title: Crystal structure and conformation of a DNA-RNA hybrid duplex with a polypurine RNA strand: d(TTCTTBr5CTTC)-r(GAAGAAGAA).
abstract: BACKGROUND
  . DNA-RNA hybrids are substrates for RNase H. This enzyme catalyzes the hydrolysis of the RNA strand in the hybrid form. The polypurine tract (PPT) in human immunodeficiency virus 1 (HIV-1) is a short stretch of purines ( approximately 15 bases) located at the 3'-end of the U3 region of the RNA genome. The PPT has the unique ability to resist digestion by RNase H and serves as a primer for plus-strand DNA synthesis.
  
  
  RESULTS
  . The crystal structure of a DNA-RNA hybrid duplex containing a polypurine RNA strand, d(TTCTTBr5CTTC)-r(GAAGAAGAA), has been determined at 1.8 A resolution. The structure was solved by molecular replacement methods and refined to a final R factor of 20.1% (R free 23.7%). The hybrid duplex adopts a standard A-form conformation. All of the sugar rings and glycosidic torsion angles are found in the standard C3'-endo/anti conformation, as seen in A-RNA or A-DNA. The crystal packing is dominated by the DNA strand, where the terminal base pairs of the hybrid abut the neighboring A-DNA sugar-phosphate backbone on the minor groove side.
  
  
  CONCLUSIONS
  . The present DNA-RNA hybrid duplex containing a polypurine RNA strand exhibits standard A-form geometry. This observation might suggest that the RNA PPT resists the RNase H activity of HIV reverse transcriptase as a result of its A-form conformation. In addition, there appears to be a correlation between the percentage purine content of the RNA and the DNA backbone conformation.
authors.affiliation: The Ohio State University Biological Macromolecular Structure Center Departments of Chemistry, Biochemistry and Biophysics Program 012 Rightmire Hall 1060 Carmack Road Columbus Ohio 43210 USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Xiong
authors.forename: M
authors.initials: M
authors.lastname: Sundaralingam
mesh_terms: D003461:Crystallography; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009692:Nucleic Acid Heteroduplexes; D011687:Purines / Q000737:chemistry*; D012313:RNA / Q000737:chemistry*
chemical_list: D009692:Nucleic Acid Heteroduplexes; D011687:Purines; D012313:RNA