Investigation of subpopulation heterogeneity and plasmid stability in recombinant escherichia coli via a simple segregated model

Many microbial and cell cultures exhibit phenomena that can best be described using a segregated modeling approach. Heterogeneties are more marked in recombinant cell cultures because subpopulations, which often exhibit different growth and productivity characteristics, are more easily identified by selective markers. A simple segregated mathematical model that simulates the growth of recombinant Escherichia coli cells is developed. Subpopulations of different growth rate, plasmid replication rate, and plasmid segregation probability are explicitly considered. Results indicate that a third mechanism of plasmid instability, referred to here as a "downward selective pressure," is significant when describing plasmid loss in batch and chemostat cultures. Also, the model agrees well with experimental data from cultures under antibiotic selective pressure. Finally, model simulations of chemostat cultures reveal the importance of initial conditions on culture stability and the possible presence of nonrandom partitioning functions. (c) 1993 John Wiley & Sons, Inc.     

Five fundamental ways in which complex food webs may spiral out of control

Theory suggests that increasingly long, negative feedback loops of many interacting species may destabilize food webs as complexity increases. Less attention has, however, been paid to the specific ways in which these ‘delayed negative feedbacks’ may affect the response of complex ecosystems to global environmental change. Here, we describe five fundamental ways in which these feedbacks might pave the way for abrupt, large-scale transitions and species losses. By combining topological and bioenergetic models, we then proceed by showing that the likelihood of such transitions increases with the number of interacting species and/or when the combined effects of stabilizing network patterns approach the minimum required for stable coexistence. Our findings thus shift the question from the classical question of what makes complex, unaltered ecosystems stable to whether the effects of, known and unknown, stabilizing food-web patterns are sufficient to prevent abrupt, large-scale transitions under global environmental change.     

Sex determination and optimal development in the Moorish gecko,Tarentola mauritanica

Under temperature sex determination (TSD), sex is determined by temperature during embryonic development. Depending on ecological and physiological traits and plasticity, TSD species may face demographic collapse due to climate change. In this context, asymmetry in bilateral organisms can be used as a proxy for developmental instability and, therefore, deviations from optimal incubation conditions. Using Tarentola mauritanica gecko as a model, this study aimed first to confirm TSD, its pattern and pivotal temperature, and second to assess the local adaptation of TSD and variation of asymmetry patterns across four populations under different thermal regimes. Eggs were incubated at different temperatures, and hatchlings were sexed and measured. The number of lamellae was counted in adults and hatchlings. Results were compatible with a TSD pattern with males generated at low and females at high incubation temperatures. Estimated pivotal temperature coincided with the temperature producing lower embryonic mortality, evidencing selection towards balanced sex ratios. The temperature of oviposition was conservatively selected by gravid females. Asymmetry patterns found were likely related to nest temperature fluctuations. Overall, the rigidity of TSD may compromise reproductive success, and demographic stability in this species in case thermal nest choice becomes constrained by climate change.     

Experimental evidence of size-selective harvest and environmental stochasticity effects on population demography,fluctuations and non-linearity

Theory and analyses of fisheries data sets indicate that harvesting can alter population structure and destabilise non-linear processes, which increases population fluctuations. We conducted a factorial experiment on the population dynamics of Daphnia magna in relation to size-selective harvesting and stochasticity of food supply. Harvesting and stochasticity treatments both increased population fluctuations. Timeseries analysis indicated that fluctuations in control populations were non-linear, and non-linearity increased substantially in response to harvesting. Both harvesting and stochasticity induced population juvenescence, but harvesting did so via the depletion of adults, whereas stochasticity increased the abundance of juveniles. A fitted fisheries model indicated that harvesting shifted populations towards higher reproductive rates and larger-magnitude damped oscillations that amplify demographic noise. These findings provide experimental evidence that harvesting increases the non-linearity of population fluctuations and that both harvesting and stochasticity increase population variability and juvenescence.     

An in silico analysis on the photoproteins Mnemiopsin 1 and Mnemiopsin 2 to explain the experimental results

Mnemiopsin 1 (Mn1) and Mnemiopsin 2 (Mn2) are photoproteins found in Mnemiopsis leidyi. We have tried to answer the question of whether the structural features of photoproteins can explain the observed activity data. According to the activity measurements data, they have the same characteristic wavelength. However, the initial intensity of Mn2 is significantly higher than that of Mn1, and decay time of Mn1 (0.92 s⁻¹) is lower than that of Mn2 (1.46 s⁻¹). The phylogenetic analysis demonstrates that, compared with Obelin and Aequorin from Obelia longissima and Aequorea victoria, respectively, a gene modification event may have caused the expansion of the N-terminal side of all photoproteins from M. leidyi. An in silico study has shown that the stability of the photoprotein–substrate complex of Mn2 is higher than that of Mn1, indicating a higher affinity of the substrate for Mn2 compared with Mn1. It was revealed that the active EF-hand loops 1 and III in Mn2 is locally more rigid compared with those in Mn1. We concluded that different stability of the photoprotein complexes leads to different initial intensity. While different patterns of the local dynamics of loops I and III may influence the decay rate.     

Eu3+-activated red phosphor Ca3YAl3B4O15 with low thermal quenching behaviour

This paper reports a sequence of a Ca₃YAl₃B₄O₁₅:xEu³⁺ red phosphor prepared using a high-temperature solid-state reaction. At the excitation of 396 nm, the samples emitted intense red emission centred at ~623 nm, which could be attributed to the ⁵D₀→⁷F₂ transition of the Eu³⁺ ion. The results showed that the optimum Eu³⁺ doping concentration of Ca₃YAl₃B₄O₁₅:Eu³⁺ phosphor was x = 80 mol%, and the concentration quenching mechanism of Ca₃YAl₃B₄O₁₅:Eu³⁺ red phosphor belonged to the exchange coupling between Eu³⁺ ions. The Commission Internationale de l'éclairage (CIE) coordinates and colour purity of Ca₃Y_0.2Al₃B₄O₁₅:0.8Eu³⁺ were calculated as (0.6375, 0.3476) and 95.5%, respectively. Moreover, the red emission of the obtained phosphor Ca₃YAl₃B₄O₁₅:0.8Eu³⁺ exhibited a low thermal quenching behaviour with an intensity retention rate of 92.85% at 150°C. The above results manifest that the Eu³⁺-activated Ca₃YAl₃B₄O₁₅ phosphor is predicted to be a promising red luminescent component for white light-emitting diodes.     

Structural engineering of the HIV-1 protease molecule with a beta-turn mimic of fixed geometry.

An important goal in the de novo design of enzymes is the control of molecular geometry. To this end, an analog of the protease from human immunodeficiency virus 1 (HIV-1 protease) was prepared by total chemical synthesis, containing a constrained, nonpeptidic type II' beta-turn mimic of predetermined three-dimensional structure. The mimic beta-turn replaced residues Gly16,17 in each subunit of the homodimeric molecule. These residues constitute the central amino acids of two symmetry-related type I' beta-turns in the native, unliganded enzyme. The beta-turn mimic-containing enzyme analog was fully active, possessed the same substrate specificity as the Gly16,17-containing enzyme, and showed enhanced resistance to thermal inactivation. These results indicate that the precise geometry of the beta-turn at residues 15-18 in each subunit is not critical for activity, and that replacement of the native sequence with a rigid beta-turn mimic can lead to enhanced protein stability. Finally, the successful incorporation of a fixed element of secondary structure illustrates the potential of a "molecular kit set" approach to protein design and synthesis.     

滇南地区桃树种植模式对土壤有机碳组分及碳库管理指数的影响

经果林种植可改变土壤质量、改善生态环境,同时具有较高的经济效益。合理的种植模式可通过物种间的互补性提高资源利用效率,改善土壤碳库质量并提高综合效益。为探讨桃树种植模式对土壤有机碳组分及碳库管理指数的影响,以云南省开远市不同桃树种植模式(桃树单种-SP和桃树南瓜套种-PP)为研究对象,以毗邻的天然林地(CK)为对照,分析不同种植模式下活性碳库,即高锰酸钾氧化有机碳(POXC)、颗粒有机碳(POC),惰性有机碳库即矿物结合态有机碳(MAOC)在0—40 cm土层的分布情况,明确土壤有机碳组分与土壤理化性质的关系;计算碳库活度指数(CPAI)、碳库指数(CPI)以及碳库管理指数(CPMI),明确不同桃树种植模式的碳库变化情况。结果表明：桃树种植模式和对照的土壤有机碳组分的含量均随着土层深度的增加而减少,平均土壤有机碳(SOC)含量为：14.68 g/kg(CK)>9.57 g/kg(PP)>8.58 g/kg(SP)。平均活性有机碳组分所占比例与POC/MAOC均表现为：SP>CK>PP,PP的活性有机碳比例较低,具有较高的有机碳稳定性。两种桃树种植模式的CPMI在1...     

游荡型黄河滩涂植物群落多样性及种间联结性

黄河干流流量季节性变化和小浪底“调水调沙”形成了黄河河南段特殊的游荡型滩涂生态系统,为揭示游荡型黄河滩涂(嫩滩、二滩、高滩和大堤)群落组成和种间关系及群落稳定性规律,于2015年9月至2018年11月,采用典型样方法在郑州黄河湿地自然保护区游荡型黄河滩涂开展群落学调查,利用149个草本层样方分析群落物种组成、多样性、种间联结性和群落稳定性。结果表明：(1)研究区植物共有90科289属445种,其中被子植物87科286属440种,裸子植物2科2属3种;蕨类植物仅见1属2种。(2)不同滩涂生境优势种具有显著差异,且总体狗牙根(Cynodon dactylon)、芦苇(Phragmites australis)和小蓬草(Erigeron canadensis)为优势种,国家二级保护植物野大豆(Glycine soja)呈斑块聚集分布,且为嫩滩和二滩亚优势种。(3)大堤物种多样性各指标取值最大,嫩滩次之,二滩和高滩较小。(4)χ²检验、Jaccard指数、Pearson相关检验和Spearman秩相关检验表明,优势种种对种间联结性不强,各物种间相对独立。(5)二滩总体联结...