNA can take up a left handed conformation in the absence of stretches of alternating pyrimidine-purine sequences, and also that like right handed DNA, left handed DNA too can exist in polymorphic forms.