The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum reveals its substrate and reaction specificity.

Cystathionine gamma-synthase catalyses the committed step of de novo methionine biosynthesis in micro-organisms and plants, making the enzyme an attractive target for the design of new antibiotics and herbicides. The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum has been solved by Patterson search techniques using the structure of Escherichia coli cystathionine gamma-synthase. The model was refined at 2.9 A resolution to a crystallographic R -factor of 20.1 % (Rfree25.0 %). The physiological substrates of the enzyme, L-homoserine phosphate and L-cysteine, were modelled into the unliganded structure. These complexes support the proposed ping-pong mechanism for catalysis and illustrate the dissimilar substrate specificities of bacterial and plant cystathionine gamma-synthases on a molecular level. The main difference arises from the binding modes of the distal substrate groups (O -acetyl/succinyl versusO -phosphate). Central in fixing the distal phosphate of the plant CGS substrate is an exposed lysine residue that is strictly conserved in plant cystathionine gamma-synthases whereas bacterial enzymes carry a glycine residue at this position. General insight regarding the reaction specificity of transsulphuration enzymes is gained by the comparison to cystathionine beta-lyase from E. coli, indicating the mechanistic importance of a second substrate binding site for L-cysteine which leads to different chemical reaction types.     

Purification and characterization of cystathionine gamma-synthase type II from Bacillus sphaericus

Cystathionine gamma-synthase type II, which catalyzes L-cystathionine synthesis from O-acetyl-L-homoserine and L-cysteine was purified from Bacillus sphaericus (IFO 3536) in seven steps. The purified enzyme appeared to be homogeneous by the results of polyacrylamide electrophoresis and ampholyte electrofocusing. The enzyme is a typical pyridoxal-P dependent enzyme, has a molecular mass of 165 kDa and consists of four subunits identical in molecular mass. The enzyme catalyzed the gamma-replacement reaction and the elimination reaction was hardly detected even when a large amount of enzyme was added. In the replacement reaction, O-acetyl-L-homoserine and the following thiol compounds: L and D-cysteine, L and D-homocysteine, sodium sulfide, various alkyl and aryl mercaptans, acted as the most suitable substrate to produce L-cystathionine and the corresponding S-substituted L-homocysteine derivatives.     

烟草FIE和MSI1基因的克隆及序列分析

采用电子克隆与实验克隆结合的方法获得了烟草胚乳发育相关基因NTFIE和NTMSI1的cDNA序列,序列号分别为EU375458和EU375459.序列分析结果表明,这两个cDNA序列均含有完整的开放读码框,分别编码370和424个氨基酸,含有保守的WD基序.氨基酸序列比对和系统发育分析结果显示,不同物种之间FIE和MSI1基因编码氨基酸序列同源性都较高.组织表达分析结果表明,这两个基因均具有一定程度的组织表达特异性,NTFIE cDNA基因在花中的表达量最多,但在根和茎中未检测到表达,而NTMSI1 cDNA基因只在离体培养的细胞和根中特异性表达.     

Cloning,over-expression,purification, and characterisation of N-acetylneuraminate synthase from Streptococcus agalactiae

N-acetylneuraminate synthase (NeuAc-synthase; E.C. 4.1.3.19) is one of the two enzymes responsible for sialic acid (N-acetylneuraminic acid) synthesis in bacteria. Potential genes encoding NeuAc synthase in Streptococcus agalactiae and Bacillus subtilis were identified from a BLAST search of the EMBL/GenBank/DDBJ database using the E. coli neuB gene sequence as a probe and the genes cloned and expressed at high level in Escherichia coli. The neuB gene of S. agalactiae was shown to encode an active NeuAc synthase, whereas the spsE gene product from B. subtilis did not have this activity. Expression of the native S. agalactiae neuB gene product enzyme in E. coli resulted in a product that was prone to proteolysis during purification so the protein was tagged with a hexa-histidine tag at its N-terminus and the enzyme was rapidly purified to homogeneity by ammonium sulphate fractionation and Ni-chelating affinity chromatography in two steps. Measurement of the subunit molecular mass by electrospray ionisation mass spectrometry (M(r) = 38, 987 +/- 3) and of the native molecular mass by gel filtration chromatography (M(r) = 78,000) clearly demonstrated that the enzyme is dimeric. The effects of EDTA, temperature, and pH on the activity of the S. agalactiae NeuAc synthase were examined. Enzyme activity was maximal at pH 7 and was dependent on the presence of metal ions such as Mg(2+), Mn(2+) or Co(2+). The purified enzyme was inhibited by the reagent phenylglyoxal and the substrates N-acetyl mannosamine or phosphoenol pyruvate afforded protection against this inhibition, suggesting that one or more arginine residues are involved in substrate recognition and binding. The ease of expression and the properties of the enzyme should now permit a thorough study of the specificity of the enzyme and provide the prerequisites for attempts to alter this specificity by directed evolution for the production of novel sialic acid analogues.     

A damascone derivative from Nicotiana tabacum

A new member of the damascone series, 4-hydroxy-β-damascone, was identified in the steam distillable oil from Virginia tobacco.     

Cloning, purification and characterisation of Staphylococcus warneri lipase 2

A gene encoding an extracellular lipase was identified in Staphylococcus warneri 863. The deduced lipase is organised as a prepro-protein and has significant similarity to other staphylococcal lipases. The mature part of the lipase was expressed with an N-terminal histidine tag in Escherichia coli, purified and biochemically characterised. The results show that the purified lipase (named SWL2) combines the properties of the staphylococcal lipases characterised so far. It has both a high preference for short chain substrates and surprisingly, it also displays phospholipase activity. Homology alignment was used to analyse sequence-function relationships of the staphylococcal lipase family with the aim to identify the structural basis underlying the different properties of the staphylococcal lipases.     

Molecular Cloning and Characterization of L-Galactose-1-phosphate Phosphatase from Tobacco (Nicotiana tabacum)

L-Galactose-1-phosphate phosphatase (GPPase) is an enzyme involved in ascorbate biosynthesis in higher plants. We isolated a cDNA encoding GPPase from tobacco, and named it NtGPPase. The putative amino acid sequence of NtGPPase contained inositol monophosphatase motifs and metal binding sites. Recombinant NtGPPase hydrolyzed not only L-galactose-1-phosphate, but also myo-inositol-1-phosphate. The optimum pH for the GPPase activity of NtGPPase was 7.5. Its enzyme activity required Mg²⁺, and was inhibited by Li⁺ and Ca²⁺. Its fluorescence, fused with green fluorescence protein in onion cells and protoplasts of tobacco BY-2 cells, was observed in both the cytosol and nucleus. The expression of NtGPPase mRNA and protein was clearly correlated with L-ascorbic acid (AsA) contents of BY-2 cells during culture. The AsA contents of NtGPPase over expression lines were higher than those of empty lines at 13 d after subculture. This suggests that NtGPPase contributes slightly to AsA biosynthesis.     

Cloning, heterologous expression, and functional characterization of 5-epi-aristolochene-1,3-dihydroxylase from tobacco (Nicotiana tabacum)

Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by several solanaceous species in response to a variety of environmental stimuli. It is the primary antimicrobial compound produced by Nicotiana tabacum in response to fungal elicitation, and it is formed via the isoprenoid pathway from 5-epi-aristolochene. Much of the biosynthetic pathway for the formation of this compound has been elucidated, except for the enzyme(s) responsible for the conversion of 5-epi-aristolochene to its dihydroxylated form, capsidiol. Biochemical evidence from previous studies with N. tabacum (Whitehead, I. M., Threlfall, D. R., and Ewing, D. F., 1989, Phytochemistry 28, 775-779) and Capsicum annuum Hoshino, T., Yamaura, T., Imaishi, H., Chida, M., Yoshizawa, Y., Higashi, K., Ohkawa, H., Mizutani, J., 1995, Phytochemistry 38, 609-613. suggested that the oxidation of 5-epi-aristolochene to capsidiol was mediated by at least one elicitor-inducible cytochrome P450 hydroxylase. In extending these observations, we developed an in vivo assay for 5-epi-aristolochene hydroxylase activity and used it to demonstrate a dose-dependent inhibition of activity by ancymidol and ketoconazole, two well characterized inhibitors of cytochrome P450 enzymes. Using degenerate oligonucleotide primers designed to the well conserved domains found within most P450 enzymes, including the heme binding domain, cDNA fragments representing four distinct P450 families (CYP71, CYP73, CYP82, and CYP92) were amplified from a cDNA library prepared against mRNA from elicitor-treated cells using PCR. The PCR fragments were subsequently used to isolate full-length cDNAs (CYP71D20 and D21, CYP73A27 and A28, CYP82E1 and CYP92A5), and these in turn were used to demonstrate that the corresponding mRNAs were all induced in elicitor-treated cells, albeit with different induction patterns. Representative, full-length cDNAs for each of the P450s were engineered into a yeast expression system, and the recombinant yeast assessed for functional expression of P450 protein by measuring the CO difference spectra of the yeast microsomes. Only microsomal preparations from yeast expressing the CYP71D20 and CYP92A5 cDNAs exhibited significant CO difference absorbance spectra at 450 nm and were thus tested for their ability to hydroxylate 5-epi-aristolochene and 1-deoxycapsidiol, a putative mono-hydroxylated intermediate in capsidiol biosynthesis. Interestingly, the CYP71D20-encoded enzyme activity was capable of converting both 5-epi-aristolochene and 1-deoxycapsidiol to capsidiol in vitro, consistent with the notion that this P450 enzyme catalyzes both hydroxylations of its hydrocarbon substrate.     

烟草NtCBL1基因的克隆、表达载体构建及表达分析

CBL是一类Ca²⁺感受蛋白,在植物适应或抵制逆境胁迫的过程中发挥重要的作用。从烟草品种K326中克隆到了一个CBL1的同源基因,该序列包含了一个642 bp的开放阅读框,编码一个由213个氨基酸残基组成的蛋白,预测分子量为24.5 kDa,等电点为5.03。同源性分析结果显示,该基因与林烟草CBL1、甜辣椒CBL1等具有较高的同源性,故命名为NtCBL1。生物信息学分析表明,NtCBL1具有CBL家族保守的EF-hand钙结合结构域。组织表达分析发现该基因在成熟期的根、茎、叶、花中均有表达,在根中的表达量最高。逆境胁迫实验表明,该基因表达受低钾、高盐、干旱、ABA和低温诱导调控,参与烟草生物与非生物逆境胁迫的响应。并成功构建了NtCBL1-pBI121过表达载体。研究结果为解析NtCBL1在响应逆境胁迫的功能奠定一定理论基础。     

Pectolytic enzyme activity from Nicotiana tabacum pollen

Ungerminated pollen of Nicotiana tabacum contains a pectolytic enzyme which has its optimal activity between pH 5.5 and 6.5. Pectic lyase was not detected.     

Isolation and characterisation of a full-length genomic clone encoding a plastidic glucose 6-phosphate dehydrogenase from Nicotiana tabacum

We describe here the isolation and characterisation of the first full-length genomic clone encoding a plant glucose 6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) from Nicotiana tabacum L. cv Samsun. The gene was expressed in all tissues, including roots, leaves, stems and flowers. Comparison of the gene with other known plant G6PDH cDNAs grouped this sequence with plastidic isoforms. The protein, minus a putative plastidic transit sequence, was overexpressed in Escherichia coli as a glutathione S-transferase fusion protein. The resulting protein was shown to be immunologically related to the potato plastidic G6PDH. This suggests that the sequence described here codes for a plastidic isoform. Plastidic G6PDH mRNA was induced in both roots and leaves in response to KNO₃, and the induction in roots was approximately 4 times the response seen in leaves. Sequence analysis of the 5′-untranslated region of the genomic clone indicated the presence of several NIT2 elements, which may contribute to the control of the expression of this gene. Plastidic G6PDH mRNA levels did not appear to respond to light. Received: 28 April 2000 / Accepted: 21 July 2000