真核mRNA 3′非翻译区在基因表达中的作用

真核生物mRNA的3′非翻译区(3′-UTR)在基因表达调控中发挥重要作用.它不仅调控mRNA在体内的稳定性和降解速率,控制着mRNA的利用效率,还参与mRNA翻译过程的调控.     

18SrRNA前体的加工及其在植物中的研究现状

核糖体RNA(rRNA)的转录和加工是真核生物细胞一项重要的生命活动,这一过程主要发生在核仁内。对前体rRNA(pre-rRNA)的加工程序包括对间隔区的剪切和通过2′-O-核糖甲基化或是假尿苷化对特定的核苷酸进行的修饰。介绍了18S前体rRNA在酵母细胞中的加工过程、主要参与因子以及在植物领域的最新研究进展。     

F-box蛋白在植物生长发育中的功能研究进展

在真核生物中,由泛素介导的蛋白降解途径与植物生长发育密切相关。F-box蛋白家族是一类含有Fbox基序(motif),在泛素介导的蛋白质水解过程中具有底物识别特性的蛋白质家族。目前,从各种植物中已鉴定出大量的F-box蛋白质,这类蛋白质在植物激素的信号转导、光信号转导、自交不亲和以及花器官发育等许多生理过程中都具有重要功能。研究发现F-box蛋白在调控植物生长发育过程中所发挥的功能与其结构及泛素蛋白酶体途径密切相关。     

真核生物mRNA降解途径

mRNA降解在真核生物的基因表达调控中发挥重要作用.目前,已经鉴定了多种参与mRNA降解 的酶和复合物,并发现细胞质处理小体可能是降解mRNA的主要位点.本文着重总结了正常和 异常mRNA降解的主要途径以及各途径相关因子和酶的功能,并讨论了细胞质处理小体在mR NA降解过程中的作用.最后对该领域今后的研究重点和方向作了探讨.     

植物microRNAs研究进展

植物microRNAs(miRNAs)是一类与RNA诱导沉默复合体相关的约由22个核苷酸组成的单链小RNA分子, 其主要功能是, 通过特异性剪切靶mRNA或阻遏靶mRNA的正常翻译在转录后水平调控基因的负表达。植物miRNAs的靶标主要是参与调控植物生长发育和防御应答的转录因子家族。文章主要综述miRNAs在植物体内的生物发生、作用机制及其调控作用研究新进展。     

八肋游仆虫中无义介导的mRNA降解途径因子的细胞共定位研究

无义介导的mRNA降解途径(nonsense-mediated mRNA decay, NMD)是一种mRNA质量监控机制,识别和降解含有提前终止密码子(premature termination codons, PTCs)的异常转录本,以保障基因的准确表达。到目前为止,有报道的从高等哺乳动物到果蝇、线虫、酵母和原生动物中,均有NMD调控途径,但其机制模型存在一定的差异。由于原生动物在生物的起源与进化上的特殊地位,以及所含的调控因子相对简单,在各种分子机制的研究领域成为热点材料。本文以八肋游仆虫(Euplotes octocarinatus)作为研究对象,从八肋游仆虫大核基因组数据库中,经过同源序列比对,分析鉴定到参与NMD途径的相关因子,包括无义mRNA识别因子poly(A)结合蛋白(poly(A) binding protein, PABP);启动NMD途径的核心因子上游移码蛋白1(up-frameshift 1, UPF1)和上游移码蛋白2(up-frameshift 2, UPF2);外显子连接复合体(exon junction complex, EJC)组分因子MAGO(Mago nashi)、Y14(Tsunagi或RMB8)、eIF4AIII(eukaryotic initiation factor 4A3)和UAP56(U2AF56 associated protein 56);降解无义mRNA的相关因子外切体(exosome)、脱帽酶(decapping mRNA 2, DCP2)、外切酶(5′-3′exoribonuclease 1, XRN1)和去腺苷酸化酶(PGK promoter directed over production, POP2)。其中,后3种蛋白质是mRNA加工小体(processing body, P-Body)的组分。通过荧光共定位分析,分别依次证实UPF1与UPF2之间、EJC组分因子之间和P-body组分因子之间的相互作用关系。随后,通过UPF2分别与MAGO和Y14的荧光共定位结果,推测八肋游仆虫依赖于EJC的NMD途径模型。通过UPF1分别与DCP2和外切体(exosome)的荧光共定位结果,推测了八肋游仆虫无义mRNA的两种降解方式:一种是无义mRNA被介导到P-body中分别被DCP2和POP2脱去5′端帽和3′端Poly(A)尾,随后在XRN1的作用下,沿着5′→3′的方向降解;另一种是在胞质中,无义mRNA直接通过招募POP2去腺苷酸化,随后又在招募来的外切体作用下,沿着3′→5′的方向降解。     

植物小G蛋白的研究进展

小G蛋白(small GTPases)是近年来细胞信号转导的研究热点,包括Ras、Rho、Rab、Arf和Ran等5个亚家族.植物中存在一种特殊的小G蛋白ROP(Rho-related GTPase from plants)是Rho家族成员,在调控细胞生长发育及植物防御反应体系的建立等方面起重要作用.在植物细胞中ROP存在两种形式,一种是与GTP结合的激活态,另一种是与GDP结合的非激活态,通过这种激活态与非激活态之间的转变,ROPs作为植物生长发育过程中重要的"分子开关"参与调控多种信号转导过程.本文主要对国内外近年来有关小G蛋白的种类及其调节机制,以及植物小G蛋白ROP在花粉管生长、根毛发育、H2O2的产生、脱落酸(ABA)以及防御应答反应中的调节作用等方面的研究进展进行综述.     

水稻NAC转录因子家族的研究进展

NAC转录因子家族是一类重要的转录调控因子,在植物中普遍存在。在水稻(Oryza sativa L.)生命历程中,NAC家族参与其细胞生长、组织发育、器官衰老等过程,且在应对外界环境刺激的响应过程中起重要作用。本研究介绍了水稻NAC转录因子家族的结构特点,并综述了水稻NAC转录因子家族参与调控植物生长发育的过程,以及在低温、高盐、病原菌等逆境胁迫中的作用与功能,并对水稻NAC家族今后的研究方向进行了展望。     

ABA分解代谢及其代谢关键酶——8'-羟化酶

脱落酸(ABA)在植物的生长发育和环境胁迫响应等过程中具有重要作用。ABA合成与分解代谢的动态平衡共同调控植物内源ABA水平。ABA8′位甲基羟基化途径是高等植物内源ABA代谢的主要途径;8′-羟化酶是该代谢途径的关键酶,属于P450酶系。生物化学和基因组学研究表明,拟南芥CYP707A家族基因编码8′-羟化酶,该基因家族广泛存在于高等植物中,调控植物内源ABA代谢,介导ABA相关的生理生化过程。本文综述了ABA分解代谢的基本途径,详细概述了ABA8′位甲基羟基化途径及该代谢途径的关键酶8′-羟化酶。同时介绍了8′-羟化酶编码基因-CYP707A家族基因的生物学特征和功能。     

幼龄小白鼠全脑5SrRNA的全核苷酸序列测定

<正> 5S rRNA是一种稳定的独立小分子,在原核和真核细胞中都牢固地结合在核糖体的大亚基上。它在蛋白质生物合成过程中具有重要作用。微生物、大鼠肝细胞及某些植物细胞5S rRNA的一级结构都已经测定,脑细胞5SrRNA的一级结构尚未见报道。因此,我们对幼龄小鼠全脑5S rRNA的序列进行了分析。我们用辜祥荣等人的方法分离和纯化5S rRNA,5S rRNA3′-末端标记参照Peattie的方法(2)。3′-末端标记5S rRNA的序列分析用化学降解-凝胶直读法及特异RNa8e降解直     

A noncoding RNA produced by arthropod-borne flaviviruses inhibits the cellular exoribonuclease XRN1 and alters host mRNA stability

All arthropod-borne flaviviruses generate a short noncoding RNA (sfRNA) from the viral 3′ untranslated region during infection due to stalling of the cellular 5′-to-3′ exonuclease XRN1. We show here that formation of sfRNA also inhibits XRN1 activity. Cells infected with Dengue or Kunjin viruses accumulate uncapped mRNAs, decay intermediates normally targeted by XRN1. XRN1 repression also resulted in the increased overall stability of cellular mRNAs in flavivirus-infected cells. Importantly, a mutant Kunjin virus that cannot form sfRNA but replicates to normal levels failed to affect host mRNA stability or XRN1 activity. Expression of sfRNA in the absence of viral infection demonstrated that sfRNA formation was directly responsible for the stabilization of cellular mRNAs. Finally, numerous cellular mRNAs were differentially expressed in an sfRNA-dependent fashion in a Kunjin virus infection. We conclude that flaviviruses incapacitate XRN1 during infection and dysregulate host mRNA stability as a result of sfRNA formation.     

RNA cleavage products generated by antisense oligonucleotides and siRNAs are processed by the RNA surveillance machinery

DNA-based antisense oligonucleotides (ASOs) elicit cleavage of the targeted RNA by the endoribonuclease RNase H1, whereas siRNAs mediate cleavage through the RNAi pathway. To determine the fates of the cleaved RNA in cells, we lowered the levels of the factors involved in RNA surveillance prior to treating cells with ASOs or siRNA and analyzed cleavage products by RACE. The cytoplasmic 5′ to 3′ exoribonuclease XRN1 was responsible for the degradation of the downstream cleavage products generated by ASOs or siRNA targeting mRNAs. In contrast, downstream cleavage products generated by ASOs targeting nuclear long non-coding RNA Malat 1 and pre-mRNA were degraded by nuclear XRN2. The downstream cleavage products did not appear to be degraded in the 3′ to 5′ direction as the majority of these products contained intact poly(A) tails and were bound by the poly(A) binding protein. The upstream cleavage products of Malat1 were degraded in the 3′ to 5′ direction by the exosome complex containing the nuclear exoribonuclease Dis3. The exosome complex containing Dis3 or cytoplasmic Dis3L1 degraded mRNA upstream cleavage products, which were not bound by the 5′-cap binding complex and, consequently, were susceptible to degradation in the 5′ to 3′ direction by the XRN exoribonucleases.     

Non-targeting siRNA induces NPGPx expression to cooperate with exoribonuclease XRN2 for releasing the stress

Short interfering RNAs (siRNAs) target specific mRNAs for their degradation mediated by RNA-induced silencing complex (RISC). Persistent activation of siRNA-RISC frequently leads to non-targeting toxicity. However, how cells mediate this stress remains elusive. In this communication, we found that the presence of non-targeting siRNA selectively induced the expression of an endoplasmic reticulum (ER)-resident protein, non-selenocysteine containing phospholipid hydroperoxide glutathione peroxidase (NPGPx), but not other ER-stress proteins including GRP78, Calnexin and XBP1. Cells suffering from constant non-targeting siRNA stress grew slower and prolonged G1 phase, while NPGPx-depleted cells accumulated mature non-targeting siRNA and underwent apoptosis. Upon the stress, NPGPx covalently bound to exoribonuclease XRN2, facilitating XRN2 to remove accumulated non-targeting siRNA. These results suggest that NPGPx serves as a novel responder to non-targeting siRNA-induced stress in facilitating XRN2 to release the non-targeting siRNA accumulation.     

The activation of the decapping enzyme DCP2 by DCP1 occurs on the EDC4 scaffold and involves a conserved loop in DCP1

The removal of the 5′-cap structure by the decapping enzyme DCP2 and its coactivator DCP1 shuts down translation and exposes the mRNA to 5′-to-3′ exonucleolytic degradation by XRN1. Although yeast DCP1 and DCP2 directly interact, an additional factor, EDC4, promotes DCP1–DCP2 association in metazoan. Here, we elucidate how the human proteins interact to assemble an active decapping complex and how decapped mRNAs are handed over to XRN1. We show that EDC4 serves as a scaffold for complex assembly, providing binding sites for DCP1, DCP2 and XRN1. DCP2 and XRN1 bind simultaneously to the EDC4 C-terminal domain through short linear motifs (SLiMs). Additionally, DCP1 and DCP2 form direct but weak interactions that are facilitated by EDC4. Mutational and functional studies indicate that the docking of DCP1 and DCP2 on the EDC4 scaffold is a critical step for mRNA decapping in vivo. They also revealed a crucial role for a conserved asparagine–arginine containing loop (the NR-loop) in the DCP1 EVH1 domain in DCP2 activation. Our data indicate that DCP2 activation by DCP1 occurs preferentially on the EDC4 scaffold, which may serve to couple DCP2 activation by DCP1 with 5′-to-3′ mRNA degradation by XRN1 in human cells.