过氧物酶体多功能酶2中固醇载体蛋白2结构域的功能

过氧物酶体多功能酶 (包括Ⅰ型、Ⅱ型 ,简称MFE1、MFE2 )在哺乳类动物的脂类代谢中发挥其重要作用 .MFE1具有 2 烯酰CoA水合酶 1和 (3S) 羟脂酰CoA脱氢酶的活性 ,而MFE2具有 2 烯酰CoA水合酶 2和 (3R) 羟脂酰CoA脱氢酶的活性 ,两者均催化烯酰CoA在过氧物酶体β 氧化途径中的第 2步和第 3步反应 .MFE1与MFE2的氨基酸序列不具有任何同源性 ,并且它们的底物特异性也不相同 .比较哺乳类MFE1及酵母MFE2发现 ,哺乳类MFE2羧基末端带有由 12 5个残基组成的固醇载体蛋白 2 (简称SCP2 )结构域 ,其功能是未知的 .为了研究SCP2结构域在MFE2中的功能 ,将人MFE2、MFE2ΔSCP2 (删除MFE2中的SCP2 )、脱氢酶结构域、水合酶结构域以及SCP2结构域分别在E .coli中表达 ,并经纯化得到相应的重组蛋白 .通过测定 2 烯酰CoA水合酶 2和 (3R) 羟脂酰CoA脱氢酶对烯酰CoA的催化活性发现 ,带有SCP2结构域的重组蛋白的酶活力及催化效率高于删除SCP2的突变体蛋白 .实验结果表明 ,SCP2结构域可能通过增强MFE2与脂酰CoA的结合力 ,使得MFE2发挥最有效的催化活力     

Mixotrophy and the toxicity of Ochromonas in pelagic food webs

1. Toxic compounds produced by many phytoplankton taxa are known to have negative effects on competitors (allelopathy), anti‐predatory effects on grazers (mortality or impaired reproduction) or both. Although mixotrophs of the genus Ochromonas are known to be toxic to zooplankton, it has often been assumed in studies of plankton community processes that all flagellates in the size range of this taxon are edible to typical zooplankton grazers (i.e. cells ≤30 μm for Daphnia, ≤6 μm for rotifers). 2. We explored the toxicity of a species of Ochromonas to other planktonic taxa, including its competitors (two species of phytoplankton and protists) and consumers (two species of zooplankton). To test if mode of nutrition by this mixotroph influences its toxicity to other taxa, we exposed each test species to Ochromonas cultured in chemostats under four different nutritional regimes: osmotrophy (labile dissolved organic carbon) and phagotrophy (bacterial prey) in both light and dark conditions (i.e. with or without photosynthesis). 3. Filtrate from osmotrophically fed Ochromonas had a significant negative effect on the population growth rate of two obligate phototrophic phytoplankton, Cryptomonasozolini and Chlamydomonas reinhardtii. The protists Tetrahymena tetrahymena and Paramecium aurelia were also negatively affected by Ochromonas filtrate. Ochromonas cells were toxic to both the rotifer Brachionus calicyflorus and the cladoceran Daphnia pulicaria, with the toxic effects significantly more severe when fed at high cell densities (75 000 cells mL⁻¹) than at low densities (7500 cells mL⁻¹). Ochromonas cultured osmotrophically in the light was more toxic to the Daphnia than cells cultured under other conditions. In contrast, Ochromonas from all nutritional conditions was equally highly toxic to Brachionus. 4. Our findings support the view that Ochromonas can be toxic to other components of the food web with which it interacts. It is especially toxic to zooplankton that directly consume it, although the effect depends upon Ochromonas cell density and whether or not a good food source is simultaneously present. Our results call into question the common practice of pooling flagellates into a single ‘functional group’ included in an ‘edible phytoplankton’ category of cells <30 μm in diameter.     

Cavity spot of carrot (Daucus carota)

Cavity spot disease of carrot (Daucus carota) has been one of the intractable problems for both growers and scientists. Carrots are rejected at grading with one or two visible lesions, and when disease incidence passes a relatively low threshold it becomes uneconomic to harvest crops. For the scientist, there has been considerable pressure to produce both information on the cause of the disease and a cure. Many putative causes have been advanced over the years, but these were almost always contradicted by subsequent work. The first solid indication of involvement of a pathogen was when three different fungicides with activity against Oomycete fungi all reduced disease. Very quickly the causal agents Pythium violae and Pythium sulcatum were isolated from cavity spot lesions and Koch's postulates satisfied. The species are not typical of the more common pythia, having slow growth at normal temperatures, which means that in the context of isolation work, plates may be overgrown by other species before they are seen. Metalaxyl fungicide was identified as the most effective in controlling cavity spot caused by P. violae, but P. sulcatum is naturally tolerant of the fungicide. Recently, metalaxyl has been shown to be subject to enhanced microbial degradation. This phenomenon has been associated with failure to control cavity spot. No other fungicide has been shown to be consistently effective in the field, and none has been registered for disease control. For the future, this means that control of cavity spot can not be based solely on fungicidal control. Other, complementary strategies are necessary for reducing disease. Calcium carbonate is known to have significant effects on cavity spot, probably by inducing a soil microflora inhibitory to filamentous fungi. Management of agronomic aspects such as irrigation, soil cultivation and the length of time for which crops are grown may all be used, while carrot cultivars with some field resistance may be beneficial. However, one of the most significant factors is disease avoidance by not selecting fields with high inoculum levels. One serology‐based risk assessment test has been produced and commercialised, and molecular probes which could be the basis of more sensitive tests are available for both pathogens. The potential for disease reduction via a management strategy combining several key components is discussed.     

Responses of potato cultivars to the common scab pathogens, Streptomyces scabies and S. turgidiscabies

A glasshouse experiment was conducted to study the responses to Streptomyces scabies and S. turgidiscabies in potato cultivars Bellona, Matilda and Sabina (Solanum tuberosum). Potatoes were grown in a peat‐sand mixture inoculated with one of the two strains of either S. scabies or S. turgidiscabies. Logit models were used to analyse the data on disease incidence and severity, whereas the data on emergence and yield were tested by analysis of variance. S. turgidiscabies, a recently described potato pathogen in Finland, possessed a high ability to cause superficial, raised and pitted lesions on all three cultivars tested. Symptoms induced by S. turgidiscabies were similar to those of S. scabies, regardless of the cultivar, which suggests that the two causal organisms of common scab cannot be distinguished based on symptoms. Infection by S. turgidiscabies and S. scabies delayed emergence, had the tendency to decrease the yield, and increased the proportion of small tubers in the yield, regardless of the potato cultivar. Differences in the levels of resistance to common scab were evident between potato cultivars, since cvs. Matilda and Bellona showed higher disease incidence and more severe scab symptoms than cv. Sabina.