Functionalizing the recently developed porous materials such as porous organic frameworks and coordination polymer networks with active homogeneous catalytic sites would offer new opportunities in the field of heterogeneous catalysis. In this regard, a novel covalent triazine framework functionalized with an Ir(III)-N-heterocyclic carbene complex was synthesized and characterized to have a coordination environment similar to that of its structurally related molecular Ir complex. Because of the strong σ-donating and poor π-accepting characters of the Nheterocyclic carbene (NHC) ligand, the heterogenized Ir-NHC complex efficiently catalyzes the hydrogenation of CO 2 to formate with a turnover frequency of up to 16 000 h −1 and a turnover number of up to 24 300; these are the highest values reported to date in heterogeneous catalysis for the hydrogenation of CO 2 to formate.

Telomerase extends the 3′-ends of linear chromosomes by adding conserved telomeric DNA repeats and is essential for cell proliferation and genomic stability. Telomerases from all organisms contain a telomerase reverse transcriptase and a telomerase RNA (TER), which together provide the minimal functional elements for catalytic activity in vitro. The RNA component of many functional ribonucleoproteins contains modified nucleotides, including conserved pseudouridines (Ψs) that can have subtle effects on structure and activity. We have identified potential Ψ modification sites in human TER. Two of the predicted Ψs are located in the loop of the essential P6.1 hairpin from the CR4-CR5 domain that is critical for telomerase catalytic activity. We investigated the effect of P6.1 pseudouridylation on its solution NMR structure, thermodynamic stability of folding and telomerase activation in vitro. The pseudouridylated P6.1 has a significantly different loop structure and increase in stability compared to the unmodified P6.1. The extent of loop nucleotide interaction with adjacent residues more closely parallels the extent of loop nucleotide evolutionary sequence conservation in the Ψ-modified P6.1 structure. Pseudouridine-modification of P6.1 slightly attenuates telomerase activity but slightly increases processivity in vitro. Our results suggest that Ψs could have a subtle influence on human telomerase activity via impact on TER–TERT or TER–TER interactions.

As a basic model study for measuring distances in RNA molecules using continuous wave (CW) EPR spectroscopy, site-directed spin-labeled 10-mer RNA duplexes and HIV-1 TAR RNA motifs with various interspin distances were examined. The spin labels were attached to the 2'-NH2 positions of appropriately placed uridines in the duplexes, and interspin distances were measured from both molecular dynamics simulations (MD) and Fourier deconvolution methods (FD). The 10-mer duplexes have interspin distances ranging from 10 A to 30 A based on MD; however, dipolar line broadening of the CW EPR spectrum is only observed for the RNAs for predicted interspin distances of 10-21 A and not for distances over 25 A. The conformational changes in TAR (transactivating responsive region) RNA in the presence and in the absence of different divalent metal ions were monitored by measuring distances between two nucleotides in the bulge region. The predicted interspin distances obtained from the FD method and those from MD calculations match well for both the model RNA duplexes and the structural changes predicted for TAR RNA. These results demonstrate that distance measurement using EPR spectroscopy is a potentially powerful method to help predict the structures of RNA molecules.

Telomerase is a ribonucleoprotein complex that extends the 3′ ends of linear chromosomes. The specialized telomerase reverse transcriptase requires a multidomain RNA (telomerase RNA, TER), which includes an integral RNA template and functionally important template-adjacent pseudoknot. The structure of the human TER pseudoknot revealed that the loops interact with the stems to form a triple helix shown to be important for activity in vitro. A similar triple helix has been predicted to form in diverse fungi TER pseudoknots. The solution NMR structure of the Kluyveromyces lactis pseudoknot, presented here, reveals that it contains a long pyrimidine motif triple helix with unexpected features that include three individual bulge nucleotides and a C + •G-C triple adjacent to a stem 2-loop 2 junction. Despite significant differences in sequence and base triples, the 3D shape of the human and K. lactis TER pseudoknots are remarkably similar. Analysis of the effects of nucleotide substitutions on cell growth and telomere lengths provides evidence that this conserved structure forms in endogenously assembled telomerase and is essential for telomerase function in vivo.RNA triplex | yeast | RNA structure | Hoogsteen T elomerase is a ribonucleoprotein complex that extends the 3′ ends of eukaryotic chromosomes by adding successive telomere DNA repeats using an internal RNA template and a specialized reverse transcriptase (1, 2). Telomeres are the protein-DNA complexes that form the ends of linear chromosomes and protect them from end-to-end fusion and degradation (3, 4). Telomerase is of significant medical interest owing to the correlation between telomere length and human health and the association of telomerase activity with cancer (5, 6). In the absence of telomerase activity, telomeres shorten with each round of cell division because of exonuclease digestion and the inability of conventional DNA polymerases to fully replicate linear chromosomes. Shortening past a critical length leads to cell cycle arrest and/or apoptosis (7). Telomerase activity is undetectable in most somatic cells, resulting in telomere attrition with each cell cycle (8, 9). On the other hand, telomerase is active in, and essential for the proliferation of, the germ line, some epithelial, haemopoietic, and stem cells, as well as ∼90% of cancer cell lines (10, 11). A number of inherited diseases are associated with telomere shortening due to telomerase insufficiency, such as dyskeratosis congenita, aplastic anemia, and pulmonary fibrosis (12-15).The telomerase holoenzyme consists of the telomerase reverse transcriptase (TERT) and telomerase RNA (TER), which are essential and sufficient for catalytic activity in vitro (16), and several species-specific accessory proteins. TERs are highly divergent in size and sequence between species, ranging from ∼150 nt in ciliates, ∼450 nt in vertebrates, to more than 2,000 nt in some fungi (17). TERs provide the template for telomeric DNA synthesis but also contain other domains that are essential for telomerase ass...