Mh. Moore et al., DIRECT OBSERVATION OF 2 BASE-PAIRING MODES OF A CYTOSINE-THYMINE ANALOG WITH GUANINE IN A DNA Z-FORM DUPLEX - SIGNIFICANCE FOR BASE ANALOG MUTAGENESIS, Journal of Molecular Biology, 251(5), 1995, pp. 665-673
The pyrimidine nucleobase analogue 6H,8H-3,4-dihydropyrimido[4,5-c][1,
2]oxazin-7-one (P) is a mimic both of cytosine and thymine, since it c
an form stable hydrogen-bonded base-pairs with either guanine or adeni
ne. To investigate the geometric properties of pairing with guanine in
a DNA double helix, the structure of d(CGCGPG)2 has been determined b
y single crystal X-ray analysis. The oligonucleotide crystallised as a
left-handed Z-DNA duplex in the orthorhombic space group P2(1)2(1)2(1
) With cell dimensions a = 18.23 Angstrom, b = 30.63 Angstrom, c = 43.
78 Angstrom. Refinement using NUCLSQ with 51 water molecules included
in the final model converged at R = 0.179 (R(w) = 0.159) for 2798 refl
ections (F > 2 sigma(F)) in the range 8 Angstrom to 1.7 Angstrom. Rema
rkably, the two P . G pairs in the hexamer duplex are different: Watso
n-Crick and wobble types separately illustrate both cytosine-like and
thymine-like behaviour. The result suggests that mutagenesis experimen
ts involving P and other analogues which display pronounced base-pairi
ng ambivalence can be used to examine the structural basis of substrat
e discrimination by polymerases that is essential to accurate genetic
replication. (C) 1995 Academic Press Limited