Chloroplast membrane biogenesis in chlamydomonas: Correlation between the formation of membrane components and membrane structure

The y-1 mutant of Chlamydomonas reinhardi, when allowed to green in the presence of chloramphenicol (CAP), an inhibitor of protein synthesis on 70s ribosomes, form photosynthetic membranes which contain somewhat less chlorophyll than those of cells greened in the absence of the drug. Photosystem I and II activities are drastically reduced in the CAP-greened cells, and specific alterations in the polypeptide composition of the thylakoid membranes are also observed. We have examined the internal structure of the thylakoid membranes from cells greened in the presence and absence of CAP, and have found that the large particles observed on the exoplasmic fracture face (EF) are substantially reduced in size and number in the CAP-greened cells. This structural defect seems related to the absence of significant photosystem activities in the CAP-greened cells, despite the presence, of most major membrane polypeptides. We suggest that CAP treatment results in a failure of the cell to organize functional reaction complexes, and is structurally reflected in the absence of large (EF) particles in such membranes. This defect can be repaired by allowing the affected cells to re-green in the absence of the drug, and the large particles reappear, paralleling an increase in photosynthetic activity.     

Spliceosomal UsnRNP biogenesis, structure and function

Significant advances have been made in elucidating the biogenesis pathway and three-dimensional structure of the UsnRNPs, the building blocks of the spliceosome. U2 and U4/U6*U5 tri-snRNPs functionally associate with the pre-mRNA at an earlier stage of spliceosome assembly than previously thought, and additional evidence supporting UsnRNA-mediated catalysis of pre-mRNA splicing has been presented.     

Plasmodesmata: structure, function and biogenesis

Plasmodesmata remain one of the outstanding mysteries in plant biology. In providing conduits for the exchange of small and large, informational molecules they are central to the growth, development and defence of all higher plants. In the past few years, strategies have been devised for the molecular dissection of plasmodesmal composition and function, and we are beginning to see how these enigmatic structures will become to be understood.     

Chloroplast fructose-1,6-bisphosphatase: structure and function

Redox regulation of photosynthetic enzymes has been a preferred research topic in recent years. In this area chloroplast fructose-1,6-bisphosphatase is probably the most extensively studied target enzyme of the CO₂ assimilation pathway. This review analyzes the structure, biosynthesis, phylogeny, action mechanism, regulation and kinetics of fructose-1,6-bisphosphatase in the light of recent findings on structure–function relationship, and from a molecular biology viewpoint. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chloroplast transit peptides: structure, function and evolution

It is thought that two to three thousand different proteins are targeted to the chloroplast, and the 'transit peptides' that act as chloroplast targeting sequences are probably the largest class of targeting sequences in plants. At a primary structural level, transit peptide sequences are highly divergent in length, composition and organization. An emerging concept suggests that transit peptides contain multiple domains that provide either distinct or overlapping functions. These functions include direct interaction with envelope lipids, chloroplast receptors and the stromal processing peptidase. The genomic organization of transit peptides suggests that these domains might have originated from distinct exons, which were shuffled and streamlined throughout evolution to yield a modern, multifunctional transit peptide. Although still poorly characterized, this evolutionary process could yield transit peptides with different domain organizations. The plasticity of transit peptide design is consistent with the diverse biological functions of chloroplast proteins.     

Chloroplast biogenesis. Molecular structure of chlorophyll b (E489 F666)

Nuclear magnetic resonance analysis established the presence of two vinyl groups/macrocycle of chlorophyll b (E489 F666). The latter is the only chlorophyll b that accumulates in a corn mutant (ON 8147). On the other hand the presence of only one vinyl group/macrocycle of authentic monovinyl chlorophyll b (E475 F660) was confirmed.     

Correlation between phylogenetic structure and function: examples from deep-sea Shewanella

The genus Shewanella is one of the typical deep-sea bacterial genera. Two isolated deep-sea Shewanella species, Shewanella benthica and Shewanella violacea, were found to be able to grow better under high hydrostatic pressure conditions than at atmospheric pressure. These species are not only piezophilic (barophilic), but also psychrophilic. Many psychrophilic and psychrotolerant Shewanella species have been isolated and characterized from cold environments, such as seawater in Antarctica or the North Sea. Some of these cold-adapted Shewanella were shown to be piezotolerant, meaning that growth occurs in a high-pressure habitat. In this review, we propose that two major sub-genus branches of the genus Shewanella should be recognized taxonomically, one group characterized as high-pressure cold-adapted species that produce substantial amounts of eicosapentaenoic acid, and the other group characterized as mesophilic pressure-sensitive species.     

Deletion of mtDNA disrupts mitochondrial function and structure, but not biogenesis

The role of nuclear DNA (nDNA)-encoded proteins in the regulation of mitochondrial fission and fusion has been documented, yet the role of mitochondrial DNA (mtDNA) and encoded proteins in mitochondrial biogenesis remains unknown. Long-term treatment of a lymphoblastoid cell line Molt-4 with ethidium bromide generated mtDNA-deficient rho0 mutants. Depletion of mtDNA in rho0 cells produced functional and morphological changes in mitochondria without affecting the nuclear genome and encoded proteins. Indeed, the gene encoding subunit II of mitochondrial cytochrome c oxidase (COX II), a prototypical mitochondrial gene, was reduced in rho0 mutants blunting the activity of mitochondrial cytochrome coxidase. Yet, the amount of the nuclear beta-actin gene and the activity of citrate synthase, a mitochondrial matrix enzyme encoded by nDNA, remained unaffected in rho0 cells. Loss of mtDNA in rho0 cells was associated with significant distortion of mitochondrial structure, decreased electron density of the matrix and disorganized inner and outer membranes, resulting in the appearance of 'ghost-like' mitochondria. However, the number of mitochondria-like structures was not significantly different between mtDNA-deficient and parental cells. Thus, we conclude that cells lacking mtDNA still generate mitochondrial scaffolds, albeit with aberrant function.     

Chloroplast biogenesis. Cell-free transfer of envelope monogalactosylglycerides to thylakoids.

An ATP- and temperature-dependent transfer of monogalactosylglycerides from the chloroplast envelope to the chloroplast thylakoids was reconstituted in a cell-free system prepared from isolated chloroplasts of garden pea (Pisum sativum) or spinach (Spinacia oleracea). Isolated envelope membranes, in which the label was present exclusively in monogalactosylglycerides, were prepared radiolabeled in vitro with [14C]galactose from UDP-[14C]galactose to label galactolipids as the donor. ATP-dependent transfer of radioactivity from donor to unlabeled acceptor thylakoids, immobilized on nitrocellulose strips, was observed. In some experiments linear transfer for longer than 30 min of incubation was facilitated by the addition of stroma proteins but in other experiments stroma was without effect or inhibitory suggesting no absolute requirements for a soluble protein carrier. Transfer was donor specific. No membrane fraction tested (plasma membrane, tonoplast, endoplasmic reticulum, nuclei, Golgi apparatus, mitochondria or thylakoids) (isolated from tissue radiolabeled in vivo with [14C]acetate) other than chloroplast envelopes demonstrated any significant ability to transfer labeled membrane lipids to immobilized thylakoids. Acceptor specificity, while not absolute, showed a 3-10-fold greater ATP-dependent transfer of labeled galactolipids from chloroplast envelopes to immobilized thylakoids than to other leaf membranes. The results provide independent confirmation of the potential for transfer of galactolipids between chloroplast envelopes and thylakoids suggested previously from ultrastructural studies and of the known location of thylakoid galactolipid biosynthetic activities in the chloroplast envelope.