title: The structure of an RNA/DNA hybrid: a substrate of the ribonuclease activity of HIV-1 reverse transcriptase.
abstract: The structure of a complementary hybrid duplex of RNA and DNA has been determined by X-ray crystallography. A ten residue DNA oligonucleotide of sequence 5'-G-G-C-G-C-C-C-G-A-A-3' was annealed to complementary RNA (5'-u-u-c-g-g-g-c-g-c-c-3') and crystallized, producing tetragonal crystals that diffract to 2.3 A resolution. The hybrid adopts a geometry that is neither strictly A nor B-form, rather the helix possesses qualities of both, reminiscent of spectroscopic descriptions of a hybrid conformation, or H-form. All of the ribonucleotides maintain the C3'-endo conformation seen in A-form, while both C3'-endo and C2'-endo conformations are found in the deoxyribonucleotides. The minor groove width (8.5 to 10.5 A) is intermediate between standard values for A (11 A) and B-form (7.4 A) DNA. The global parameters rise and base-pairs tilt (or inclination) are like that of A-DNA, however the slide and x displacement (Dx) are more like that of A-RNA, thus giving the hybrid a unique conformation. In addition, the 10-mer crystallizes in a manner that allows the formation of dimers that stack end-to-end, thereby providing a glimpse of how an extended (20 base-pair) helix of RNA-DNA hybrid might appear. This duplex sequence was selected for study because it is specifically recognized by the ribonuclease H function of HIV reverse transcriptase. A structure of a substrate of this enzyme is of potential value in understanding requirements for the selectivity of this important drug target. The minor groove of the hybrid duplex, lined with the 2-OH of the ribose rings, is the single distinguishing characteristic of the RNA/DNA hybrid, undoubtedly an important structural feature conferring selectivity.
authors.affiliation: Structural, Analytical & Medicinal Chemistry, Pharmacia & Upjohn, Inc., Kalamazoo, MI 49007, USA.
authors.forename: N C
authors.initials: NC
authors.lastname: Horton
authors.forename: B C
authors.initials: BC
authors.lastname: Finzel
mesh_terms: D018360:Crystallography, X-Ray; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D054303:HIV Reverse Transcriptase / Q000378:metabolism*; D015497:HIV-1 / Q000201:enzymology; D006801:Humans; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009692:Nucleic Acid Heteroduplexes / Q000737:chemistry* / Q000378:metabolism; D012313:RNA / Q000737:chemistry* / Q000378:metabolism; D016914:Ribonuclease H / Q000378:metabolism
chemical_list: D009692:Nucleic Acid Heteroduplexes; D012313:RNA; D004247:DNA; D054303:HIV Reverse Transcriptase; D016914:Ribonuclease H