Distinct classes of lagging chromosome underpin age-related oocyte aneuploidy in mouse

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Novel Functions for the Endocytic Regulatory Proteins MICAL‐L1 and EHD1 in Mitosis

During interphase, recycling endosomes mediate the transport of internalized cargo back to the plasma membrane. However, in mitotic cells, recycling endosomes are essential for the completion of cytokinesis, the last phase of mitosis that promotes the physical separation the two daughter cells. Despite recent advances, our understanding of the molecular determinants that regulate recycling endosome dynamics during cytokinesis remains incomplete. We have previously demonstrated that Molecule Interacting with CasL Like‐1 (MICAL‐L1) and C‐terminal Eps15 Homology Domain protein 1 (EHD1) coordinately regulate receptor transport from tubular recycling endosomes during interphase. However, their potential roles in controlling cytokinesis had not been addressed. In this study, we show that MICAL‐L1 and EHD1 regulate mitosis. Depletion of either protein resulted in increased numbers of bi‐nucleated cells. We provide evidence that bi‐nucleation in MICAL‐L1‐ and EHD1‐depleted cells is a consequence of impaired recycling endosome transport during late cytokinesis. However, depletion of MICAL‐L1, but not EHD1, resulted in aberrant chromosome alignment and lagging chromosomes, suggesting an EHD1‐independent function for MICAL‐L1 earlier in mitosis. Moreover, we provide evidence that MICAL‐L1 and EHD1 differentially influence microtubule dynamics during early and late mitosis. Collectively, our new data suggest several unanticipated roles for MICAL‐L1 and EHD1 during the cell cycle.      

论湿地生态系统服务的多维度价值评估方法

为了维系湿地生态系统的功能并制定合理的保护与开发规划,有必要量化湿地生态系统服务的价值。目前学术界尚未形成统一的湿地价值评估方法,而不同的选取方法、评估角度、评估对象等,会导致计算结果存在很大差异,尤其是在对时空差异性服务价值评估和湿地生态系统服务的总价值评估中存在难以量化和重复计算现象。针对上述问题,从湿地生态系统服务的定义、分类和受益人出发,提出了多维度价值评估方法,以定量计算时空差异性服务价值和系统的总服务价值,并探讨了该方法在淮河流域八里河湿地生态系统中的应用以及对其他类型生态系统服务研究的借鉴意义。     

HIV-1 viral RNA is selected in the form of monomers that dimerize in a three-step protease-dependent process; the DIS of stem-loop 1 initiates viral RNA dimerization

We have characterized the viral RNA conformation in wild-type, protease-inactive (PR-) and SL1-defective (DeltaDIS) human immunodeficiency virus type 1 (HIV-1), as a function of the age of the viruses, from newly released to grown-up (>or=24 h old). We report evidence for packaging HIV-1 genomic RNA (gRNA) in the form of monomers in PR- virions, viral RNA rearrangement (not maturation) within PR- HIV-1, protease-dependent formation of thermolabile dimeric viral RNAs, a new form of immature gRNA dimer at about 5 h post virion release, and slow-acting dimerization signals in SL1-defective viruses. The rates of gRNA dimer formation were >or=3-fold and >or=10-fold slower in DeltaDIS and PR- viruses than in wild-type, respectively. Thus, the DIS, i.e. the palindrome in the apical loop of SL1, is a dimerization initiation signal, but its role can be masked by one or several slow-acting dimerization site(s) when grown-up SL1-inactive virions are investigated. Grown-up PR- virions are not flawless models for immature virions because gRNA dimerization increases with the age of PR- virions, indicating that the PR- mutation does not "freeze" gRNA conformation in a nascent primordial state. Our study is the first on gRNA conformation in newly released mutant or primate retroviruses. It shows for the first time that the packaged retroviral gRNA matures in more than one step, and that formation of immature dimeric viral RNA requires viral protein maturation. The monomeric viral RNAs isolated from budding HIV-1, as modeled by newly released PR- virions, may be seen as dimers that are much more fragile than thermolabile dimers.     

Analysis of bilateral inverse symmetry in whole bacterial chromosomes

The positions of the 64 DNA tri-nucleotides (triplets) along the Borrelia burgdorferi chromosome were determined and cumulative position plots (CPP) were obtained. Analysis of CPP for complementary triplets revealed close correlations in complementary triplet frequencies (CTF) between opposing leading and lagging strands. Such bilateral inverse symmetry (BIS) applied also to complementary mono- and di-nucleotides and to some >3 n-tuples. At the level of individual bases BIS explains Chargaff's second parity rule for whole bacterial chromosomes. Using shuffled control sequences we show that single-base BIS was not the source of higher-order BIS. Analysis of CTF in 45 other chromosomes suggests that BIS is a general property of eubacteria. BIS at the various levels may be due to the very similar numbers of codons used in chromosomal halves. Evolutionarily, BIS could have resulted from asymmetric substitution of bases combined with genetic rearrangements. However, the provocative theoretical alternative of whole-genome inverse duplication is here considered.     

Mammalian Mitochondrial DNA Replication Intermediates Are Essentially Duplex but Contain Extensive Tracts of RNA/DNA Hybrid

We demonstrate, using transmission electron microscopy and immunopurification with an antibody specific for RNA/DNA hybrid, that intact mitochondrial DNA replication intermediates are essentially duplex throughout their length but contain extensive RNA tracts on one strand. However, the extent of preservation of RNA in such molecules is highly dependent on the preparative method used. These findings strongly support the strand-coupled model of mitochondrial DNA replication involving RNA incorporation throughout the lagging strand.     

Replication fidelity in E. coli: Differential leading and lagging strand effects for dnaE antimutator alleles

DNA Pol III holoenzyme (HE) is the major DNA replicase of Escherichia coli. It is a highly accurate enzyme responsible for simultaneously replicating the leading- and lagging DNA strands. Interestingly, the fidelity of replication for the two DNA strands is unequal, with a higher accuracy for lagging-strand replication. We have previously proposed this higher lagging-strand fidelity results from the more dissociative character of the lagging-strand polymerase. In support of this hypothesis, an E. coli mutant carrying a catalytic DNA polymerase subunit (DnaE915) characterized by decreased processivity yielded an antimutator phenotype (higher fidelity). The present work was undertaken to gain deeper insight into the factors that influence the fidelity of chromosomal DNA replication in E. coli. We used three different dnaE alleles (dnaE915, dnaE911, and dnaE941) that had previously been isolated as antimutators. We confirmed that each of the three dnaE alleles produced significant antimutator effects, but in addition showed that these antimutator effects proved largest for the normally less accurate leading strand. Additionally, in the presence of error-prone DNA polymerases, each of the three dnaE antimutator strains turned into mutators. The combined observations are fully supportive of our model in which the dissociative character of the DNA polymerase is an important determinant of in vivo replication fidelity. In this model, increased dissociation from terminal mismatches (i.e., potential mutations) leads to removal of the mismatches (antimutator effect), but in the presence of error-prone (or translesion) DNA polymerases the abandoned terminal mismatches become targets for error-prone extension (mutator effect). We also propose that these dnaE alleles are promising tools for studying polymerase exchanges at the replication fork.     

Distribution of repetitive sequences on the leading and lagging strands of the Escherichia coli genome: comparative study of Long Direct Repeat (LDR) sequences.

In the present study, we developed a method for detecting sequences whose similarity to a target sequence is statistically significant and we examined the distribution of these sequences in the E. coli K-12 genome. Target sequences examined are as follows: (i) short repeat: Crossover hot-spot instigator (Chi) sequence, replication termination (Ter) sequence, and DnaA binding sequence (DnaA box); (ii) potential stem-loop structure repeats: palindromic unit (PU), boxC sequences, and intergenic repeat unit (IRU); (iii) potential RNA coding repeats: rRNAs, PAIR, TRIP, and QUAD; and (iv) potential protein coding repeats: insertion elements (ISs) and Long Direct Repeats (LDRs). We also examined the distribution of these sequences on leading and lagging strands. We obtained another four statistically significant LDR sequences with more than 187 bp matched to LDR-A near the LDR loci, suggesting that these regions might be used as high recombination hot spots for LDR. Adaptation of individual LDRs to E. coli genome is also discussed on the basis of codon usage.