Solution structure of N-(2-deoxy-D-erythro-pentofuranosyl)urea frameshifts, one intrahelical and the other extrahelical, by nuclear magnetic resonance and molecular dynamics.

The presence of a N-(2-deoxy-D-erythro pentofuranosyl)urea (henceforth referred to as deoxyribosylurea) residue, ring fragmentation product of a thymine, in a frameshift situation in the sequence 5'd(AGGACCACG).d(CGTGGurTCCT) has been studied by 1H and 31P nuclear magnetic resonance and molecular dynamics. At equilibrium, two species are found in slow exchange. We observe that the deoxyribosylurea residue can be either intra- or extrahelical within structures which otherwise do not deviate strongly from that of a B-DNA as observed by NMR. Our study suggests that this is determined by the nature and number of hydrogen bonds which this residue can form as a function of two possible isomers. There are two possible structures for the urea side chain, either cis or trans for the urido bond which significantly changes the hydrogen bonding geometry of the residue. In the intrahelical species, the cis isomer can form two good hydrogen bonds with the bases on the opposite strand in the intrahelical species, A4 and C5, which is not the case for the trans isomer. This results in a kink in the helical axis. For the major extrahelical species, the situation is reversed. The trans isomer is able to form two good hydrogen bonds, with G13 on the same strand and A7 on the opposite strand. For the extrahelical species, the cis isomer can form only one hydrogen bond. In this major structure the NMR data show that the bases which are on either side of the deoxyribosylurea residue in the sequence, G14 and T16, are stacked over each other in a way similar to a normal B-DNA structure. This requires the formation of a loop for the backbone between these two residues. This loop can belong to one of two families, right- or left-handed. In a previous study of an abasic frameshift [Cuniasse et al. (1989) Biochemistry 28, 2018-2026], a left-handed loop was observed, whereas in this study a right-handed loop is found for the first time in solution. The deoxyribosylurea residue lies in the minor groove and can form both an intra- and an interstrand hydrogen bond.