title: Crystal structure of archaeal RNase HII: a homologue of human major RNase H.
abstract: BACKGROUND
  RNases H are present in all organisms and cleave RNAs in RNA/DNA hybrids. There are two major types of RNases H that have little similarity in sequence, size and specificity. The structure of RNase HI, the smaller enzyme and most abundant in bacteria, has been extensively studied. However, no structural information is available for the larger RNase H, which is most abundant in eukaryotes and archaea. Mammalian RNase H participates in DNA replication, removal of the Okazaki fragments and possibly DNA repair.
  
  
  RESULTS
  The crystal structure of RNase HII from the hypothermophile Methanococcus jannaschii, which is homologous to mammalian RNase H, was solved using a multiwavelength anomalous dispersion (MAD) phasing method at 2 A resolution. The structure contains two compact domains. Despite the absence of sequence similarity, the large N-terminal domain shares a similar fold with the RNase HI of bacteria. The active site of RNase HII contains three aspartates: Asp7, Asp112 and Asp149. The nucleotide-binding site is located in the cleft between the N-terminal and C-terminal domains.
  
  
  CONCLUSIONS
  Despite a lack of any detectable similarity in primary structure, RNase HII shares a similar structural domain with RNase HI, suggesting that the two classes of RNases H have a common catalytic mechanism and possibly a common evolutionary origin. The involvement of the unique C-terminal domain in substrate recognition explains the different reaction specificity observed between the two classes of RNase H.
authors.affiliation: Department of Chemistry, University of California, Berkeley 94720, USA.
authors.forename: L
authors.initials: L
authors.lastname: Lai
authors.forename: H
authors.initials: H
authors.lastname: Yokota
authors.forename: L W
authors.initials: LW
authors.lastname: Hung
authors.forename: R
authors.initials: R
authors.lastname: Kim
authors.forename: S H
authors.initials: SH
authors.lastname: Kim
mesh_terms: D000595:Amino Acid Sequence; D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics; D004260:DNA Repair; D006801:Humans; D017017:Methanococcus; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016914:Ribonuclease H / Q000737:chemistry* / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D019843:Archaeal Proteins; C091139:ribonuclease HII; D016914:Ribonuclease H