During translation termination, class II release factor RF3 binds to the ribosome to promote rapid dissociation of a class I release factor (RF) in a GTP-dependent manner. We present the crystal structure of E. coli RF3*GDP, which has a three-domain architecture strikingly similar to the structure of EF-Tu*GTP. Biochemical data on RF3 mutants show that a surface region involving domains II and III is important for distinct steps in the action cycle of RF3. Furthermore, we present a cryo-electron microscopy (cryo-EM) structure of the posttermination ribosome bound with RF3 in the GTP form. Our data show that RF3*GTP binding induces large conformational changes in the ribosome, which break the interactions of the class I RF with both the decoding center and the GTPase-associated center of the ribosome, apparently leading to the release of the class I RF.

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Ursolic acid (UA) is a major pentacyclic triterpenoid in plants, vegetables and fruits, which has been reported to have a potential anti-diabetic activity. Despite various semi-synthetic ursolic acid derivatives already described, new derivatives still need to be designed and synthesized to further improve the anti-diabetic activity. In the present study, two series of novel UA derivatives, were synthesized and their structures were confirmed. The enzyme inhibition activities of semi-synthesized analogues against α-glucosidase were screened in vitro. The results indicated that most of UA derivatives showed a significant inhibitory activity, especially analogues UA-O-i with the IC50 values of 0.71 ± 0.27 μM, which was more potential than other analogues and the positive control. Furthermore, molecular docking studies were also investigated to verify the in vitro study. Structure modification at the C-3 and C-2 positions of UA was an effective approach to obtain the desired ligand from UA, whose structure was in accordance with the active pocket. Besides, suitable hydrophobic group at the position of C-2 might play an important role for the docking selectivity and binding affinity between the ligand and the homology modelling protein. These results could be helpful for designing more potential α-glucosidase inhibitors from UA in the future.

Genetic instability can be induced by unusual DNA structures and sequence repeats. We have previously demonstrated that a large palindrome in the mouse germ line derived from transgene integration is extremely unstable and undergoes stabilizing rearrangements at high frequency, often through deletions that produce asymmetry. We have now characterized other palindrome rearrangements that arise from complex homologous recombination events. The structure of the recombinants is consistent with homologous recombination occurring by a noncrossover gene conversion mechanism in which a break induced in the palindrome promotes homologous strand invasion and repair synthesis, similar to mitotic break repair events reported in mammalian cells. Some of the homologous recombination events led to expansion in the size of the palindromic locus, which in the extreme case more than doubled the number of repeats. These results may have implications for instability observed at naturally occurring palindromic or quasipalindromic sequences.palindrome ͉ hairpin ͉ sequence repeats ͉ sister chromatid recombination

Four novel isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) were found in rat aorta smooth muscle. Two of them were related to gamma-isoform of brain CaM kinase II (gamma-a). Differences in the primary structure of these isoforms were located in the variable region. One of them (gamma-b) contained 23 unique amino acid residues, whereas the other (gamma-c) did not contain this sequence. Both isoforms lacked the two segments (Val-316 to Gln-337 and Lys-353 to Leu-362) present in gamma-a. The DNA sequence of these gamma-isoforms except the variable region was exactly the same, suggesting that they are produced by alternative splicing. Another two isoforms were related to the delta-isoform of brain CaM kinase II (delta-a). delta-b contained a unique 11-residue sequence in the variable region whereas delta-c did not. As found for gamma-isoforms, the sequence analysis suggested that the three delta-isoforms are also produced by alternative splicing. Analysis of RNA by reverse transcription PCR confirmed the existence of specific messages for gamma-b, delta-a and delta-b. The variety of isoforms of CaM kinase II suggest that each isoform may play a specialized role in cell regulation.

Glycyl-tRNA synthetase (GlyRS) is the enzyme that covalently links glycine to cognate tRNA for translation. It is of great research interest because of its nonconserved quaternary structures, unique species-specific aminoacylation properties, and noncanonical functions in neurological diseases, but none of these is fully understood. We report two crystal structures of human GlyRS variants, in the free form and in complex with tRNA Gly respectively, and reveal new aspects of the glycylation mechanism. We discover that insertion 3 differs considerably in conformation in catalysis and that it acts like a "switch" and fully opens to allow tRNA to bind in a cross-subunit fashion. The flexibility of the protein is supported by molecular dynamics simulation, as well as enzymatic activity assays. The biophysical and biochemical studies suggest that human GlyRS may utilize its flexibility for both the traditional function (regulate tRNA binding) and alternative functions (roles in diseases). Aminoacyl-tRNA synthetases (aaRSs)2 play essential roles in mediating genetic information transfer from mRNAs to proteins and attach amino acids to their cognate tRNA molecules in a two-step reaction. In the first step of the reaction, the enzymes catalyze the condensation between ATP and the specific amino acid to generate an aminoacyl-adenylate intermediate. In the second, they transfer the activated amino acid to the acceptor stem of cognate tRNA to form the product aminoacyl-tRNA. The 20 aaRSs can be grouped into two distinct classes based of the conservation of primary sequences and quaternary structures (1-7). Glycyl-tRNA synthetase (GlyRS) is a class II enzyme and possesses three conserved signature motifs at the active site. However, different from other aaRSs, the quaternary structures of GlyRSs are not conserved phylogenetically. Two oligomeric forms have been discovered in nature: whereas eukaryotic and archaeal GlyRSs form ␣2 homodimers and belong to subclass IIa, their eubacterial counterparts form ␣2␤2 heterotetramers and belong to subclass . No significant sequence homology can be found between the two subtypes. In addition to the sequence and structure diversity, studies also show that GlyRSs only aminoacylate tRNA molecules within their own kingdoms, and crossspecies glycylation is rare. This phenomenon may be attributed to the distinct discriminator base at position 73. In eukaryotes, it is an adenosine that precedes the 3Ј-CCA end, whereas in prokaryotes, a uridine is present (9,11,14,15). Therefore, the tRNA recognition mode is an interesting problem, but it is poorly understood.The human GlyRS (hGlyRS) is a class IIa synthetase and forms a homodimer using motif 1. Motifs 2 and 3, on the other hand, are responsible for recognizing the substrates glycine and ATP. Additionally, hGlyRS features an N-terminal WHEP-TRS domain, as well as several insertion domains named insertions 1-3 (9, 16), most of which are flexible in structure (17). Recent studies have shown that aaRSs have developed functions other than aminoacyl...

Two-photon absorption and two-photon induced blue emission characteristics of a series of heterocycle-based organic molecules are investigated experimentally and by quantum-chemical computations. The molecules consist of a typical A-p-A' structure, where heterocycle, styryl and formyl groups are employed as A, p-conjugated and A' moieties, respectively. Experimental results indicate that significant enhancements in the blue emission efficiency and two-photon absorption cross-sections can be achieved by replacing S and O atoms with an N atom in the heterocycle acceptor moiety, which is also supported by the quantum-chemical computations. Additionally, larger two-photon absorption cross-sections can be obtained by choosing appropriate solvents, as indicated by the computations.