In vitro selection was used to identify deoxyribozymes that ligate two RNA substrates. In the ligation reaction, a 2'-5' RNA phosphodiester linkage is created from a 2',3'-cyclic phosphate and a 5'-hydroxyl group. The new Mg(2+)-dependent deoxyribozymes provide 50-60% yield of ligated RNA in overnight incubations at pH 7.5 and 37 degrees C, and they afford 40-50% yield in 1 h at pH 9.0 and 37 degrees C. Various RNA substrate sequences may be joined by simple Watson-Crick covaration of the DNA binding arms that interact with the two RNA substrates. The current deoxyribozymes have some RNA substrate sequence requirements at the nucleotides immediately surrounding the ligation junction (either UAUA GGAA or UAUN GGAA, where the arrow denotes the ligation site and N equals any nucleotide). One of the new deoxyribozymes was used to prepare by ligation the Tetrahymena group I intron RNA P4-P6 domain, a representative structured RNA. Nondenaturing gel electrophoresis revealed that a 2'-5' linkage between nucleotides A233 and G234 of P4-P6 does not disrupt its Mg(2+)-dependent folding (DeltaDeltaG degrees ' < 0.2 kcal/mol). This demonstrates that a 2'-5' linkage does not necessarily interfere with structure in a folded RNA. Therefore, these non-native linkages may be acceptable in modified RNAs when structure/function relationships are investigated. Deoxyribozymes that ligate RNA should be particularly useful for preparing site-specifically modified RNAs for studies of RNA structure, folding, and catalysis.

The RMI subcomplex (RMI1/RMI2) functions with the BLM helicase and topoisomerase IIIα in a complex called the "dissolvasome," which separates double-Holliday junction DNA structures that can arise during DNA repair. This activity suppresses potentially harmful sister chromatid exchange (SCE) events in wild-type cells but not in cells derived from Bloom syndrome patients with inactivating BLM mutations. The RMI subcomplex also associates with FANCM, a component of the Fanconi anemia (FA) core complex that is important for repair of stalled DNA replication forks. The RMI/ FANCM interface appears to help coordinate dissolvasome and FA core complex activities, but its precise role remains poorly understood. Here, we define the structure of the RMI/FANCM interface and investigate its roles in coordinating cellular DNA-repair activities. The X-ray crystal structure of the RMI core complex bound to a well-conserved peptide from FANCM shows that FANCM binds to both RMI proteins through a hydrophobic "knobsinto-holes" packing arrangement. The RMI/FANCM interface is shown to be critical for interaction between the components of the dissolvasome and the FA core complex. FANCM variants that substitute alanine for key interface residues strongly destabilize the complex in solution and lead to increased SCE levels in cells that are similar to those observed in blm-or fancm-deficient cells. This study provides a molecular view of the RMI/FANCM complex and highlights a key interface utilized in coordinating the activities of two critical eukaryotic DNA-damage repair machines.

Summary BLM, the protein product of the gene mutated in Bloom syndrome, is one of five human RecQ helicases. It functions to separate double Holliday junction DNA without genetic exchange as a component of the “dissolvasome”, which also includes topoisomerase IIIα and the RMI (RecQ-mediated genome instability) subcomplex (RMI1 and RMI2). We describe the crystal structure of the RMI core complex, comprising RMI2 and the C-terminal OB domain of RMI1. The overall RMI core structure strongly resembles two-thirds of the trimerization core of the eukaryotic single-strand DNA-binding protein, Replication Protein A. Immunoprecipitation experiments with RMI2 variants confirm key interactions that stabilize the RMI core interface. Disruption of this interface leads to a dramatic increase in cellular sister chromatid exchange events similar to that seen in BLM-deficient cells. The RMI core interface is therefore crucial for BLM dissolvasome assembly and may have additional cellular roles as a docking hub for other proteins.

Deoxyribozymes that ligate RNA expand the scope of nucleic acid catalysis and allow preparation of site-specifically modified RNAs. Previously, deoxyribozymes that join a 5'-hydroxyl and a 2',3'-cyclic phosphate were identified by in vitro selection from random DNA pools. Here, the alternative strategy of in vitro evolution was used to transform the 8-17 deoxyribozyme that cleaves RNA into a family of DNA enzymes that ligate RNA. The parent 8-17 DNA enzyme cleaves native 3'-5' phosphodiester linkages but not 2'-5' bonds. Surprisingly, the new deoxyribozymes evolved from 8-17 create only 2'-5' linkages. Thus, reversing the direction of the DNA-mediated process from ligation to cleavage also switches the selectivity in forming the new phosphodiester bond. The same change in selectivity was observed upon evolution of the 10-23 RNA-cleaving deoxyribozyme into an RNA ligase. The DNA enzymes previously isolated from random pools also create 2'-5' linkages. Therefore, deoxyribozyme-mediated formation of a non-native 2'-5' phosphodiester linkage from a 5'-hydroxyl and a 2',3'-cyclic phosphate is strongly favored in many different contexts.

We report Zn 2+ -dependent deoxyribozymes that ligate RNA. The DNA enzymes were identified by in vitro selection and ligate RNA with k obs up to 0.5 min −1 at 1 mM Zn 2+ and 23 °C, pH 7.9, which is substantially faster than our previously reported Mg 2+ -dependent deoxyribozymes. Each new Zn 2+ -dependent deoxyribozyme mediates the reaction of a specific nucleophile on one RNA substrate with a 2′,3′-cyclic phosphate on a second RNA substrate. Some of the Zn 2+ -dependent deoxyribozymes create native 3′-5′ RNA linkages (with k obs up to 0.02 min −1 ), whereas all of our previous Mg 2+ -dependent deoxyribozymes that use a 2′,3′-cyclic phosphate create non-native 2′-5′ RNA linkages. On this basis, Zn 2+ -dependent deoxyribozymes have promise for synthesis of native 3′-5′-linked RNA using 2′,3′-cyclic phosphate RNA substrates, although these particular Zn 2+ -dependent deoxyribozymes are likely not useful for this practical application. Some of the new Zn 2+ -dependent deoxyribozymes instead create non-native 2′-5′ linkages, just like their Mg 2+ counterparts. Unexpectedly, other Zn 2+ -dependent deoxyribozymes synthesize one of several unnatural linkages arising from reaction of an RNA nucleophile other than a 5′-hydroxyl group. Two of these unnatural linkages are the 3′-2′ and 2′-2′ linear junctions created when the 2′-hydroxyl of the 5′-terminal guanosine of one RNA substrate attacks the 2′,3′-cyclic phosphate of the second RNA substrate. The third unnatural linkage is a branched RNA resulting from attack of a specific internal 2′-hydroxyl of one RNA substrate at the 2′,3′-cyclic phosphate. When compared with the consistent creation of 2′-5′ linkages by Mg 2+ -dependent ligation, formation of this variety of RNA ligation products by Zn 2+ -dependent deoxyribozymes highlights the versatility of transition metals like Zn 2+ for mediating nucleic acid catalysis. † This research was supported by the Burroughs Wellcome Fund (New Investigator Award in the Basic Pharmacological Sciences to S.K.S.), the March of Dimes Birth Defects Foundation Many of the Mg 2+ -dependent deoxyribozymes mediate RNA ligation via reaction of a 2′,3′-cyclic phosphate, which may be opened by an attacking nucleophile with cleavage of either the P-O 2′ bond or the P-O 3′ bond. If the nucleophilic group is a 5′-hydroxyl, then these two reaction pathways lead to the isomeric native 3′-5′ linkage or non-native 2′-5′ linkage, respectively ( Figure 1). Of course, the attacking nucleophile is not necessarily a 5′-hydroxyl, but this functional group is typically the most nucleophilic of those present in RNA. Indeed, in our previous studies, all of the Mg 2+ -dependent deoxyribozymes that use a 2′,3′-cyclic phosphate RNA substrate were found to create only non-native 2′-5′ linkages (path ii in Figure 1), although the selection strategy itself did not compel this high selectivity against 3′-5′ linkages (46-50). Therefore, understanding the distribution of ligation products upon DNAcatalyzed reaction of a 2′,3′-cyclic phosphate RNA substrate i...