The integrated and free states of Streptomyces griseus plasmid pSG1

A 16.6-kb plasmid-pSG1-was isolated from Streptomyces griseus following transformation of protoplasts with unrelated plasmids. Southern hybridization experiments with radioactive probes prepared from pSG1 fragments and immobilized S. griseus DNA fragments indicated that the plasmid was present in the progenitor strain, in an integrated state. In the pSG1+ isolates plasmid sequences existed both as integrated sequences and as free plasmids. The integrated state of maintenance persisted in strains which have been cured of the free plasmid. The junction site on the plasmid was located on a 0.5-kb EcoRI-SalI fragment. The chromosomal integration site was demonstrated to be the same in all strains derived from S. griseus NRRL3851. The occurrence of both states of plasmid maintenance in the same clones indicates that an integrated pSG1 sequence does not interfere with free plasmid replication and partition. It suggests that the establishment of the free state may involve a replicative excision of pSG1 from the S. griseus chromosome.     

Stoichiometry of proton uptake in isolated pea chloroplasts under different light intensities

The number of protons released inside the chloroplast thylakoids per electron which is transferred through the electron transport chain (H⁺/e^– ratio) was measured in isolated pea chloroplasts at pH 6.0 under continuous illumination and with methyl viologen as an electron acceptor. At saturating light intensity (200 W · m^–2) (strong light) the H⁺/e^– ratio was 3. At low intensity (0.9 W · m^–2) (weak light) the H⁺/e^– ratio was 2 with dark-adapted chloroplasts, but it was close to 3 with chloroplasts that were preilluminated with strong light. It is shown that the presence of azide in the reaction mixture leads to errors in the determination of the H⁺/e^– ratio due to underestimation of the initial rate of H⁺ efflux on switching off the light. To explain the above data, we assume that transformation of the electron transport chain occurs during illumination with strong light, namely, the Q cycle becomes operative.     

Effect of light intensity on ammonia assimilation in maize leaves

The effect of light on the metabolism of ammonia was studied by subjecting detached maize leaves to 150 or 1350 mol m^–2 s^–1 PAR during incubation with the leaf base in 2 mM ¹⁵NH₄Cl. After up to 60 min, leaves were extracted. Ammonia, glutamine, glycine, serine, alanine, and aspartate were separated by isothermal distillation and ion exchange chromatography. ¹⁵N enrichments were analyzed by emission spectroscopy. The uptake of ammonium chloride did not influence CO₂ assimilation (8.3 and 17.4 mol m^–1 s^–1 at 150 and 1350 mol m^–2 s^–1 PAR, respectively). Leaves kept at high light intensity contained more serine and less alanine than leaves from low light treatments. Within 1 h of incubation the enrichment of ammonia extracted from leaves rose to approximately 20% ¹⁵N. In the high light regime the amino acids contained up to 15% ¹⁵N, whereas in low light ¹⁵N enrichments were small (up to 6%). The kinetics of ¹⁵N incorporation indicated that NH₃ was firstly assimilated into glutamine and then into glutamate. After 15 min ¹⁵N was also found in glycine, serine and alanine. At high light intensity nearly half of the ¹⁵N was incorporated in glycine. On the other hand, at low light intensity alanine was the predominant ¹⁵N sink. It is concluded that light influences ammonia assimilation at the glutamine synthetase reaction.     

Energetic aspects of the light activation of two chloroplast enzymes: fructose-1,6-bisphosphatase and NADP-malate dehydrogenase

The light energy requirements for photoactivation of two chloroplast enzymes: fructose-1,6-bisphosphatase and NADP-malate dehydrogenase were studied in a reconstituted chloroplast system. This system comprised isolated pea thylakoids, ferredoxin (Fd), ferredoxin-thioredoxin reductase (FTR) thioredoxin_m and _f (Td_m, Td_f) and the photoactivatable enzyme. Light-saturation curves of the photoactivation process were established with once washed thylakoids which did not require the addition of Td for light activation. They exhibited a plateau at 10 W·m^–2 under nitrogen and 50 W·m^–2 under air, while NADP photoreduction was saturated at 240 W·m^–2. Cyclic and pseudocyclic phosphorylations saturated at identical levels as enzyme photoactivations. All these observations suggested that the shift of the light saturation plateau towards higher values under air was due to competing oxygen-dependent reactions. With twice washed thylakoids, which required Td for enzyme light-activation, photophosphorylation was stimulated under N₂ by the addition of the components of the photoactivation system. Its rate increased with increasing Td concentrations, just as did the enzyme photoactivation rate, while varying the target enzyme concentration had only a weak effect. Considering that Td concentrations were in a large excess over target enzyme concentrations, it may be assumed that the observed ATP synthesis was essentially dependent on the rate of Td reduction.Under air, Fd-dependent pseudo-cyclic photophosphorylation was not stimulated by the addition of the other enzyme photoactivation components, suggesting that an important site of action of O₂ was located at the level of Fd.Abbreviations Fd ferredoxin - FBPase fructose-1,6-bisphosphatase - FTR ferredoxin-thioredoxin reductase - LEM light effect mediator - NADP-MDH NADP-malate dehydrogenase - Td thioredoxin     

Molecular cloning of a pea mRNA encoding an early light induced,nuclear coded chloroplast protein

Summary cDNA clones were isolated for a chloroplast protein, the mRNA of which is induced to maximum levels within 2–4 h after onset of illumination in five day old, etiolated pea seedlings. The cDNA library was constructed from poly(A)⁺-mRNA which was isolated from 4 h illuminated seedlings. The extremely short induction period of the early light induced protein(ELIP)-mRNA established the basis of our screening procedure. Colony hybridization experiments were performed with³²P-labelled cDNA probes, synthesized from RNA of seedlings which had been exposed to different programs of illumination. Plasmid DNAs were isolated from colonies showing strong hybridization signals exclusively with cDNA corresponding to the 4 h-mRNA. Hybrid released translation of preselected plasmids p 17/C2 and p17/C4 revealed a peptide of M_r 24 000. After posttranslational importin vitro, the processed product of M_r 17 000 appears in the chloroplast. Using these clones, the expression of the ELIP-mRNA was investigated by DOT-hybridization. The ELIP-mRNA reaches maximum levels within 2–4 hours after onset of illumination. Our results correspond precisely to thein vivo characteristics and indicate positive identification of the sought clones.     

Antigenic variation is associated with DNA rearrangements in a relapsing fever Borrelia

Borrelia hermsii, an agent of relapsing fever, undergoes antigenic variation in its host. Surface-exposed proteins with differing primary structures determine the serotype of each organism. Using amino acid sequence data from two of these variable proteins, we synthesized two mixed-sequence oligonucleotides and then used the oligonucleotides to probe mRNA and DNA of three isogenic serotypes of B. hermsii. In Northern blots the probes were specific for the mRNA of the homologous serotype. Southern blots revealed two classes of hybridizing fragments: those common to the three serotypes and those specific for a particular serotype. A serotype-specific DNA fragment, which had hybridized to both oligonucleotide probes, was cloned. Subsequent use of the cloned fragment as a probe provided further evidence that antigenic variation in B. hermsii is associated with DNA rearrangements and with occurrence of expression-linked copies of all, or part, of an antigen-specifying gene.     

Seed germination in Chenopodium album L.: further evidence for the dependence of the effects of growth regulators on nitrate availability

Abstract Chenopodium album L. plants, grown under controlled environmental conditions on different levels of soil nitrate, produced seeds with proportionately different NO^?₃ contents. Regardless of the endogenous NO^?₃ content, few seeds germinated in water or upon treatment with KNO₃. Ethylene promoted germination, and the extent of germination was positively correlated with the endogenous seed NO^?₃ content. Combined application of ethylene and KNO₃ in the dark had a synergistic effect on NO^?₃ -deficient seed. The synergism between ethylene and KNO₃ was attributable to the NO^?₃ moiety of the nitrate salt. Ethylene and light showed moderate synergism in seeds with low or high endogenous nitrate. Addition of nitrate, however, masked the interaction between ethylene and light. Gibberellic acid₄₊₇ (GA₄₊₇) or red light, each alone or combined with KNO₃, had little effect on germination. When applied together in the dark, ethylene and GA₄₊₇ synergistically enhanced the germination of NO^?₃-deficient seed. The combined effects of the two hormones on this seed were further enhanced by the addition of KNO₃. There was no synergism between ethylene and GA₄₊₇ in NO^?₃-rich seed. These interactions among GA₄₊₇, ethylene and KNO₃ were not affected by light. The results confirm and further elaborate our earlier finding that the sensitivity of C. album seeds to ethylene may depend on nitrate availability.     

明适应条件下鲤属鱼L-型外水平细胞反应的给光-瞬变成分

在明适应条件下鲤属鱼 L-型外水平细胞的反应显示明显的给光-瞬变成分(on-transient)、它与刺激波长有关——对蓝、绿光的反应比对红光的反应有更明显的瞬变成分,其光谱特性提示它与绿敏锥细胞的输入信号有关。与已报道的其它动物 L 型水平细胞的给光-瞬变成分不同,它的出现在一定范围内与网膜受照射的面积无关。绿色(502nm)和红色(706nm)闪光同时照射所引起反应的给光-瞬变成分比各自单独刺激时要显著得多,提示它也与绿敏锥细胞和红敏锥细胞输入的相互作用有关。     

单管黑白双光灯的诱虫效应

在棉田单管黑白双光灯比黑光灯诱虫量有较显著的提高,对害虫天敌昆虫的诱获量却明显比黑光灯少。本文并分析了单管黑白双光灯的诱虫原理及诱虫超过黑光灯的原因。