Stability of ribosomal protein mRNA and translational feedback regulation in Escherichia coli

Summary It was previously observed that the stability of ribosomal protein (r-protein) mRNA in Escherichia coli decreases under the conditions where its translation is feedback inhibited by repressor r-protein. We have now demonstrated that the stability of mRNA for r-proteins S13, S11 and S4 increases in a strain carrying a mutation in the gene for S4, a translational repressor regulating these r-proteins. The results confirm the previous observations that translational repression increases the decay rate of r-protein mRNA, and in addition, show that the half-life of S13-S4 r-protein mRNA in cells growing under ordinary conditions is significantly shorter than its inherent stability would predict, due to the operation of translational feedback regulation.     

Ribosome recycling,diffusion, and mRNA loop formation in translational regulation

We explore and quantify the physical and biochemical mechanisms that may be relevant in the regulation of translation. After elongation and detachment from the 3' termination site of mRNA, parts of the ribosome machinery can diffuse back to the initiation site, especially if it is held nearby, enhancing overall translation rates. The elongation steps of the mRNA-bound ribosomes are modeled using exact and asymptotic results of the totally asymmetric exclusion process. Since the ribosome injection rates of the totally asymmetric exclusion process depend on the local concentrations at the initiation site, a source of ribosomes emanating from the termination end can feed back to the initiation site, leading to a self-consistent set of equations for the steady-state ribosome throughput. Additional mRNA binding factors can also promote loop formation, or cyclization, bringing the initiation and termination sites into close proximity. The probability distribution of the distance between the initiation and termination sites is described using simple noninteracting polymer models. We find that the initiation, or initial ribosome adsorption binding required for maximal throughput, can vary dramatically depending on certain values of the bulk ribosome concentration and diffusion constant. If cooperative interactions among the loop-promoting proteins and the initiation/termination sites are considered, the throughput can be further regulated in a nonmonotonic manner. Experiments that can potentially test the hypothesized physical mechanisms are discussed.     

同义密码子使用偏嗜性对mRNA半衰期及翻译调控的影响北大核心CSCD

随着基因组学和转录组学在不同生物体遗传和细胞生物学领域的广泛应用,同义密码子使用的偏嗜性逐渐被接受,并且在研究生物进化与生物表型之间的深层联系时,同义密码子使用模式受到相关领域研究人员的重视。信使RNA(messenger RNA,mRNA)最终表达出具有正常生物活性的蛋白产物是生命活动的重要环节。被称为“第二遗传密码”的同义密码子使用模式,可以通过精微调控翻译选择压力等分子机制,从转录调控、翻译调控及代谢活动等水平表达其承载的遗传信息。研究表明,mRNA半衰期的长短对mRNA活性以及转录和翻译过程有显著的影响。因此,系统地归纳同义密码子使用模式在基因转录、翻译调控及翻译后修饰等生命活动中所扮演的角色,将有助于全方位审视生物体如何巧妙利用密码子使用模式所产生的遗传效应来精准合成不同种类蛋白质,并以此保障生长或分化的特定基因表达程序顺利执行、维持正常的生命周期。     

Diversity in translational regulation

Translational control of individual mRNAs relies on cis-regulatory elements, which are often found in the 3' untranslated region. The best characterized of these regulate cytoplasmic polyadenylation, and much of this process can now be defined in terms of molecular interactions, protein modifications and their consequences. Biochemical and genetic approaches have advanced the understanding of the many instances of translational regulation that are crucial for body patterning in Drosophila. For example, in vitro translation systems have been used to study the regulatory mechanisms, and genetic interactions have been instrumental in establishing a link between a regulatory factor and a component of the translational apparatus. Although most examples of control are thought to affect the initiation of translation, two classes of regulatory factors, one a protein and one a short non-coding RNA now appear to inhibit protein synthesis during elongation. Diversity seems to be a central feature of translational control, both in the mechanisms themselves and in the situations where this form of regulation is used.     

A minimal element in 5'UTR of insulin mRNA mediates its translational regulation by glucose

Glucose induced translation of insulin in pancreatic beta cells is mediated by the 5'UTR of insulin mRNA. We determined the minimal sequence/structure in the 5'UTR of rat insulin gene1 for this regulation. We show that specific factors in the pancreatic islets bind to the 5'UTR of the insulin mRNA upon glucose stimulation. We identified a minimal 29-nucleotide element in the 5'UTR that is sufficient to form the complex, and confer glucose mediated translation activation. Conserved residues in the predicted stem loop region of the un-translated region (UTR) seem to be important for the complex formation and the translation regulation.     

Endogenous and exogenous glucocorticoid regulation of ENaC mRNA expression in developing kidney and lung

Lung liquid absorption at birthis crucial for the successful onset of respiration. Na absorption bythe renal collecting duct plays an important role in renal fluid andelectrolyte homeostasis during the early postnatal period. Theepithelial Na channel (ENaC) plays a central role in mediating thesefunctions, and its subunit expression is developmentally regulated in atemporal and tissue specific pattern. Several lines of evidence suggestthat the prenatal increase in circulating glucocorticoids may play animportant role in increasing ENaC expression during maturation. Wetested the role of the prenatal surge using corticotropin-releasinghormone (CRH) knockout (KO) mice. Relative ENaC expression in lungs of KO mice increased at the same rate as in wild-type (WT) mice, butabsolute expression was only 20-30% of WT. In contrast, relative and absolute expression of all three subunits in kidneys was not different between KO and WT mice. Dexamethasone (Dex) increased -ENaC mRNA in fetal lung and kidney explants within 24 h but had different effects on - or -ENaC. Dex increased - and-ENaC in lung, but only after >48 h of exposure, and had no effecton kidney. The results suggest that the kidney metabolizes endogenous glucocorticoids, but the lung does not. Furthermore, the marked difference between lung and kidney responsiveness to glucocorticoids in- and -ENaC expression suggests that factors other than steroids may be important in regulating functional ENaC expression during development.

     

Mechanisms of translational regulation in Drosophila

Translational regulation plays an essential role in many phases of the Drosophila life cycle. During embryogenesis, specification of the developing body pattern requires co-ordination of the translation of oskar, gurken and nanos mRNAs with their subcellular localization. In addition, dosage compensation is controlled by Sex-lethal-mediated translational regulation while dFMR1 (the Drosophila homologue of the fragile X mental retardation protein) controls translation of various mRNAs which function in the nervous system. Here we describe some of the mechanisms that are utilized to regulate these various processes. Our review highlights the complexity that can be involved with multiple factors employing different mechanisms to control the translation of a single mRNA.     

Microtubules and filaments in developing axons and optic stalk cells

Fibrous structures have been studied in the developing optic nerve of chick embryos. The first ganglion cell axons (3-day embryos) were of moderate size, with both neurofilaments and microtubules. Subsequently (4- and 5-day embryos), very small axons were also present. In thesc embryos and in the 4-day hatched chick, the density of microtubules fell within the same range for all but the very small axons, which tended to have more microtubules per unit area. Filaments similar to those previously thought to represent neurofilaments in other parts of the embryonic nervous system were present in the early optic stalk cells, calling into question the reliability of identifying early nerve cells on the basis of 'neurofilaments'.