Signal transduction and phosphatidylinositol turnover in plants

Transduction of light and hormone signals in plants may utilize a mechanism similar to one widely used in animal systems, that of accelerated phosphatidylinositol (PI) turnover. The elements of this central mechanism and consequences of the acceleration as well as recent evidence for its operation in plants are described.     

Distribution of folates and modified folates in extremely thermophilic bacteria.

Analyses were made of the structures and levels of folates and modified folates present in extremely thermophilic bacteria. These procedures involved the chemical analysis of products resulting from the oxidative cleavage of the 6-substituted, folatelike tetrahydropterins present in the cells. Air-oxidized cell extracts of extreme thermophiles from two members of the archaebacterial order Thermococcales, Thermococcus celer and Pyrococcus furiosus, contained only 7-methylpterin, indicating that these cells contain a modified folate with a methylated pterin. Cell extracts also contained 6-acetyl-7-methyl-7,8-dihydropterin, another product derived from the oxidative cleavage of a dimethylated folate, demonstrating that both the C-7 and C-9 carbons of the pterin were methylated. Extracts, however, contained neither p-aminobenzoylpolyglutamates nor methaniline, the oxidative cleavage products of folates and methanopterin, respectively, indicating that they contain a previously undescribed C1 carrier(s). On the basis of the level of the 7-methylpterin isolated, the levels of modified folate were 2 to 10 times higher than those typically found in mesophilic bacteria and 10 to 100 times less than the level of methanopterin found in the methanogenic bacteria. Oxidized cell extracts of Sulfolobus spp. of the archaebacterial order Sulfolobales contained only pterin, and, like members of the order Thermococcales, they contained neither-p-aminobenzoylpolyglutamates nor methaniline. Oxidized cell extracts of the extreme thermophiles Pyrobaculum sp. strain H10 and Pyrodictium occultum, from the archaebacterial orders Thermoproteales and Pyrodictiales, respectively, and Thermotoga maritima from the eubacterial order Thermotogales, contained pterin and p-aminobenzoylpolyglutamates, indicating that these cells contained unmodified folates. The levels of p-aminobenzoylpolyglutamates in these archaebacterial cell extracts indicate that the folates were present in the cells at levels 4 to 10 times higher than generally found in those mesophilic eubacteria which do not folates in energy metabolism. The levels and chain lengths of the of p-aminobenzoylpolyglutamates present in Thermotoga maritima were typical of those found in mesophilic eubacteria.     

Stem turnover strategy of multiple-stemmed woody plants

We have investigated stem turnover strategy for Lindera umbellata, an understory shrub that sprouts from its rootstock under natural conditions to replace constituent stems, on the basis of the hypothesis that the multiple-stemmed form of woody species is an adaptation enabling efficient reproduction in high-stress environments. We tested the hypothesis that the timing of stem replacement maximizes sexual reproduction for the shrub. We developed a model for the time of optimum replacement of a stem by a daughter stem which maximizes the sexual reproduction of a shrub and tested the model using L. umbellata growing in the field. From the model, the optimum time of replacement of a stem with a daughter stem is when cumulative sexual reproduction per unit time for the stem is maximum. In practice, this will be the last age (t _opt) at which annual sexual reproduction in a stem can potentially exceed cumulative sexual reproduction per unit time for the stem. Half of the stems died at almost t _opt and had sexually mature daughter stems at that time. Other stems, however, died at times more remote from t _opt when daughter stems were sexually immature. It is thought that normal replacement of the latter stems was prevented by accidents such as breakage. We conclude that clumps of L. umbellata achieve efficient sexual reproduction by stem replacement at the optimum time, although accidents can, to some extent, determine when the stem actually dies.     

Synthesis and turnover of non-polar lipids in yeast

In the yeast Saccharomyces cerevisiae as in other eukaryotic cells non-polar lipids form a reservoir of energy and building blocks for membrane lipid synthesis. The yeast non-polar lipids, triacylglycerol (TAG) and steryl ester (STE), are synthesized by enzymes with overlapping function. Recently, genes encoding these enzymes were identified and gene products were partially characterized. Once formed, TAG and STE are stored in so-called lipid particles/droplets. This compartment which is reminiscent of mammalian lipoproteins from the structural viewpoint is, however, not only a lipid depot but also an organelle actively contributing to lipid metabolism. Non-polar lipid degrading enzymes, TAG lipases and STE hydrolases, also occur in redundancy in the yeast. These proteins, which are components of the lipid particle surface membrane with the exception of one plasma membrane localized STE hydrolase, mobilize non-polar lipids upon requirement. In this review, we describe the coordinate pathways of non-polar lipid synthesis, storage and mobilization in yeast with special emphasis on the role of the different enzymes and organelles involved in these processes. Moreover, we will discuss non-polar lipid homeostasis and its newly discovered links to various cell biological processes in the yeast.     

Management defines species turnover of bees and flowering plants in vineyards

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Synthesis and turnover of collagen precursors in rabbit skin

1. The rate of synthesis of [(14)C]hydroxyproline by rabbit skin was studied in vitro and in vivo. 2. The soluble collagen fractions were shown to have a very rapid turnover. The 0.15m-sodium chloride-extractable collagen showed t((1/2)) values of 1.2hr. in vitro and 12hr. in vivo. The 0.5m-sodium chloride-extractable collagen exhibited a t((1/2)) value of 20hr. in vivo. 3. Under the conditions used it was not possible to obtain radioactive insoluble collagen in vitro. 4. A significant amount of soluble collagen is lost before it becomes insoluble. 5. These observations may help to explain why large amounts of peptide-bound hydroxyproline appear in the urine during periods of rapid collagen synthesis.     

Perennation in woody fruit plants and its relationship to carbohydrate turnover

Carbohydrates constitute the principal reserves in temperate woody fruit plants and are depleted during winter, particularly around the time of bud-break before new growth makes good the losses. Movement of reserves between underground and aerial parts occurs during winter. All perennial parts show alternate depletion and replenishment but root extremities may undergo irreversible depletion and die. Interconversions between soluble carbohydrates and polysaccharides indicate a balance between degradation and synthesis. High growth potential is associated with high starch/starch + sugar ratio.     

Synthesis and turnover of nitrate reductase in corn roots

The induction and reinduction of nitrate reductase in root tip or mature root sections show essentially a similar pattern: a lag, a period of rapid increase in enzyme activity and finally a period of relatively minor change. Both inductions are sensitive to 6-methylpurine and cycloheximide. Kinetic studies with 6-methylpurine suggest that the half-life of the messenger RNA for nitrate reductase in both sections is about 20 minutes. The rate of decay of nitrate reductase activity induced by transfer to a nitrate-free medium is slower in root tips (t½ = 3 hours) than in mature root sections (t½ = 2 hours). The enzyme from mature root sections is also less stable to mild heat treatments (27 C; 40 C) than the enzyme from root tip sections. The results indicate that factors regulating enzyme turnover show important changes as root cells mature and may be significant in determining steady state levels of the enzyme.     

Purine biosynthesis in the domain Archaea without folates or modified folates.

The established pathway for the last two steps in purine biosynthesis, the conversion of 5-aminoimidazole-4-carboxamide ribonucleotide (ZMP) to IMP, is known to utilize 10-formyl-tetrahydrofolate as the required C1 donor cofactor. The biosynthetic conversion of ZMP to IMP in three members of the domain Archaea, Methanobacterium thermoautotrophicum deltaH, M. thermoautotrophicum Marburg, and Sulfolobus solfataricus, however, has been demonstrated to occur with only formate and ATP serving as cofactors. Thus, in these archaea, which use methanopterin (MPT) or another modified folate in place of folate as the C1 carrier coenzyme, neither folate nor a modified folate serves as a cofactor for this biosynthetic transformation. It is concluded that archaea, which function with modified folates such as MPT, are able to carry out purine biosynthesis without the involvement of folates or modified folates.     

Species immigration, extinction and turnover of vascular plants in boreal lakes

Dictated by limited resource availability for land acquisition, a central question in conservation biology is the ability of areas of different size to maintain species diversity. The selected reserves should not only be species rich at the moment, but should also maintain species diversity in the long run. We used two sets of data on vascular plant species in boreal lakes collected in 1933/34 and 1996 to test the relationships between lake area and the extinction, immigration and turnover rates of the species. Moreover, we investigated, whether the number of species in 1933/34 or water connection between lakes was related to extinction, immigration and turnover rates of species. We found that lake area or shoreline length was not correlated with immigration or turnover rate. But extinction rate was slightly negatively correlated with shoreline length. The original number of species was positively related to the number of species extinctions and to the absolute turnover rate in the lakes, which indicates that species richness does not create stability in these communities. Species number was not correlated with immigration rate. Upstream water connections in the lakes did not affect immigration, extinction or turnover rates. We conclude that length of the shoreline is a better measure of suitable area for water plants than the lake area, and that because the correlation between shoreline length and extinction rate was slight, also small lakes can be valuable for conservation.     

Synthesis and turnover of Euglena gracilis mitochondrial DNA

Replication of mitochondrial DNA was investigated by a density transfer experiment in a strain of Euglena gracilis lacking chloroplast DNA. DNA was uniformly labeled in a medium containing ³²P-labeled inorganic phosphate and [³H]adenine in the presence of the heavy-density label and transferred to a medium containing ³²P-labeled inorganic phosphate but no [³H]adenine following removal of the heavy-density label. Replication of nuclear DNA within these cells was used as an internal control. The densities and ratios of the peaks of nuclear DNA were those expected for a strict semiconservative replication. In contrast, replication of mitochondrial DNA was dispersive, as illustrated by the following results: (1) both native and denatured mitochondrial DNA exhibited a single density peak at 1.1 and 2.2 cell doublings after the density transfer. (2) The specific activity of ³H-labeled DNA varied across the peak of native or denatured DNA, indicating a heterogeneous population of molecules exhibiting different degrees of density and radioisotope labeling. This dispersive replication could involve either multiple recombination events or extensive turnover of the DNA or a mixture of both. Extensive dispersion of the sample obtained at 1.1 cell doublings after the density transfer is shown by the persistence of the same peak density for duplex DNA reduced to a molecular weight of 6 × 10⁵ by shearing.Two measures of the rate of replication of mitochondrial DNA were obtained from the densities of native duplex DNA and the rate of decrease in ³H-specific activities of duplex DNA during the experiment. The average of these rates indicates that mitochondrial DNA replicates at least 1.5 times as fast as nuclear DNA. Since there is a constant ratio of mitochondrial DNA:nuclear DNA in a logarithmic culture, mitochondrial DNA was calculated to have a half-life of 1.8 cell doublings.     

Synthesis and turnover of polysomal mRNAs in sea urchin embryos

The synthesis and turnover kinetics of polysomal mRNA have been measured in sea urchin embryos. Polysomes were isolated from stages ranging between mesenchyme blastula and late gastrula Strongylocentrotus purpuratus embryos which had been exposed to exogenous ³H-guanosine. The amount of radioactivity incorporated into messenger and ribosomal RNAs was determined separately as a function of time, and the precursor pool specific activity was measured in the same embryos. Synthesis and decay rate constants were extracted from the data by a leastsquares procedure. Per embryo, the rate of mRNA synthesis was calculated to be about 0.13 pg min^?1, while the rate of rRNA synthesis is about 0.022 pg min^?1. The newly synthesized mRNA turns over with a half-time of 5.7 hr. The data support only a single decay rate for the mRNA, but small fractions of mRNA decaying at different rates cannot be excluded. Previous studies have shown that a minor fraction of the mRNA includes the least abundant, most highly diverse set of messages (“complex class” mRNAs). To determine whether mRNAs of the complex class are synthesized and degraded at similar rates, labeled mRNA was measured in hybrids formed in mRNA excess reactions with single copy DNA. These experiments showed that complex class mRNAs represent an approximately proportional amount of the new mRNA synthesis, and turn over at the same average rate as does the bulk of the mRNA. Most of the mRNAs in the embryo polysomes are newly synthesized, rather than maternal. This statement refers both to complex class mRNAs and to prevalent mRNAs. Considering the sequence homology between embryo and oocyte mRNAs shown earlier, these results indicate that many of the same structural genes active during oogenesis are being transcribed in embryos at these stages.     

Interconnections between mRNA degradation and RDR-dependent siRNA production in mRNA turnover in plants

Journal of Plant Research - Accumulation of an mRNA species is determined by the balance between the synthesis and the degradation of the mRNA. Individual mRNA molecules are selectively and...     

Retained folates in the rat.

The retention of radioactivity after doses of 14C- and 3H-labelled folic acid is described. Radioactivity was retained in liver, kidney and gut of rats for some time after administration of the dose. The retained radioactivity could not be displaced by large doses of unlabelled folic acid or unlabelled 5-methyltetrahydrofolate. 14C- and 3H-labbelled folates showed similar chromatographic behaviour onion-exchange chromatography to 5-methyltetrahydrofolate, and on ion-exchange and gel-permeation chromatography to synthetic pteroylhepta-gamma-glutamate.     

Absence of rapid terpene turnover in several diverse species of terpene-accumulating plants

Terpenes are commonly believed to undergo rapid metabolic turnover in plants, but the evidence for this process comes largely from studies that used detached organs or applied radiolabeled precursors in unnatural ways. When ¹⁴CO₂ pulse labeling experiments were carried out with intact plants of four taxonomically distant, terpene-accumulating species, no significant turnover of monoterpenes, sesquiterpenes or diterpenes was detected in young foliage over a two week period after exposure to ¹⁴CO₂. These results are consistent with those of other investigations performed under physiologically realistic conditions, and caution against the uncritical incorporation of turnover into models or theories concerning plant chemical defense.