Modulating active sites in MOFs for improved Lewis acid or base catalysis

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Locus-Specific Ribosomal RNA Gene Silencing in Nucleolar Dominance

The silencing of one parental set of rRNA genes in a genetic hybrid is an epigenetic phenomenon known as nucleolar dominance. We showed previously that silencing is restricted to the nucleolus organizer regions (NORs), the loci where rRNA genes are tandemly arrayed, and does not spread to or from neighboring protein-coding genes. One hypothesis is that nucleolar dominance is the net result of hundreds of silencing events acting one rRNA gene at a time. A prediction of this hypothesis is that rRNA gene silencing should occur independent of chromosomal location. An alternative hypothesis is that the regulatory unit in nucleolar dominance is the NOR, rather than each individual rRNA gene, in which case NOR localization may be essential for rRNA gene silencing. To test these alternative hypotheses, we examined the fates of rRNA transgenes integrated at ectopic locations. The transgenes were accurately transcribed in all independent transgenic Arabidopsis thaliana lines tested, indicating that NOR localization is not required for rRNA gene expression. Upon crossing the transgenic A. thaliana lines as ovule parents with A. lyrata to form F1 hybrids, a new system for the study of nucleolar dominance, the endogenous rRNA genes located within the A. thaliana NORs are silenced. However, rRNA transgenes escaped silencing in multiple independent hybrids. Collectively, our data suggest that rRNA gene activation can occur in a gene-autonomous fashion, independent of chromosomal location, whereas rRNA gene silencing in nucleolar dominance is locus-dependent

The MCM-Binding Protein ETG1 Aids Sister Chromatid Cohesion Required for Postreplicative Homologous Recombination Repair

The DNA replication process represents a source of DNA stress that causes potentially spontaneous genome damage. This effect might be strengthened by mutations in crucial replication factors, requiring the activation of DNA damage checkpoints to enable DNA repair before anaphase onset. Here, we demonstrate that depletion of the evolutionarily conserved minichromosome maintenance helicase-binding protein ETG1 of Arabidopsis thaliana resulted in a stringent late G2 cell cycle arrest. This arrest correlated with a partial loss of sister chromatid cohesion. The lack-of-cohesion phenotype was intensified in plants without functional CTF18, a replication fork factor needed for cohesion establishment. The synergistic effect of the etg1 and ctf18 mutants on sister chromatid cohesion strengthened the impact on plant growth of the replication stress caused by ETG1 deficiency because of inefficient DNA repair. We conclude that the ETG1 replication factor is required for efficient cohesion and that cohesion establishment is essential for proper development of plants suffering from endogenous DNA stress. Cohesion defects observed upon knockdown of its human counterpart suggest an equally important developmental role for the orthologous mammalian ETG1 protein

Counteranion effects on the catalytic activity of copper salts immobilized on the 2,2 '-bipyridine-functionalized metal-organic framework MOF-253

© 2014 Elsevier B.V. All rights reserved. The charge-neutral, aluminium-based metal-organic framework containing accessible 2,2′-bipyridine (bpy) sites, MOF-253, is a suitable host material for the immobilization of various copper catalysts. The catalytic performance of CuCl 2 , Cu(NO 3 ) 2 , Cu(BF 4 ) 2 and Cu(CF 3 SO 3 ) 2 before and after coordination to the bpy ligands in MOF-253 was studied in the Meinwald rearrangement of α-pinene oxide to campholenic aldehyde (CA). The coordination environment of Cu 2+ in MOF-253 was further studied via EPR spectroscopy. Although the catalytic activity of the copper salts decreased upon heterogenization through coordination with the bpy linker, the selectivity to campholenic aldehyde markedly increased. Furthermore, the catalytic performance of the MOF loaded with copper salts was shown to vary greatly with the choice of charge compensating anion, allowing for improvement of the heterogeneous catalyst.status: publishe