This study examines the importance of N source and concentration on plant response to distinct CO2 concentrations and root temperatures. The experimental design of this work was a factorial combination of: CO2 concentration, nitrogen concentration, nitrogen source and root temperature. Carob (Ceratonia siliqua L.) was assessed as a potential model of a slow growing Mediterranean species.
The results showed that: 1) biomass increment under high CO2 varied between 13 and 100 percnt; in relation to plants grown under the same conditions but at ambient CO2 concentrations, depending on the root temperature and nitrogen source; 2) nitrate-fed plants attained a larger increase in biomass production compared to ammonium-fed ones. This performance seems to be linked to the co-ordinated regulation of the activities of glutamine synthetase and sucrose phosphate synthase. The variations in the magnitude and nature of growth responses to elevated CO2 observed resulted in substantial changes in the chemical composition of the plant material and consequently in plant nitrogen use efficiency.
Although performed with seedlings and under controlled conditions, this work emphasizes the importance of the nitrogen source used by the plants, a factor rarely taken into consideration when forecasting plant responses to global changes. Particularly, the results presented here, highlight the potential for uncoupling biomass accumulation from increment of air CO2 concentration and show that more than nitrogen availability N source may offset positive plant growth responses under elevated CO2 and root temperature. 相似文献
MbeA is a 60 kDa protein encoded by plasmid ColE1. It plays a key role in conjugative mobilization. MbeA*, a slightly truncated version of MbeA, was purified for in vitro analysis. MbeA* catalysed DNA cleavage and strand-transfer reactions using oligonucleotides embracing the ColE1 nic site, which was mapped to 5'-(1469)CTGG/CTTA(1462)-3'. Thus MbeA is the relaxase for ColE1 conjugal mobilization, in spite of the fact that it lacks a three histidine motif considered the invariant signature of conjugative relaxases. Amino acid sequence comparisons suggest MbeA is nevertheless related to the common relaxase protein family. For instance, MbeA residue Y19 could correspond to the invariant tyrosine in Motif I, whereas H97, E104 and N106 may constitute the equivalent residues to the histidine triad in Motif III. This hypothesis was tested by site-directed mutagenesis. MbeA amino acid residues Y19, H97, E104 and N106 were changed to alanine. MbeA mutant N106A showed reduced oligonucleotide cleavage and strand-transfer activities, whereas mutation in the other three residues resulted in proteins without detectable activity, suggesting they are directly implicated in catalysis of DNA-cleavage and strand-transfer reactions. A double substitution of E104 and N106 by histidines, therefore reconstituting the canonical histidine triad, restored relaxase activities to 1% of wild type. Thus, MbeA is a variant of the common relaxase theme with a HEN signature motif, which has to be added to the canonical three histidine motif of previously reported relaxases. 相似文献
Suspensions of dipalmitoylphosphatidylcholine (DPPC) bilayers containing 5, 10 or 20% (w/w) surfactant protein SP-B have
been reconstituted and spread at air-liquid interfaces. Compression isotherms of DPPC/SP-B monolayers spread from these preparations
were qualitatively comparable to the isotherms of the corresponding DPPC/SP-B monolayers spread from solvents. SP-B was squeezed-out
at higher pressures from vesicle-spread films than from solvent-spread monolayers. SP-B caused a marked decrease on the rate
of relaxation of DPPC collapse phases to equilibrium pressures in all the lipid/protein films assayed. This stabilizing effect
was higher in vesicle-spread than in solvent-spread monolayers. Inclusion in the films of traces of the fluorescent probe
NBD-PC (1 mol%) and use of a fluorescent derivative of SP-B labeled with a rhodamine derivative, Texas Red, allowed for direct
observation of protein and lipid domains at the interface by epifluorescence microscopy. Upon compression, SP-B altered the
packing of phospholipids in the bilayer-spread films, observed as a SP-B-induced reduction of the area of liquid-condensed
domains, in a way similar to its effect in solvent-spread monolayers. SP-B was not associated with condensed regions of the
films. Fluorescence images from vesicle-spread films showed discrete fluorescent aggregates that could be consistent with
the existence of lipid-protein vesicles in close association with the monolayer. Both the retention of SP-B at higher surface
pressures and the greater stability of collapse phases of DPPC/SP-B films prepared by spreading from liposomes in comparison
to those spread from solvents can be interpreted as a consequence of formation of complex bilayer-monolayer interacting systems.
Received: 1 December 1999 / Revised version: 2 March 2000 / Accepted: 2 March 2000 相似文献
Solid cultures of the producing strain grown on Bennett medium develop abundant mycelium and intense sporulation. Under these
conditions biosynthesis of APHE antibiotics (APHE-1 to -3) is accomplished. Further studies show that APHE-3 is basically
produced during spore formation and mostly present in spores, while APHE-1 and APHE-2 are the predominant antibiotics in the
mycelium. APHE compounds are present in almost all streptomycetes tested, indicating a possible role in the life cycle of
these microorganisms.
Received: 19 July 1999 / Received revision: 22 October 1999 / Accepted: 22 October 1999 相似文献
Niemann-Pick type C (NPC) is a disease that affects intracellular cholesterol-trafficking pathways. By cloning the hamster ortholog of NPC1, we identified the molecular lesions in two independently isolated Chinese hamster ovary cell mutants, CT60 and CT43. Both mutants lead to premature translational terminations of the NPC1 protein. Transfecting hamster NPC1 cDNA complemented the defects of the mutants. Investigation of the CT mutants, their parental cells, and an NPC1-stable transfectant allow us to present evidence that NPC1 is involved in a post-plasma membrane cholesterol-trafficking pathway. We found that the initial movement of low density lipoprotein (LDL)-derived cholesterol to the plasma membrane (PM) did not require NPC1. After reaching the PM and subsequent internalization, however, cholesterol trafficking back to the PM did involve NPC1. Both LDL-derived cholesterol and cholesterol originating from the PM accumulated in a dense, intracellular compartment in the CT mutants. Cholesterol movement from this compartment to the PM or endoplasmic reticulum was defective in the CT mutants. Our results functionally distinguish the dense, intracellular compartment from the early endocytic hydrolytic organelle and imply that NPC1 is involved in sorting cholesterol from the intracellular compartment back to the PM or to the endoplasmic reticulum. 相似文献
Cone snails are tropical marine mollusks that envenomate prey with a complex mixture of neuropharmacologically active compounds. We report the discovery and biochemical characterization of a structurally unique peptide isolated from the venom of Conus marmoreus. The new peptide, mr10a, potently increased withdrawal latency in a hot plate assay (a test of analgesia) at intrathecal doses that do not produce motor impairment as measured by rotarod test. The sequence of mr10a is NGVCCGYKLCHOC, where O is 4-trans-hydroxyproline. This sequence is highly divergent from all other known conotoxins. Analysis of a cDNA clone encoding the toxin, however, indicates that it is a member of the recently described T-superfamily. Total chemical synthesis of the three possible disulfide arrangements of mr10a was achieved, and elution studies indicate that the native form has a disulfide connectivity of Cys1-Cys4 and Cys2-Cys3. This disulfide linkage is unprecedented among conotoxins and defines a new family of Conus peptides. 相似文献
Protein TrwC is the relaxase-helicase responsible for the initiation and termination reactions of DNA processing during plasmid R388 conjugation. Site-directed mutagenesis was used to change to phenylalanine each of a set of four conserved tyrosyl residues in the sequence of the N-terminal relaxation domain of the protein. Simultaneous mutation of both Y18 and Y26 was required to abolish in vitro cleavage and strand-transfer reactions catalyzed by protein TrwC on oligonucleotides containing the nic site. Thus, both Y18 and Y26 could be involved independently in the formation of oligonucleotide-protein covalent complexes that constitute presumed intermediates of these reactions. This hypothesis was confirmed by the observation of Y18 and Y26-specific peptide-oligonucleotide adducts after protease digestion of TrwC and mutant derivatives. Finally mutation Y18F, but not mutation Y26F, abolished nic-cleavage of a supercoiled DNA containing the R388 origin of transfer (oriT). These data allowed the construction of a model for conjugative DNA processing in which Y18 specifically catalyzes the initial cleavage reaction, while Y26 is used for the second strand-transfer reaction, which terminates conjugation. The model suggests a control mechanism that can be effective at each conjugative replication cycle. 相似文献