Published online 8 November 2004
Nucleic Acids Research, Vol. 32 No. 19 © Oxford University Press 2004; all rights reserved
Cobalt hexammine induced tautomeric shift in Z-DNA: the structure of d(CGCGCA).d(TGCGCG) in two crystal forms
Department of Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai 600 025, India
* To whom correspondence should be addressed. Tel: +91 44 2230 0122; Fax: +91 44 2235 2494; Email: gautham{at}unom.ac.in or n_gautham{at}hotmail.com
We dedicate this paper to the memory of late Prof. R. Srinivasan, former head of our department
Received September 4, 2004; Revised and Accepted October 14, 2004
We report here the crystal structure of the DNA hexamer duplex d(CGCGCA).d(TGCGCG) at 1.71 Å resolution. The crystals, in orthorhombic space group, were grown in the presence of cobalt hexammine, a known inducer of the left-handed Z form of DNA. The interaction of this ion with the DNA helix results in a change of the adenine base from the common amino tautomeric form to the imino tautomer. Consequently the A:T base pair is disrupted from the normal Watson–Crick base pairing to a ‘wobble’ like base pairing. This change is accommodated easily within the helix, and the helical parameters are those expected for Z-DNA. When the cobalt hexammine concentration is decreased slightly in the crystallization conditions, the duplex crystallizes in a different, hexagonal space group, with two hexamer duplexes in the asymmetric unit. One of these is situated on a crystallographic 6-fold screw axis, leading to disorder. The tautomeric shift is not observed in this space group. We show that the change in inter-helix interactions that lead to the two different space groups probably arise from the small decrease in ion concentration, and consequently disordered positions for the ion.