Isolation and characterization of Photosystem I from two strains of the marine oxychlorobacterium Prochlorococcus

Photosystem I (PS I) complexes from two strains of the marine photosynthetic prokaryote Prochlorococcus, MED4 (= clone CCMP1378) and SS120 (= clone CCMP1375), were isolated by centrifugation on sucrose gradients after detergent treatment. The PS I-enriched fractions of both strains contained about 100 chlorophyll molecules per P700. Electron microscopy showed that the PS I complexes were in a trimeric form. The characteristic long wavelength fluorescence emission of PS I at 77 K, currently observed in chloroplasts and most cyanobacteria was absent both in intact cells and in PS I preparations of both strains. The major proteins of the PS I-enriched fractions were identified immunologically as PsaA and PsaB. Two proteins with apparent molecular masses of about 21 and 25 kDa were present in PS I preparations of Prochlorococcus, whereas the small PS I subunits in cyanobacteria all have molecular masses below 18 kDa. The 25 kDa protein showed a strong cross-reaction with a heterologous antibody against PsaL. Relatedness of the 21 kDa protein to PsaF was demonstrated by internal protein sequencing. Although only trace amounts of the major divinyl-Chl a/b-binding antenna complexes were present in the PS I preparations, significant amounts of divinyl-Chl b were observed in this fraction. The putative organization of this Chl b in PS I is discussed.     

生物组织光传输理论概论

组织光学是一个光学与生命科学相互交叉相互渗透的新兴研究领域,其根本任务是为医用光子技术的发展提供理论基础。本文集中介绍了现有的生物组织光传输理论,它是组织光学研究的基本内容：同时评述了现有光传输理论,如漫射近似,Monte Carlo模拟等的优缺点及适用条件;最后,探讨了处理实际光传输问题时近似理论的合理选择问题。     

Mutagenesis and carcinogenesis in nucleotide excision repair-deficient XPA knock out mice

Mice with a defect in the xeroderma pigmentosum group A (XPA) gene have a complete deficiency in nucleotide excision repair (NER). As such, these mice mimic the human XP phenotype in that they have a >1000-fold higher risk of developing UV-induced skin cancer. Besides being UV-sensitive, XPA^−/− mice also develop internal tumors when they are exposed to chemical carcinogens. To investigate the effect of a total NER deficiency on the induction of gene mutations and tumor development, we crossed XPA^−/− mice with transgenic lacZ/pUR288 mutation-indicator mice. The mice were treated with various agents and chemicals like UV-B, benzo[a]pyrene and 2-aceto-amino-fluorene. Gene mutation induction in several tumor target- and non-target tissues was determined in both the bacterial lacZ reporter gene and in the endogenous Hprt gene. Furthermore, alterations in the p53- and ras genes were determined in UV-induced skin tumors of XPA^−/− mice. In this work, we review these results and discuss the applicability and reliability of enhanced gene mutant frequencies as early indicators of tumorigenesis.     

Shrub Cover Influence on Seedling Growth and Survival Following Logging of a Tropical Forest

Logging in tropical forests may create large canopy openings. These gaps provide suitable conditions for some opportunistic shrubs and herbs to take advantage of the surge in resources and rapidly colonize disturbed sites. This dense plant cover may limit forest regeneration by interfering with tree seedling establishment, growth, and survival by altering the light and nutrients available to seedlings, modifying herbivore behavior, or a number of other factors. In Kibale National Park (Uganda), old logging sites are mainly covered by dense stands of Acanthus pubescens Engl., which appear to inhibit tree regeneration. We wanted to identify the ecological processes underlying this regeneration collapse. To do so, we designed a factorial experiment to evaluate the influences of herbivory and vegetation cover on the growth and survival of tree seedlings. We compared the survival and growth of transplanted tree seedlings in A. pubescens stands and logged forests, in the presence or absence of the understory vegetation layer (logged forest) or vegetation cover (A. pubescens), and with or without herbivory. We found no evidence to support the hypothesis that herbivory is significantly higher under dense A. pubescens cover. Seedling survival was not influenced by the environment. Seedling growth, however, was positively influenced by the removal of A. pubescens, suggesting that changes in resource availability associated with the presence of A. pubescens, may be important for regeneration. Our results suggest that sustained cutting of A. pubescens cover could foster the growth of established seedlings and could lead to tree regeneration and habitat restoration.     

Change in light quality due to a blue-green pigment, marennine, released in oyster-ponds: effect on growth and photosynthesis in two diatoms, Haslea ostrearia and Skeletonema costatum

Two prominent diatoms encountered in oyster-ponds,Haslea ostrearia and Skeletonema costatum,were grown in batch and in a semi-continuous modeunder light of different spectral quality, white, blueor blue-green. The last corresponded to white lightmodified by a water-soluble pigment, marennine,produced by H. ostrearia. After acclimation tothe different light treatments, the growth rates ofboth species showed little variation with respect tolight quality. The parameters for photosynthesisvs irradiance curves were very similar in H. ostrearia grown under the three light conditions,whereas S. costatum the maximum photosyntheticcapacity (on a chlorophyll a basis) wassignificantly reduced under blue-green light. Fluorescence analyses confirmed the data forphotosynthesis, with the operational fluorescenceyield decreasing faster with increasing irradiance inS. costatum grown under blue-green light. InH. ostrearia, fluorescence yields undersaturating irradiance were closely similar in thethree light conditions. The results are discussed inrelation with the prominent development of H.ostrearia that can outcompete other diatoms inoyster-ponds.     

Osmotic Water Permeability of Vacuolar and Plasma Membranes Isolated from Maize Roots

The method of stopped flow was used to follow the changes in light scattering by the vesicles of plasmalemma and tonoplast isolated from maize (Zea maysL.) roots and treated by osmotic pressure. In both membrane preparations, the rate of the process depended on the osmotic gradient and was described with the simple exponential function. The rate constants derived from these functions were the following: the coefficient of water permeability in the tonoplast (P= 165 ± 7 m/s) exceeded by an order of magnitude the corresponding index for plasmalemma (11 ± 2 m/s). The presence of HgCl₂(1.6 nmol/g membrane protein) decreased the tonoplast water permeability by 80%. Microviscosity studies of the hydrocarbon zone in the isolated membranes by using a fluorescent diphenylhexatriene probe demonstrated that the two membranes do not differ in the phase state of their lipid bilayer. The authors conclude that the observed difference in water permeability does not depend on the state of the lipid phase and probably reflects the dissimilar functional activity of plasmalemma and tonoplast aquaporins.     

光/暗对玉米叶片果糖-6-磷酸激酶-2和果糖-2,6-二磷酸酯酶活力的影响

比较了照光和黑暗条件下玉米叶片果糖—6—磷酸激酶—2(PFK-2)和果糖—2,6—二磷酸酯酶(FBPase-2)的活力变化。当玉米植株从暗中转入光下后,其叶片PFK—2的活力随光照时间的延长而逐渐降低,而FBPase-2活力变化不明显;从光下转入暗后叶片PFK-2活力明显上升,FBPase-2活力仍无明显变化;其PFK-2/FBPase-2比值在光处理时下降,暗处理时上升。同时叶片中果糖—2,6—二磷酸的含量与PFK-2/FBPase-2活力比值的变化趋势一致。连续光照 20 h,PFK-2活力持续下降,表明PFK-2的光钝化现象与玉米植株的昼夜节律变化无关。