L-异亮氨酸和甘氨酸对大肠杆菌表达与分泌邻苯二酚2,3-双加氧酶的作用

证实了甘氨酸与L-异亮氨酸对大肠杆菌表达邻苯二酚2,3-双加氧酶(CatO_2ase)的促进作用和甘氨酸促使该酶分泌至胞外培养基中的作用.产酶量高低和分泌量多少与培养基种类、甘氨酸和L-异亮氨酸的浓度以及培养时间等因素有关.在甘氨酸存在的情况下,胞壁对溶菌酶的敏感性有所增加,超微形态似有变化,还存在其他物质的伴随分泌,故甘氨酸可能是引起细胞壁结构的改变而导致邻苯二酚2,3-双加氧酶等胞内容物被动分泌至胞外.     

3-氰基吡啶水合酶的反应条件及影响因子

研究了芳腈水合酶催化水合3-氰基吡啶生成尼克酰胺的反应条件及影响因子.酶反应的最适pH为8.0,最适温度为25℃.酶在pH8.5于25℃保温4小时或在25—30℃于pH8.0保温3小时是稳定的.反应液中加入Fe~(3 )(1.5 mmol/L)可使酶活力增加 50%,而加入NH_4~ (300 mmol/L)则使酶活降低了67%.Ag~ 和 Hg(2 )”强烈地抑制酶反应活性,在浓度均为 5mmol/L时,抑制率分别为99.7%和100%.NaCN(50 mmol/L)和苯甲腈(100 mmol/L)对酶活性的抑制率分别为78%和85%.该酶作用于 3-氰基吡啶的Km为62.5 mmol/L,V_(max)为85.8 μmol·min~(-1)·mg~(-1).     

新疆地区小麦白粉病菌寄主范围的研究

用小麦白粉病菌11个生理小种的混合菌种,对新疆地区的小麦近缘植物的7个属22个种的47份材料进行接种,除6份免疫外,其余均接种成功.用其中6个属19个种的29份小麦近缘植物产生的白粉病菌,对小麦回接,参试的29份材料全部回接成功.小麦白粉病菌对小麦近缘植物的寄生像在小麦上一样,有明显的寄生专化性.感病的小麦近缘植物的78.0%对小麦白粉病菌的感病性,随生育期增长而急剧下降.文中并对小麦白粉病中间寄主的作用进行了讨论.     

Concordance between Parental Origin of Chromosome 13q Loss and 6p Duplication in Sporadic Retinoblastoma

Two hypotheses are capable of explaining nonrandom loss of one parent's alleles at tumor suppressor loci in sporadic cases of several pediatric cancers, including retinoblastoma—namely, preferential germ-line mutation or chromosome imprinting. We have examined 74 cases of sporadic retinoblastoma for tumors in which at least two genetic events—loss of heterozygosity for chromosome 13q markers and formation of an isochromosome 6p—have occurred. Sixteen cases were found to contain both events. In 13 of 16 such tumors, the chromosomes 13q that were lost and chromosomes 6p that were duplicated are derived from the same parent. These data may be explained within the framework of the genome imprinting model but are not predicted by preferential germ-line mutation.     

Decreased susceptibility of melanized Cryptococcus neoformans to UV light.

Melanized Cryptococcus neoformans cells were less susceptible than nonmelanized cells to the fungicidal effects of UV light. Phenoloxidase-catalyzed production of melanin-like pigments may serve to protect the fungus against ionizing radiation.     

Parallel formation and synergism of hydrolytic enzymes and peptaibol antibiotics, molecular mechanisms involved in the antagonistic action of Trichoderma harzianum against phytopathogenic fungi.

Chitinase, beta-1,3-glucanase, and protease activities were formed when Trichoderma harzianum mycelia, grown on glucose as the sole carbon source, were transferred to fresh medium containing cell walls of Botrytis cinerea. Chitobiohydrolase, endochitinase, and beta-1,3-glucanase activities were immunologically detected in culture supernatants by Western blotting (immunoblotting), and the first two were quantified by enzyme-linked immunosorbent assay. Under the same conditions, exogenously added [U-14C]valine was incorporated in acetone-soluble compounds with an apparent M(r) of < 2,000. These compounds comigrated with the peptaibols trichorzianines A1 and B1 in thin-layer chromatography and released [U-14C]valine after incubation in 6N HCl. Incorporation of radioactive valine into this material was stimulated by the exogenous supply of alpha-aminoisobutyric acid, a rare amino acid which is a major constituent of peptaibols. The obtained culture supernatants inhibited spore germination as well as hyphal elongation of B. cinerea. Culture supernatants from mycelia placed in fresh medium without cell walls of B. cinerea did not show hydrolase activities, incorporation of [U-14C]valine into peptaibol-like compounds, and inhibition of fungal growth. Purified trichorzianines A1 and B1 as well as purified chitobiohydrolase, endochitinase, or beta-1,3-glucanase inhibited spore germination and hyphal elongation, but at concentrations higher than those observed in the culture supernatants. However, when the enzymes and the peptaibols were tested together, an antifungal synergistic interaction was observed and the 50% effective dose values obtained were in the range of those determined in the culture supernatants. Therefore, the parallel formation and synergism of hydrolytic enzymes and antibiotics may have an important role in the antagonistic action of T. harzianum against fungal phytopathogens.     

Multiple genotypes of mitochondrial DNA within a horse population from a small region in Yunnan province of China

mtDNA genotypes of six domestic horses (three adult short horses whose heights are under 1 m and three common domestic horses) from a small region of 15 km² in Malipo county of Yunnan province of China were investigated by the technique of restriction fragment length polymorphism (RFLP) with 16 restriction endonucleases which recognize 6-bp sequences. An average of 56 fragments for an individual was obtained. Unlike other domestic animals, this population of horses exhibits high mtDNA genetic diversity. Each of the six horses has a specific mtDNA genotype showing a pattern of multiple maternal origins, as suggested by fossil and literature records. We think the population of horses is an amazing seed-resource pool of horses and hence deserves to be paid more attention from the view of conservation genetics. However, it is also remarkable that we did not find any typical mtDNA genetic markers which would discriminate between short horses and common domestic horses.     

De novo synthesis of thymidylate via deoxycytidine in dcd (dCTP deaminase) mutants of Escherichia coli.

dcd (dCTP deaminase) mutants of Escherichia coli were reported not to require thymidine for growth even though most of the thymidylate that is synthesized de novo arises from cytosine nucleotides through a pathway involving dCTP deaminase. We found, however, that the fresh introduction of dcd mutations into many strains of E. coli produced a requirement for thymidine for optimum aerobic growth, but the mutants readily reverted to prototrophy via mutations in other genes. One such mutation was in deoA, the gene for deoxyuridine phosphorylase. However, a dcd deo mutant became thymidine dependent once again if a cdd mutation (affecting deoxycytidine deaminase) were introduced. The results indicate that dcd mutants utilize an alternative pathway of TMP synthesis in which deoxycytidine and deoxyuridine are intermediates. A cdd mutation blocks the pathway by preventing the conversion of deoxycytidine to deoxyuridine, whereas a deoA mutation enhances it by sparing deoxyuridine from catabolism. The deoxycytidine must arise from dCTP or dCDP via unknown steps. It is not known to what extent this pathway is utilized in wild-type cells, which, unlike the dcd mutants, do not accumulate dCTP.     

Water potentials for developing cladodes and fruits of a succulent plant, including xylem-versus-phloem implications for water movement

Developing cladodes had lower water potentials and developingfruits had higher water potentials than the underlying cladodesof the widely cultivated prickly pear cactus, Opuntia ficus-indica.The 0.06 MPa lower value in 4-week-old daughter cladodes indicateda typical water potential gradient from the underlying clad-odealong the xylem of –0.2 MPa m^–1; the 0.17 MPa highervalue in 4-week-old fruits, which decreased to 0.07 MPa by 10weeks, implicated the phloem as their supplier of water. Thephloem sap of the underlying cladodes had an osmotic pressureof only 0.90 to 0.98 MPa, so the phloem could supply a relativelydilute solution to the photosynthetically dependent fruits (daughtercladodes of O. ficus-indica are photosynthetically independentat 4 weeks). Although the water potentials were similar foradjacent tissues, the osmotic pressures were lower for the water-storagecompared with the photosynthetic tissue; the osmotic pressureswere higher for xylem sap from fruits, for which xylary flowapparently occurred toward the underlying cladodes, than fordaughter cladodes. The relative capacitance (change in relativewater content divided by change in tissue water potential) wasapproximately 0.71 MPa^–1 for the water-storage tissueand the photosynthetic tissue of both daughter cladodes andfruits at 4 weeks of age. When these organs approached maturityat 10 weeks, the relative capacitance increased about 40% fortheir water-storage tissue, but decreased 30% for their photosynthetictissue. As the plant water content decreases during drought,about twice as much water will thus be lost per unit volumeof the water-storage tissue compared with the photosynthetictissue of maturing fruits and cladodes. Key words: Opuntia ficus-indica, phloem, relative water content, water capacitance, water potential