Changes in the Levels of Chlorophyll and Light-Harvesting Chlorophyll a/b Protein of PS II in Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

Effects of irradiance on changes in the amounts of chlorophyll(Chl) and light-harvesting chlorophyll a/b protein of PS II(LHCII) were examined in senescing leaves of rice (Oryza sativaL.). Results of treatments at two irradiances (100% and 20%natural sunlight) were examined after the full expansion ofthe 13th leaf throughout the course of senescence. With 20%sunlight, the Chl content decreased only a little during leafsenescence, while with 100% sunlight it decreased appreciably.Similarly, the amount of LHCII protein during treatment with20% sunlight remained almost constant. However, the ratio ofChl a/b during the shade treatment decreased significantly andthe rate of decrease was greater than during the full-sunlighttreatment. The ratio of Chl a/b for Chl a and b bound to LHCIIwas about 1.2, irrespective of leaf age or irradiance treatment.When the amounts of Chl bound to LHCII were calculated fromthe total leaf content of Chl and the ratio of Chl a/b, assuminga ratio of Chl a/b bound to LHCII of 1.2, they were well correlatedwith the amounts of LHCII protein. Changes in the amounts of LHCII synthesized during the two irradiancetreatments were examined using an ¹⁵ tracer. Incorporation of¹⁵N into LHCII declined dramatically during both treatmentsfrom full expansion through senescence, suggesting that therewas little synthesis of LHCII protein during that time. In addition,the amount of LHCII synthesized during senescence was lowerduring the shade treatment than during the 100% sunlight treatment.These results indicate that the absence of an apparent changein levels of LHCII with shade treatment during senescence wascaused by the very low rate of turnover of LHCII protein. (Received June 17, 1992; Accepted September 28, 1992)     

Versatile roles of plastids in plant growth and development

Plastids, found in plants and some parasites, are of endosymbiotic origin. The best-characterized plastid is the plant cell chloroplast. Plastids provide essential metabolic and signaling functions, such as the photosynthetic process in chloroplasts. However, the role of plastids is not limited to production of metabolites. Plastids affect numerous aspects of plant growth and development through biogenesis, varying functional states and metabolic activities. Examples include, but are not limited to, embryogenesis, leaf development, gravitropism, temperature response and plant-microbe interactions. In this review, we summarize the versatile roles of plastids in plant growth and development.     

Solution structure of the RWD domain of the mouse GCN2 protein

GCN2 is the alpha-subunit of the only translation initiation factor (eIF2alpha) kinase that appears in all eukaryotes. Its function requires an interaction with GCN1 via the domain at its N-terminus, which is termed the RWD domain after three major RWD-containing proteins: RING finger-containing proteins, WD-repeat-containing proteins, and yeast DEAD (DEXD)-like helicases. In this study, we determined the solution structure of the mouse GCN2 RWD domain using NMR spectroscopy. The structure forms an alpha + beta sandwich fold consisting of two layers: a four-stranded antiparallel beta-sheet, and three side-by-side alpha-helices, with an alphabetabetabetabetaalphaalpha topology. A characteristic YPXXXP motif, which always occurs in RWD domains, forms a stable loop including three consecutive beta-turns that overlap with each other by two residues (triple beta-turn). As putative binding sites with GCN1, a structure-based alignment allowed the identification of several surface residues in alpha-helix 3 that are characteristic of the GCN2 RWD domains. Despite the apparent absence of sequence similarity, the RWD structure significantly resembles that of ubiquitin-conjugating enzymes (E2s), with most of the structural differences in the region connecting beta-strand 4 and alpha-helix 3. The structural architecture, including the triple beta-turn, is fundamentally common among various RWD domains and E2s, but most of the surface residues on the structure vary. Thus, it appears that the RWD domain is a novel structural domain for protein-binding that plays specific roles in individual RWD-containing proteins.     

20S proteasome prevents aggregation of heat-denatured proteins without PA700 regulatory subcomplex like a molecular chaperone

The eukaryotic 20S proteasome is the multifunctional catalytic core of the 26S proteasome, which plays a central role in intracellular protein degradation. Association of the 20S core with a regulatory subcomplex, termed PA700 (also known as the 19S cap), forms the 26S proteasome, which degrades ubiquitinated and nonubiquitinated proteins through an ATP-dependent process. Although proteolytic assistance by this regulatory particle is a general feature of proteasome-dependent turnover, the 20S proteasome itself can degrade some proteins directly, bypassing ubiquitination and PA700, as an alternative mechanism in vitro. The mechanism underlying this pathway is based on the ability of the 20S proteasome to recognize partially unfolded proteins. Here we show that the 20S proteasome recognizes the heat-denatured forms of model proteins such as citrate synthase, malate dehydrogenase. and glyceraldehydes-3-phosphate dehydrogenase, and prevents their aggregation in vitro. This process was not followed by the refolding of these denatured substrates into their native states, whereas PA700 or the 26S proteasome generally promotes their reactivation. These results indicate that the 20S proteasome might play a role in maintaining denatured and misfolded substrates in a soluble state, thereby facilitating their refolding or degradation.     

Characterization of two genes encoding putative cysteine synthase required for cysteine biosynthesis in Schizosaccharomyces pombe

Cysteine synthase catalyzes the formation of cysteine from O-acetylserine, and is the key enzyme for de novo cysteine biosynthesis in Schizosaccharomyces pombe. An examination of the S. pombe database revealed that two gene products are predicted to encode proteins homologous to eukaryotic cysteine synthases. Disruption of one of these candidates, cys1a+ (SPBC36.04), caused an obvious cysteine auxotrophy, while disruption of cys1b+ (SPAC3A12.17c) had no effect on the growth phenotype. Furthermore, overexpression of cys1b+ did not complement the cysteine auxotrophic phenotype of cys1a mutant cells. These results indicated that cys1a+, not cys1b+, primarily functions in the biosynthesis of cysteine in S. pombe cells. We constructed a bacterial-S. pombe shuttle vector containing cys1a+ as a selective marker gene. The combination of the cysteine auxotroph and new vector could be useful for the expression of a heterologous protein.     

Dystrophin in rod spherules; submembranous dense regions facing bipolar cell processes

 It is known that the retina contains the protein dystrophin in the ribbon synapse, but the ultrastructural analysis is not yet fully elucidated. Our previous study reported that dystrophin is localized under the rod cell membranes in rat retinas. In the present study, we have investigated the relationship between dystrophin-rich regions of rod cell membranes and other neuronal processes in mouse retinas with a monoclonal antibody raised against the human dystrophin C-terminus. Immunoblotting, immunofluorescence stainings, and immunoelectron microscopy were employed. Immunoblotting analysis indicated that mouse retinas possessed some of the dystrophin isoforms of approximately 260 kDa, 140 kDa, and 70 kDa molecular weight. Confocal images showed a punctate appearance in the outer plexiform layer, as previously described. Immunoelectron microscopy showed that dystrophin immunoreactive products were always observed at submembranous dense regions of the rod spherule abutting bipolar processes. These results suggest that retinal dystrophin may be closely involved in signal transmission from rods to bipolar cells. Accepted: 7 May 1997     

The Electrophoretical Analysis of Esterase and Catalase and Its Use in Taxonomical Studies of Mycobacteria

The zymogram patterns of esterases and catalases of mycobacterial strains were studied using the thin layer agar electrophoresis. Though there were some variations, Mycobacterium hominis, M. bovis, M. kansasii, M. fortuitum, M. runyonii, M. avium, M. phlei and M. smegmatis seemed to show species-specific patterns consisting of 2 to 6 esterase bands and one or more catalase bands. The patterns of scotochromogens and nonchromogens were rather variable.     

Comparison of neuronal responses in primate inferior-temporal cortex and feed-forward deep neural network model with regard to information processing of faces

Journal of Computational Neuroscience - Feed-forward deep neural networks have better performance in object categorization tasks than other models of computer vision. To understand the relationship...