抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Copper is an essential trace element in living systems, present in the parts per million concentration range. It is a key cofactor in a diverse array of biological oxidation-reduction reactions. These involve either outer-sphere electron transfer, as in the blue copper proteins and the Cu{sub A} site of cytochrome oxidase and nitrous oxide redutase, or inner-sphere electron transfer in the binding, activation, and reduction of dioxygen, superoxide, nitrite, and nitrous oxide. Copper sites have historically been divided into three classes based on their spectroscopic features, which reflect the geometric and electronic structure of the active site: type 1 (T1) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled binuclear copper centers. 428 refs.

Multicopper Oxidases and Oxygenases

Rapid generation of superoxide and accumulation of H2O2 is a characteristic early feature of the hypersensitive response following perception of pathogen avirulence signals. Emerging data indicate that the oxidative burst reflects activation of a membrane-bound NADPH oxidase closely resembling that operating in activated neutrophils. The oxidants are not only direct protective agents, but H2O2 also functions as a substrate for oxidative cross-linking in the cell wall, as a threshold trigger for hypersensitive cell death, and as a diffusible signal for induction of cellular protectant genes in surrounding cells. Activation of the oxidative burst is a central component of a highly amplified and integrated signal system, also involving salicylic acid and perturbations of cytosolic Ca2+, which underlies the expression of disease-resistance mechanisms.

The oxidative burst in plant disease resistance

Streptomyces coelicolor is a representative of the group of soil-dwelling, filamentous bacteria responsible for producing most natural antibiotics used in human and veterinary medicine. Here we report the 8,667,507 base pair linear chromosome of this organism, containing the largest number of genes so far discovered in a bacterium. The 7,825 predicted genes include more than 20 clusters coding for known or predicted secondary metabolites. The genome contains an unprecedented proportion of regulatory genes, predominantly those likely to be involved in responses to external stimuli and stresses, and many duplicated gene sets that may represent 'tissue-specific' isoforms operating in different phases of colonial development, a unique situation for a bacterium. An ancient synteny was revealed between the central 'core' of the chromosome and the whole chromosome of pathogens Mycobacterium tuberculosis and Corynebacterium diphtheriae. The genome sequence will greatly increase our understanding of microbial life in the soil as well as aiding the generation of new drug candidates by genetic engineering.

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Complete genome sequence of the model actinomycete Streptomyces coelicolor A3(2)

Plant growth is greatly affected by drought and low temperature. Expression of a number of genes is induced by both drought and low temperature, although these stresses are quite different. Previous experiments have established that a cis-acting element named DRE (for dehydration-responsive element) plays an important role in both dehydration- and low-temperature-induced gene expression in Arabidopsis. Two cDNA clones that encode DRE binding proteins, DREB1A and DREB2A, were isolated by using the yeast one-hybrid screening technique. The two cDNA libraries were prepared from dehydrated and cold-treated rosette plants, respectively. The deduced amino acid sequences of DREB1A and DREB2A showed no significant sequence similarity, except in the conserved DNA binding domains found in the EREBP and APETALA2 proteins that function in ethylene-responsive expression and floral morphogenesis, respectively. Both the DREB1A and DREB2A proteins specifically bound to the DRE sequence in vitro and activated the transcription of the b-glucuronidase reporter gene driven by the DRE sequence in Arabidopsis leaf protoplasts. Expression of the DREB1A gene and its two homologs was induced by low-temperature stress, whereas expression of the DREB2A gene and its single homolog was induced by dehydration. Overexpression of the DREB1A cDNA in transgenic Arabidopsis plants not only induced strong expression of the target genes under unstressed conditions but also caused dwarfed phenotypes in the transgenic plants. These transgenic plants also revealed freezing and dehydration tolerance. In contrast, overexpression of the DREB2A cDNA induced weak expression of the target genes under unstressed conditions and caused growth retardation of the transgenic plants. These results indicate that two independent families of DREB proteins, DREB1 and DREB2, function as trans-acting factors in two separate signal transduction pathways under low-temperature and dehydration conditions, respectively.

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Two transcription factors, DREB1 and DREB2, with an EREBP/AP2 DNA binding domain separate two cellular signal transduction pathways in drought- and low-temperature-responsive gene expression, respectively, in Arabidopsis.

Chalcone synthase and stilbene synthase are plant-specific polyketide synthases. They catalyze three common consecutive decarboxylative condensations and specific cyclization reactions. They are highly homologous to each other, and are likely to fall into a family of polyketide synthases along with acridone synthase and bibenzyl synthase. Two cDNA clones (named HmC and HmS), both of which show high homology to the known chalcone synthases, were obtained from leaves of Hydrangea macrophylla var. thunbergii. They were expressed in Escherichia coli in order to determine their enzyme functions. Detection of chalcone formation clearly indicated that HmC encoded chalcone synthase, while HmS protein catalyzed the formation of neither chalcone nor stilbene. However, a novel pyrone, a lactonization product of a linear tetraketide was detected in reaction products of HmS protein. This proves that HmS encodes a novel polyketide synthase that catalyzes only chain elongation without cyclization.

p-Coumaroyltriacetic acid synthase, a new homologue of chalcone synthase, from Hydrangea macrophylla var. thunbergii.

The cultured cells and intact plants of Glycyrrhiza glabra (Fabaceae) produce betulinic acid and oleanane-type triterpene saponins (soyasaponins and glycyrrhizin). To elucidate the regulation of triterpenoid biosynthesis in G. glabra, the cDNA of lupeol synthase, an oxidosqualene cyclase (OSC) responsible for betulinic acid biosynthesis, was cloned, and expression patterns of lupeol synthase and two additional OSCs, β-amyrin synthase and cycloartenol synthase, were compared. The mRNA expression levels of lupeol synthase and β-amyrin synthase were consistent with the accumulation of betulinic acid and oleanane-type triterpene saponins, respectively. The transcript of lupeol synthase was highly expressed in the cultured cells and root nodules. The transcript of β-amyrin synthase, an OSC responsible for oleanane-type triterpene biosynthesis, was highly expressed in the cultured cells, root nodules and germinating seeds, where soyasaponin accumulates, and in the thickened roots where glycyrrhizin accumulates. In the cultured cells, the addition of methyl jasmonate up-regulated β-amyrin synthase mRNA and soyasaponin biosynthesis, but down-regulated lupeol synthase mRNA. Furthermore, the addition of gibberellin A3 down-regulated β-amyrin synthase mRNA but not lupeol synthase mRNA in the cultured cells. The mRNA levels of cycloartenol synthase, an additional OSC responsible for sterol biosynthesis, in the intact plant and cultured cells were relatively constant in these experiments.

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Differential Expression of Three Oxidosqualene Cyclase mRNAs in Glycyrrhiza glabra

Properties and Substrate Specificity of RppA, a Chalcone Synthase-related Polyketide Synthase in Streptomyces griseus

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2890%2980410-K

Reaction mechamism of oxidative rearrangement of flavanone in isoflavone biosynthesis

Betula platyphylla var. japonica is a rich source of triterpenoid as it contains dammarane type triterpenes in the leaves, and lupane type and oleanane type triterpenes in the bark. Four oxidosqualene cyclase cDNAs (BPX, BPX2, BPW and BPY) were cloned by homology based PCR methods from cell suspension cultures of B. platyphylla var. japonica. Open reading frames consisting of 2274, 2304, 2268 and 2340 bp were ligated into yeast expression plasmid pYES2 under the control of GAL1 promoter and introduced into lanosterol synthase deficient (erg7) Saccharomyces cerevisiae strain GIL77. Analyses of in vitro enzyme activities and/or accumulated products in the transformants demonstrated that they encode cycloartenol synthase (BPX and BPX2), lupeol synthase (BPW) and β-amyrin synthase (BPY) proteins. Phylogenetic tree was constructed for all the known oxidosqualene cyclases (OSCs) including the clones obtained in this study, revealing that OSCs having the same enzyme function form respective branches in the tree even though they derive from different plant species. Intriguing correlation was found between reaction mechanism and molecular evolution of OSCs in higher plants.

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Oxidosqualene cyclases from cell suspension cultures of Betula platyphylla var. japonica: molecular evolution of oxidosqualene cyclases in higher plants.

Yutaka Ebizuka

Papers

p-Coumaroyltriacetic acid synthase, a new homologue of chalcone synthase, from Hydrangea macrophylla var. thunbergii.

Differential Expression of Three Oxidosqualene Cyclase mRNAs in Glycyrrhiza glabra

Properties and Substrate Specificity of RppA, a Chalcone Synthase-related Polyketide Synthase in Streptomyces griseus

Reaction mechamism of oxidative rearrangement of flavanone in isoflavone biosynthesis

Oxidosqualene cyclases from cell suspension cultures of Betula platyphylla var. japonica: molecular evolution of oxidosqualene cyclases in higher plants.