The structure of the DNA.RNA hybrid (GTCACATG).(caugugac), where lower
case letters designate RNA residues, has been determined on the basis
of J-coupling analysis and 2D-NOE studies. The central hexamer in this
sequence has been previously studied [Reid, D. G., Salisbury, S. A.,
Brown, T., Williams, D. H., Vasseur, J.-J., Rayner, B., & Imabach, J.-
L. (I 983) Eur. J. Biochem. 135, 307-314] via one-dimensional NOE meth
ods and circular dichroism studies. Contrary to their results, we find
that this duplex does not assume a B-form conformation in solution. I
nstead, the RNA residues retain their C3'-endo (A-form) conformation,
as indicated by the absence of H1'-H2' couplings and by strong H6/H8 t
o (n - 1) H2' NOEs. The sugars of the DNA residues, on the other hand,
do not assume an A-form (or a B-form) conformation but an intermediat
e conformation in the O4'-endo range(P almost-equal-to 72-110-degrees)
, as indicated by the presence of strong H1'-H4' NOEs, medium-strength
H2''-H3' COSY cross peaks, strong H3'-H4' DQF-COSY cross peaks, and H
1'-H2' coupling constants that are of approximately the same magnitude
as the H1'-H2'' coupling constants. These results suggest that the RN
A strand not only retains its N-type structure but also exerts an infl
uence on the conformation of the DNA strand. Our results provide stron
g evidence that DNA.RNA hybrid duplexes do not assume an all-C2'-endo
B-type conformation; neither do they assume an all-C3'-endo A-type con
formation in solution. Furthermore, although not the main focus of thi
s study, a comparison of the longitudinal relaxation times of the DNA
and RNA residues indicates the need for extended relaxation delays in
two-dimensional NMR spectra of hybrid duplexes, as has been previously
observed for DNA.RNA chimeric duplexes (Wang, A. C., Kim, S.-G., Chou
, S.-H., Orban, J., Flynn, P., & Reid, B. R. (1992) Biochemistry 31, 3
940-3946).