Floral expression of a gene encoding an E2-relatedubiquitin-conjugating protein from Arabidopsis thaliana

An Arabidopsis thaliana gene (UBC6) encoding a homologue to ubiquitin-conjugating enzymes has been isolated which is capable of encoding a protein of 183 amino acids of ca. 21 kDa. Northern analysis indicates that the gene is expressed in flowers, seeds and, to a somewhat lesser extent, in 10-day seedlings but not in mature leaves, callus and pre-flowering plants. This pattern of expression is confirmed using transgenic Arabidopsis plants containing a UBC6 promoter-GUS gene fusion construct. These plants displey GUS activity in mature anthers prior to dehiscence, in developing embryos, sepals and the style after pollination.     

Genotypic differences in cold tolerance are masked by high sucrose and cytokinin in shoot cultures of sugarbeet

Cold tolerance of field grown plants and shoot cultures of a commercial sugarbeet cultivar, Hilma, was compared with that of two cultivars bred for improved cold tolerance, Monofeb and Winter Hybrid 88619. Leaves of Monofeb and Winter Hybrid 88619 showed an increase in frost tolerance compared to Hilma, as assessed by electrolyte leakage measurements, in both July, and November. However, all varieties exhibited acclimation in the latter month. Similar qualitative differences between cultivars were detected in shoot cultures only when maintained on low (1%) sucrose medium, without added plant growth regulators. The use of high (3%) sucrose and benzyladenine, which releases apical dominance producing multiple shoots, each contributed to a substantial lowering of the temperature at which cold-induced damage occurred in leaves. Under these conditions varietal differences were masked. The implications of these findings in regard to in vitro selection for improved cold tolerance in organized cultures are discussed.Abbreviations BA benzyladenine - MS Murashige & Skoog (1962)     

Fine root growth phenology,production, and turnover in a northern hardwood forest ecosystem

A large part of the nutrient flux in deciduous forests is through fine root turnover, yet this process is seldom measured. As part of a nutrient cycling study, fine root dynamics were studied for two years at Huntington Forest in the Adirondack Mountain region of New York, USA. Root growth phenology was characterized using field rhizotrons, three methods were used to estimate fine root production, two methods were used to estimate fine root mortality, and decomposition was estimated using the buried bag technique. During both 1986 and 1987, fine root elongation began in early April, peaked during July and August, and nearly ceased by mid-October. Mean fine root ( 3 mm diameter) biomass in the surface 28-cm was 2.5 t ha^–1 and necromass was 2.9 t ha^–1. Annual decomposition rates ranged from 17 to 30% beneath the litter and 27 to 52% at a depth of 10 cm. Depending on the method used for estimation, fine root production ranged from 2.0 to 2.9 t ha^–1, mortality ranged from 1.8 to 3.7 t ha^–1 yr^–1, and decomposition was 0.9 t ha^–1 yr^–1. Thus, turnover ranged from 0.8 to 1.2 yr^–1. The nutrients that cycled through fine roots annually were 4.5–6.1 kg Ca, 1.1–1.4 kg Mg, 0.3–0.4 kg K, 1.2–1.7 kg P, 20.3–27.3 kg N, and 1.8–2.4 kg S ha^–1. Fine root turnover was less important than leaf litterfall in the cycling of Ca and Mg and was similar to leaf litterfall in the amount of N, P, K and S cycled.     

Regeneration dynamics of alluvial gaps in a warm temperate rain forest in New Zealand

Gaps formed by stream laid gravels in a warm temperate Podocarp-Angiosperm rain forest in New Zealand were quickly invaded by 41 free-standing woody species. Height-age curves for the eight most frequent species were used to construct a 12 yr chronosequence which showed 18 species arrived by 0.9 yr and 33 by 1.9 yr. This rapid invasion was supplied from both gaps and mature forest and fitted the Initial Floristic Model coupled with a Facilitation Mechanism. Over the 12 yr period, stem density and births peaked at 0.9 yr and deaths at 1.9 yr with a natality of 39.5/m², a mortality of 38.5/m² and nearly 99% dying as seedlings. Plant height increased to 5.1 m at 12.1 yr with height increment peaking at 43 cm/yr at 7.3 yr. Maximum potential heights ranged from 4 to 60 m and were positively correlated with longevity which varied from 20 to 1150 yr for 19 known species. A decline in height increment to 0.9 yr showed a below-ground component in intraspecific competition with significant negative correlations between stem density and height indicating a later above-ground component. There was no significant height competition amongst the 11 most frequent species and only one significant negative interspecific association. This suggests high species coexistence for which Equal chance was probably the major mechanism. Given this coexistence, rapid species arrival, early sexual maturity and similar density and height-age patterns, the main dynamic variables may be maximum height, longevity and height increment, all of which are, partly, genetically programmed. Most dynamic affinities are with tropical forest, although height growth and species richness are warm temperate.     

The Rhodobacter capsulatus chlorin reductase-encoding locus, bchA, consists of three genes, bchX, bchY, and bchZ.

The bchA locus of Rhodobacter capsulatus codes for the chlorin reductase enzyme in the bacteriochlorophyll synthesis pathway. Previous work has suggested that this locus might encompass a single gene. We have sequenced the bchA locus and found it to contain three coding segments, which we designate bchX, bchY, and bchZ. Each coding segment contains its own translational initiation sequence and follows codon utilization patterns consistent with those of previously published R. capsulatus genes. When various regions of the bchA locus and flanking sequences were subcloned into an expression vector and expressed in Escherichia coli, the three coding segments were all expressed as separate peptides. Finally, conservation of amino acid sequences between bchX and a subunit of the protochlorophyllide reductase (bchL, 34% identity) and the nitrogenase Fe protein (nifH, 30 to 37% identity) suggests structural and mechanistic commonalities among all three proteins.     

Cell Cycle Arrest of Proliferating Neuronal Cells by Serum Deprivation Can Result in Either Apoptosis or Differentiation

Abstract: Apoptotic cell death plays a critical role in the development of the nervous system. The death of mature nondividing neurons that fail to receive appropriate input from the target field has been extensively studied. However, the mechanisms mediating the extensive cell death occurring in areas of the developing brain where proliferating neuroblasts differentiate into mature nondividing neurons have not been analyzed. We show here that the cell cycle arrest of a proliferating cell of neuronal origin by removal of serum results in either apoptotic cell death or differentiation to a mature nondividing neuronal cell. The proportion of cells undergoing death or differentiation is influenced in opposite directions by treatment of the cells with cyclic AMP and retinoic acid. This suggests that following the withdrawal of signals stimulating neuroblast cell division, neuronal cells either can cease to suppress a constitutive suicide pathway and hence die by apoptosis or, alternatively, can differentiate into a mature neuronal cell. Regulation of the balance between apoptosis and neuronal differentiation could therefore play a critical role in controlling the numbers of mature neurons that form.     

Inhibition of tobacco NADH-hydroxypyruvate reductase by expression of a heterologous antisense RNA derived from a cucumber cDNA: Implications for the mechanism of action of antisense RNAs

Tobacco plants were genetically transformed to generate antisense RNA from a gene construct comprised of a full-length cucumber NADH-dependent hydroxypyruvate reductase (HPR) cDNA placed in reverse orientation between the cauliflower mosaic virus 35S promoter and a nopaline synthase termination/polyadenylation signal sequence. In vivo accumulation of antisense HPR RNA within eight independent transgenic tobacco plants resulted in reductions of up to 50% in both native HPR activity and protein accumulation relative to untransformed tobacco plants (mean transgenote HPR activity=67% wild type, mean transgenote HPR protein=63% wild type). However, in contrast to previous reports describing antisense RNA effects in plants, production of the heterologous HPR antisense RNA did not systematically reduce levels of native tobacco HPR mRNA (mean transgenote HPR mRNA level=135% wild type). Simple regression comparison of the steady-state levels of tobacco HPR mRNA to those of HPR antisense RNA showed a weak positive correlation (r value of 0.548, n=9 ; n is wild type control plus eight independent transformants; significant at 85% confidence level), supporting the conclusion that native mRNA levels were not reduced within antisense plants. Although all transgenic antisense plants examined displayed an apparent reduction in both tobacco HPR protein and enzyme activity, there is no clear correlation between HPR activity and the amount of either sense (r=0.267, n=9) or antisense RNA (r=0.175, n=9). This compares to a weak positive correlation between HPR mRNA levels and the amount of HPR activity observed in wild-type SRI tobacco plants (r=0.603, n=5). The results suggest that in vivo production of this heterologous HPR antisense RNA is inhibitory at the level of HPR-specific translation and produces its effect in a manner not dependent upon, nor resulting in, a reduction in steady-state native HPR mRNA levels. In this context, the observed antisense effect appears to differ mechanistically from most antisense systems described to date.     

Solid-state nuclear magnetic resonance investigation of solvent dependence of tyrosyl ring motion in an enzyme

Tyrosyl ring motions in alpha-lytic protease were investigated by solid-state deuterium nuclear magnetic resonance (NMR) spectroscopy in lyophilized enzyme powder, in powder suspended in organic solvents, and in aqueous crystals. Ring flipping rates were determined by examining deuterium quadrupole echo line shapes. Of the four Tyr residues in the enzyme, one was flipping at the slow (< or =10(3) s(-1)) and one at the fast (> or =10(7) s(-1)) exchange limit of the line shape experiment in all the environments tested. Flipping rates of the remaining two Tyr residues depended markedly on the solvent, with the lowest flipping rates (< or =10(3) s(-1) for both residues) observed in the enzyme powder, whether dry or suspended in hydrophobic tert-butyl methyl ether. In hydrophilic dioxane and acetonitrile, the mobility of these residues increased to 10(4) and 10(5) s(-1). The latter rate rose further to 10(6) s(-1) in the hydrated hydrophilic solvents and to > or =10(7) s(-1) in aqueous crystals. The deuterium spectrum of native alpha-lytic protease was compared with that of the enzyme whose active center was covalently modified with an inhibitor, which binds next to Tyr-123, constraining its ring. This experiment revealed that water addition to acetonitrile specifically increased the flipping rate of this active center residue. Librational motions ("wobbling"), estimated by their effect on spin-lattice relaxation times, were slowest in the anhydrous solvents, intermediate in the hydrated solvents, and fastest in the aqueous crystals. Thus, alpha-lytic protease is more rigid in organic solvents than in water, as judged by mobility of its tyrosyl residues. Water stripping by hydrophilic solvents did not increase enzyme rigidity, nor were there clear correlations between mobility and either enzymatic activity or solvent dielectric constant.