SummaryPlant isoprenoids represent a heterogeneous group of compounds which play essential roles not only in growth and development, but also in the interaction of plants with their environment. Higher plants contain two pathways for the biosynthesis of isoprenoids: the mevalonate pathway, located in the cytosol/endoplasmic reticulum, and the recently discovered mevalonate-independent pathway (Rohmer pathway), located in the plastids. In order to evaluate the function of the Rohmer pathway in the regulation of the synthesis of plastidial isoprenoids, we have isolated a tomato cDNA encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS), the ®rst enzyme of the pathway. We demonstrate in vivo activity and plastid targeting of plant DXS. Expression analysis of the tomato DXS gene indicates developmental and organ-speci®c regulation of mRNA accumulation and a strong correlation with carotenoid synthesis during fruit development. 1-Deoxy-D-xylulose feeding experiments, together with expression analysis of DXS and PSY1 (encoding the fruit-speci®c isoform of phytoene synthase) in wildtype and yellow¯esh mutant fruits, indicate that DXS catalyses the ®rst potentially regulatory step in carotenoid biosynthesis during early fruit ripening. Our results change the current view that PSY1 is the only regulatory enzyme in tomato fruit carotenogenesis, and point towards a coordinated role of both DXS and PSY1 in the control of fruit carotenoid synthesis.
For many years it was accepted that isopentenyl diphosphate, the common precursor of all isoprenoids, was synthesized through the well known acetate͞mevalonate pathway. However, recent studies have shown that some bacteria, including Escherichia coli, use a mevalonate-independent pathway for the synthesis of isopentenyl diphosphate. The occurrence of this alternative pathway has also been reported in green algae and higher plants. The first reaction of this pathway consists of the condensation of (hydroxyethyl)thiamin derived from pyruvate with the C1 aldehyde group of D-glyceraldehyde 3-phosphate to yield D-1-deoxyxylulose 5-phosphate. In E. coli, D-1-deoxyxylulose 5-phosphate is also a precursor for the biosynthesis of thiamin and pyridoxol. Here we report the molecular cloning and characterization of a gene from E. coli, designated dxs, that encodes D-1-deoxyxylulose-5-phosphate synthase. The dxs gene was identified as part of an operon that also contains ispA, the gene that encodes farnesyl-diphosphate synthase. D-1-Deoxyxylulose-5-phosphate synthase belongs to a family of transketolase-like proteins that are highly conserved in evolution.Isoprenoids are ubiquitous compounds found in all living organisms. Some isoprenoids play essential roles in particular cell functions such as sterols, contributing to eukaryotic membrane architecture, acyclic polyprenoids found in the side chain of ubiquinone, plastoquinone, and chlorophylls, sugar carriers for polysaccharide biosynthesis, or carotenoids in photosynthetic organisms. Although the physiological role of other isoprenoids is less evident, like that of the vast array of plant secondary metabolites, some are known to play key roles in the adaptative responses to different environmental challenges. In spite of the remarkable diversity of structure and function, all isoprenoids originate from a single metabolic precursor, isopentenyl diphosphate (1, 2).For many years, it was accepted that isopentenyl diphosphate was synthesized through the well known acetate͞mevalonate pathway. However, recent studies have demonstrated that the mevalonate-dependent pathway does not operate in all living organisms (3, 4). An alternative mevalonate-independent pathway for isopentenyl diphosphate biosynthesis was initially characterized in bacteria (4, 5) and later also in green algae (6) and higher plants (7-11). The first reaction of the novel mevalonateindependent pathway involves the condensation of (hydroxyethyl)thiamin derived from pyruvate with the C1 aldehyde group of D-glyceraldehyde 3-phosphate to yield D-1-deoxyxylulose 5-phosphate (5, 12). In Escherichia coli, D-1-deoxyxylulose (most likely in the form of D-1-deoxyxylulose 5-phosphate) is efficiently incorporated into the prenyl side chain of menaquinone and ubiquinone (12,13). In plants, the incorporation of D-1-deoxyxylulose into isoprenoids has also been reported (11,14). In addition, D-1-deoxyxylulose has also been described as a precursor for the biosynthesis of thiamin and pyridoxol. D-1-Deoxyxylulose is the...
Skeletal muscle is recognized as vital to physical movement, posture, and breathing. In a less known but critically important role, muscle influences energy and protein metabolism throughout the body. Muscle is a primary site for glucose uptake and storage, and it is also a reservoir of amino acids stored as protein. Amino acids are released when supplies are needed elsewhere in the body. These conditions occur with acute and chronic diseases, which decrease dietary intake while increasing metabolic needs. Such metabolic shifts lead to the muscle loss associated with sarcopenia and cachexia, resulting in a variety of adverse health and economic consequences. With loss of skeletal muscle, protein and energy availability is lowered throughout the body. Muscle loss is associated with delayed recovery from illness, slowed wound healing, reduced resting metabolic rate, physical disability, poorer quality of life, and higher health care costs. These adverse effects can be combatted with exercise and nutrition. Studies suggest dietary protein and leucine or its metabolite β-hydroxy β-methylbutyrate (HMB) can improve muscle function, in turn improving functional performance. Considerable evidence shows that use of high-protein oral nutritional supplements (ONS) can help maintain and rebuild muscle mass and strength. We review muscle structure, function, and role in energy and protein balance. We discuss how disease- and age-related malnutrition hamper muscle accretion, ultimately causing whole-body deterioration. Finally, we describe how specialized nutrition and exercise can restore muscle mass, strength, and function, and ultimately reverse the negative health and economic outcomes associated with muscle loss.
A beta-glucoside encoded by a cloned Zea mays complementary DNA (Zm-p60.1) cleaved the biologically inactive hormone conjugates cytokinin-O-glucosides and kinetin-N3-glucoside, releasing active cytokinin. Tobacco protoplasts that transiently expressed Zm-p60.1 could use the inactive cytokinin glucosides to initiate cell division. The ability of protoplasts to sustain growth in response to cytokinin glucosides persisted indefinitely after the likely disappearance of the expression vector. In the roots of maize seedlings, Zm-p60.1 was localized to the meristematic cells and may function in vivo to supply the developing maize embryo with active cytokinin.
Recent studies have suggested a role for neurotrophins in the growth and refinement of neural connections, in dendritic growth, and in activity-dependent adult plasticity. To unravel the role of endogenous neurotrophins in the development of neural connections in the CNS, we studied the ontogeny of hippocampal afferents in trkB (Ϫ/Ϫ) and trkC (Ϫ/Ϫ) mice. Injections of lipophilic tracers in the entorhinal cortex and hippocampus of newborn mutant mice showed that the ingrowth of entorhinal and commissural/associational afferents to the hippocampus was not affected by these mutations. Similarly, injections of biocytin in postnatal mutant mice (P10-P16) did not reveal major differences in the topographic patterns of hippocampal connections.In contrast, quantification of biocytin-filled axons showed that commissural and entorhinal afferents have a reduced number of axon collaterals (21-49%) and decreased densities of axonal varicosities (8-17%) in both trkB (Ϫ/Ϫ) and trkC (Ϫ/Ϫ) mice. In addition, electron microscopic analyses showed that trkB (Ϫ/Ϫ) and trkC (Ϫ/Ϫ) mice have lower densities of synaptic contacts and important structural alterations of presynaptic boutons, such as decreased density of synaptic vesicles. Finally, immunocytochemical studies revealed a reduced expression of the synaptic-associated proteins responsible for synaptic vesicle exocytosis and neurotransmitter release (v-SNAREs and t-SNAREs), especially in trkB (Ϫ/Ϫ) mice. We conclude that neither trkB nor trkC genes are essential for the ingrowth or layer-specific targeting of hippocampal connections, although the lack of these receptors results in reduced axonal arborization and synaptic density, which indicates a role for TrkB and TrkC receptors in the developmental regulation of synaptic inputs in the CNS in vivo. The data also suggest that the genes encoding for synaptic proteins may be targets of TrkB and TrkC signaling pathways.
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