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G•U wobble base pair frequently occurs in RNA structures. The unique chemical, thermodynamic, and structural properties of the G•U pair are widely exploited in RNA biology. In several RNA molecules, the G•U pair plays key roles in folding, ribozyme catalysis, and interactions with proteins. G•U may occur as a single pair or in tandem motifs with different geometries, electrostatics, and thermodynamics, further extending its biological functions. The metal binding affinity, which is essential for RNA folding, catalysis, and other interactions, differs with respect to the tandem motif type due to the different electrostatic potentials of the major grooves. In this work, we present the crystal structure of an RNA 8-mer duplex r[UCGUGCGA] 2 , providing detailed structural insights into the tandem motif I (5′UG/3′GU) complexed with Ba 2+ cation. We compare the electrostatic potential of the presented motif I major groove with previously published structures of tandem motifs I, II (5′GU/3′UG), and III (5′GG/3′UU). A local patch of a strongly negative electrostatic potential in the major groove of the presented structure forms the metal binding site with the contributions of three oxygen atoms from the tandem. These results give us a better understanding of the G•U tandem motif I as a divalent metal binder, a feature essential for RNA functions. 

Solid phase synthesis of RNA oligonucleotides which are over 100-nt in length remains challenging due to the complexity of purification of the target strand from failure sequences. This work describes a non-chromatographic strategy that will enable routine solid phase synthesis of long RNA strands. 

The global public health concerns and economic impact caused by emerging outbreaks of RNA viruses call for the search for new direct acting antiviral agents. Herein, we describe the synthesis, DFT calculations, and antiviral evaluation of a series of novel 2-hydroxyimino-6-aza-pyrimidine ribonucleosides. DFT//B3LYP/6-311+G** calculations of the tautomeric distributions of the 2-hydroxyimino nucleosides 7 , 8 , and 9 in aqueous environments indicate a predominance of the canonical 2-( E )-hydroxyimino structure, where the hydroxyl group points away from the sugar moiety. The conformer distributions of the latter geometrical isomers of 7 , 8 , and 9 support the formation of five membered rings via hydrogen bonding between the ( E )- C 2 N–O–H moiety and N 3 –H of 7 and 8 and between ( E )- C 2 N–O–H and N 3 of 9 , creating purine shaped nucleosides with the glycosidic linkage at the pyrimidine ring. The newly synthesized nucleosides were screened against an RNA viral panel, of which moderate antiviral activity was observed against Zika virus (ZIKV) and human respiratory syncytial virus (HRSV). 6-Aza-2-hydroxyimino-5-methyluridine derivative 18 showed activity against ZIKV (EC 50 3.2 μM), while its peracetylated derivative 19 showed activity against HRSV (EC 50 5.2 μM). The corresponding 4-thiono-2-hydroxyimino derivative 8 showed activity against HRSV (EC 50 6.1 μM) and against ZIKA (EC50 2.4 μM). This study shows that the 6-aza-2-hydroxyimino-5-methyluracil derived nucleosides can be further optimized to provide potent antiviral agents. 

Abstract The N4-methylation of cytidine (m4C and m42C) in RNA plays important roles in both bacterial and eukaryotic cells. In this work, we synthesized a series of m4C and m42C modified RNA oligonucleotides, conducted their base pairing and bioactivity studies, and solved three new crystal structures of the RNA duplexes containing these two modifications. Our thermostability and X-ray crystallography studies, together with the molecular dynamic simulation studies, demonstrated that m4C retains a regular C:G base pairing pattern in RNA duplex and has a relatively small effect on its base pairing stability and specificity. By contrast, the m42C modification disrupts the C:G pair and significantly decreases the duplex stability through a conformational shift of native Watson-Crick pair to a wobble-like pattern with the formation of two hydrogen bonds. This double-methylated m42C also results in the loss of base pairing discrimination between C:G and other mismatched pairs like C:A, C:T and C:C. The biochemical investigation of these two modified residues in the reverse transcription model shows that both mono- or di-methylated cytosine bases could specify the C:T pair and induce the G to T mutation using HIV-1 RT. In the presence of other reverse transcriptases with higher fidelity like AMV-RT, the methylation could either retain the normal nucleotide incorporation or completely inhibit the DNA synthesis. These results indicate the methylation at N4-position of cytidine is a molecular mechanism to fine tune base pairing specificity and affect the coding efficiency and fidelity during gene replication.