A general method for site-directed mutagenesis in prokaryotes

The genetic analysis of genes from prokaryotic species for which experimental genetic systems have not yet been developed is often limited by the difficulty of producing mutations in those genes. We report here a general technique applicable to Gram-negative prokaryotes for site-directed mutagenesis of cloned DNA fragments which we have applied to the study of the symbiotic nitrogen fixation genes of Rhizobium meliloti. In particular, we mutagenized cloned R. meliloti restriction fragments in Escherichia coli with transposon Tn5 and then replaced the wild-type parental DNA sequences with the mutant DNA sequences in the R. meliloti genome. Using this method we show that an R. meliloti DNA restriction fragment, cloned previously on the basis of homology to Klebsiella pneumoniae nif genes, contains gene(s) essential for symbiotic nitrogen fixation. In addition, we use this method to construct a physical genetic map of a subset of the R. meliloti nif genes.     

A possible unifying principle for mechanosensation

Of Aristotle's five senses, we know that sight, smell and much of taste are initiated by ligands binding to G-protein-coupled receptors; however, the mechanical sensations of touch and hearing remain without a clear understanding of their molecular basis. Recently, the relevant force-transducing molecules--the mechanosensitive ion channels--have been identified. Such channel proteins purified from bacteria sense forces from the lipid bilayer in the absence of other proteins. Recent evidence has shown that lipids are also intimately involved in opening and closing the mechanosensitive channels of fungal, plant and animal species.     

A unifying framework for dinitrogen fixation in the terrestrial biosphere

Dinitrogen (N(2)) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N(2) fixation across major sectors of the land biosphere. A question remains as to why symbiotic N(2)-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here we present a unifying framework for terrestrial N(2) fixation that can explain the geographic occurrence of N(2) fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, we find a clear advantage to symbiotic N(2) fixers in phosphorus-limited tropical savannas and lowland tropical forests. The ability of N(2) fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N(2) fixation in phosphorus-limited tropical ecosystems. In contrast, modern-day temperatures seem to constrain N(2) fixation rates and N(2)-fixing species from mature forests in the high latitudes. We propose that an analysis that couples biogeochemical cycling and biophysical mechanisms is sufficient to explain the principal geographical patterns of symbiotic N(2) fixation on land, thus providing a basis for predicting the response of nutrient-limited ecosystems to climate change and increasing atmospheric CO(2).     

A G1 rate model accounts for cell-cycle kinetics attributed to 'transition probability'

Smith and his colleagues have proposed that the duration of the cell cycle is determined by a random transition, analogous to the random decay of a radioactive nucleus, by which a cell passes from an 'A state' within the G1 phase to a 'B phase' that includes the rest of the cycle. The experimental support for this transition probability hypothesis is the tendency of a cumulative plot of differences of cycle times of sibling cells (the beta curve) to be exponential ad parallel to the exponential tail of a cumulative plot of the cycle times themselves (the alpha curve). However, a close examination of four of the most extensive sets of experimental data now suggests that the two beta curves with the steepest slopes may not, in fact, be exponential. These and all the other characteristics of the experimental curves are best matched by computer simulations using a cell-cycle model that will be termed here a G1 rate model. This model is consistent with differences in cell metabolism, rather than a transition at an inherently unpredictable time, being the physiological basis for differences in cycle times within a cell population.     

带启动-关闭期和N策略的单重休假M/G/1排队

为了研究带启动-关闭期和N策略的单重休假M/G/1排队系统,考虑顾客服务完成后离去时刻系统中的顾客数,推导出其嵌入马尔可夫链的状态转移概率矩阵;再利用拟生灭过程与矩阵几何解的方法,给出稳态队长的母函数及其数学期望的表达式;采用LST变换处理卷积,求出条件等待时间和稳态等待时间的LST变换;采用经典随机分解方法,得到了稳态队长和条件等待时间的随机分解结果;同时,给出了忙期的母函数及数学期望的表达式,讨论了服务员处于忙期、休假期、空闲期、启动期和关闭期的概率等性能指标。丰富了排队系统的研究内容,也为该模型在实际背景下的应用提供了理论基础。     

多重休假的带启动期和关闭期的Geom~X/G/1排队

研究了批量到达的多重休假带启动期和关闭期的Geom^X／G／1排队。给出了系统稳态队长和等待时间的母函数及其随机分解结果,最后给出该模型的几个特例。     

A mouse knock-in model exposes sequential proteolytic pathways that regulate p27Kip1 in G1 and S phase

The protein p27Kip1 is an inhibitor of cell division. An increase in p27 causes proliferating cells to exit from the cell cycle, and a decrease in p27 is necessary for quiescent cells to resume division. Abnormally low amounts of p27 are associated with pathological states of excessive cell proliferation, especially cancers. In normal and tumour cells, p27 is regulated primarily at the level of translation and protein turnover. Phosphorylation of p27 on threonine 187 (T187) by cyclin-dependent kinase 2 (Cdk2) is thought to initiate the major pathway for p27 proteolysis. To critically test the importance of this pathway in vivo, we replaced the murine p27 gene with one that encoded alanine instead of threonine at position 187 (p27T187A). Here we show that cells expressing p27T187A were unable to downregulate p27 during the S and G2 phases of the cell cycle, but that this had a surprisingly modest effect on cell proliferation both in vitro and in vivo. Our efforts to explain this unexpected result led to the discovery of a second proteolytic pathway for controlling p27, one that is activated by mitogens and degrades p27 exclusively during G1.     

带启动期的GI/G/1排队的瞬时分布

研究了带启动期的GI／G／1排队，利用马尔可夫骨架过程法得到系统队长{L（t），θ1（t），θ2（t）}的瞬时分布所满足的方程，并证明了它的概率分布是一线性方程的唯一最小非负解．     

有限总体M／G／1排队系统的解析模型

This paper studies the stochastic transition behavior of M|G|1 queueing system with a finite population by means of generalized Markov renewal processes.Formulas for the steady-state performance measures, such as the distribution of queue sizes, average waiting time, mean busy period lengths and so on, are then derived, Finally, the queueing model is applied to reliability analysis of the parallel repairable system with an arbitrary service time distribution.     

Rough集与Vague集集成模型

不确定条件下信息系统的知识获取是智能信息处理研究的一个热点问题.对前一阶段研究工作进行了总结,重点介绍了Rough Vague集与Vague Rough集模型,并分别讨论了这2种模型的代数性质.这些理论与方法对经典Rough集进行了扩展,为不确定条件下Vague信息系统的知识获取提供了新思路.     

Rough集与Vague集集成模型

不确定条件下信息系统的知识获取是智能信息处理研究的一个热点问题。对前一阶段研究工作进行了总结,重点介绍了Rough Vague集与Vague Rough集模型,并分别讨论了这2种模型的代数性质。这些理论与方法对经典Rough集进行了扩展,为不确定条件下Vague信息系统的知识获取提供了新思路。     

一类潜伏期和染病期均传染且具一般接触率的SEIS模型

研究了一类潜伏期和染病期均传染且具有一般接触率的SEIS流行病模型,确定了各类平衡点存在的阈值,讨论了各类平衡点的稳定性,揭示了潜伏期和染病期传染对流行病发展趋势的共同影响.     

应用于5G无线通信的微带周期结构带通滤波器设计

提出一种应用于5G无线通信的周期结构微带带通滤波器设计方法。在周期结构低通滤波器的矩形环中靠近电路中心一侧加载金属过孔,使其在截止频率附近发生谐振,以形成中心频率位于截止频率附近的带通滤波器。通过调整金属过孔的位置,实现带通滤波器带宽的控制,并且在矩形环偏离电路中心一侧连接矩形金属贴片,通过调整矩形金属贴片的尺寸可以适当调整滤波器的中心频率,而不改变滤波器的整体尺寸。实验测试结果表明,该滤波器的中心频率为3.33 GHz,3 dB相对带宽为6.9%。其带外抑制特性较好,具有较低的插损,适用于5G无线通信系统。     

潜伏期和染病期均传染的SEIS模型的分析

研究了潜伏期和染病期均传染的SEIS模型.给出了各类平衡点存在的条件阈值,证明了无病平衡点全局渐近稳定性的条件,并且利用第二加性复合矩阵给出了地方平衡点的存在性和全局渐近稳定性的充分条件.     

M/G/1及休假式M/G/1排队模型的解析

骼建立向前微分方程的方法解析了Ｍ／Ｇ／１／∞／ＦＣＦＳ及多重休假式Ｍ／Ｇ／１／∞／ＦＣＦＳ,得到了顾客的平均等待队长和平均等待时间。     

一类具潜伏期和非线性传染率的流行病模型

研究了一类具潜伏期和非线性传染率βSqIp的SEIS流行病模型,确定了各类平衡点存在的条件阈值,讨论了各平衡点的稳定性.     

Reassessing the first appearance of eukaryotes and cyanobacteria

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from approximately 2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10-20 per thousand) of (13)C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ (13)C/(12)C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in (13)C, indicating that indigenous petroleum is 10-20 per thousand lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism approximately 2.2 Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68 Gyr ago and approximately 2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis approximately 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (approximately 300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45-2.32 Gyr ago.