Crystal structure of archaeal RNase HII: a homologue of human major RNase H

Background: RNases H are present in all organisms and cleave RNAs in RNA/DN A hybrids. There are two major types of RNases H that have little similarit y in sequence, size and specificity. The structure of RNase HI, the smaller enzyme and most abundant in bacteria, has been extensively studied. Howeve r, 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 Methan ococcus jannaschii, which is homologous to mammalian RNase H, was solved us ing a multiwavelength anomalous dispersion (MAD) phasing method at 2 Angstr om resolution. The structure contains two compact domains. Despite the abse nce of sequence similarity, the large N-terminal domain shares a similar fo ld with the RNase HI of bacteria. The active site of RNase HII contains thr ee aspartates: Asp7, Asp112 and Asp149. The nucleotide-binding site is loca ted in the cleft between the N-terminal and C-terminal domains. Conclusions: Despite a lack of any detectable similarity in primary structu re, RNase HII shares a similar structural domain with RNase HI, suggesting that the two classes of RNases H have a common catalytic mechanism and poss ibly a common evolutionary origin. The involvement of the unique C-terminal domain in substrate recognition explains the different reaction specificit y observed between the two classes of RNase H.