RNAi and microRNA: breakthrough technologies for the improvement of plant nutritional value and metabolic engineering

This article raises the complex issue of improving plant nutritional value through metabolic engineering and the potential of using RNAi and micro RNA technologies to overcome this complexity, focusing on a few key examples. It also highlights current knowledge of RNAi and microRNA functions and discusses recent progress in the development of new RNAi vectors and their applications. RNA interference (RNAi) and microRNA (miRNA) are recent breakthrough discoveries in the life sciences recognized by the 2006 Nobel Prize in Physiology or Medicine. The importance of these discoveries relates not only to elucidating the fundamental regulatory aspects of gene expression, but also to the tremendous potential of their applications in plants and animals. Here, we review recent applications of RNAi and microRNA for improving the nutritional value of plants, discuss applications of metabolomics technologies in genetic engineering, and provide an update on the related RNAi and microRNA technologies.     

A functionalized 20-residue peptaibol derivative for nucleic acid delivery

Based on the membrane-modifying peptaibol trichocellin-A-I (1) from Trichoderma viride, we designed a vehicle for the cellular delivery of antisense oligodeoxynucleotides by attaching a (Lys)10 stretch to the C-terminus of 1. The resulting transporter peptide 2, prepared by solid-phase synthesis using Fmoc protocol in combination with amino acid fluorides, was found to be mainly alpha-helical in solution, in contrast to its precursors 1 and 3. The uptake of the complex formed between carrier 2 and a fluorescence-tagged oligonucleotide, i.e., 4, was studied at different charge ratios by confocal laser-scanning microscopy, using two different eukaryotic cell lines: mouse embryonal fibroblast (NIH3T3) and human lung carcinoma (A549) cells. Peptide 2 readily translocated 4 into the cytoplasms of NIH3T3 cells. However, the peptide/oligonucleotide complex was accumulated around the plasma membrane of the A549 cells.     

Escherichia coli “TatExpress” strains export several g/L human growth hormone to the periplasm by the Tat pathway

Escherichia coli is a heavily used platform for the production of biotherapeutic and other high-value proteins, and a favored strategy is to export the protein of interest to the periplasm to simplify downstream processing and facilitate disulfide bond formation. The Sec pathway is the standard means of transporting the target protein but it is unable to transport complex or rapidly folding proteins because the Sec system can only transport proteins in an unfolded state. The Tat system also operates to transport proteins to the periplasm, and it has significant potential as an alternative means of recombinant protein production because it transports fully folded proteins. Here, we have tested the Tat system's full potential for the production of biotherapeutics for the first time using fed-batch fermentation. We expressed human growth hormone (hGH) with a Tat signal peptide in E. coli W3110 “TatExpress” strains that contain elevated levels of the Tat apparatus. This construct contained four amino acids from TorA at the hGH N-terminus as well as the initiation methionine from hGH, which is removed in vivo. We show that the protein is efficiently exported to the periplasm during extended fed-batch fermentation, to the extent that it is by far the most abundant protein in the periplasm. The protein was shown to be homogeneous, disulfide bonded, and active. The bioassay showed that the yields of purified periplasmic hGH are 5.4 g/L culture whereas an enzyme-linked immunosorbent assay gave a figure of 2.39 g/L. Separate analysis of a TorA signal peptide linked to hGH construct lacking any additional amino acids likewise showed efficient export to the periplasm, although yields were approximately two-fold lower.     

Synthesis of a new disulfide Fmoc monomer for creating biologically susceptible linkages in peptide nucleic acid oligomers

Peptide nucleic acids (PNA) are one of many synthetic mimics of DNA and RNA that have found applications as biological probes, as nano-scaffold components, and in diagnostics. In an effort to use PNA as constructs for cellular delivery we investigated the possibility of installing a biologically susceptible disulfide bond in the backbone of a PNA oligomer. Here we report the synthesis of a new abasic Fmoc monomer containing a disulfide bond that can be incorporated into a PNA oligomer (DS-PNA) using standard solid phase peptide synthesis. The disulfide bond survives cleavage from the resin and DS-PNA forms duplexes with complementary PNA oligomers. Initial studies aimed at determining if the disulfide bond is cleavable to reducing agents while in a duplex are explored using UV thermal analysis and HPLC.     

Thylakoid DeltapH-dependent precursor proteins bind to a cpTatC-Hcf106 complex before Tha4-dependent transport

The thylakoid DeltapH-dependent pathway transports folded proteins with twin arginine-containing signal peptides. Identified components of the machinery include cpTatC, Hcf106, and Tha4. The reaction occurs in two steps: precursor binding to the machinery, and transport across the membrane. Here, we show that a cpTatC-Hcf106 complex serves as receptor for specific binding of twin arginine-containing precursors. Antibodies to either Hcf106 or cpTatC, but not Tha4, inhibited precursor binding. Blue native gel electrophoresis and coimmunoprecipitation of digitonin-solubilized thylakoids showed that Hcf106 and cpTatC are members of an approximately 700-kD complex that lacks Tha4. Thylakoid-bound precursor proteins were also associated with an approximately 700-kD complex and were coimmunoprecipitated with antibodies to cpTatC or Hcf106. Chemical cross-linking revealed that precursors make direct contact with cpTatC and Hcf106 and confirmed that Tha4 is not associated with precursor, cpTatC, or Hcf106 in the membrane. Precursor binding to the cpTatC-Hcf106 complex required both the twin arginine and the hydrophobic core of the signal peptide. Precursors remained bound to the complex when Tha4 was sequestered by antibody, even in the presence of DeltapH. These results indicate that precursor binding to the cpTatC-Hcf106 complex constitutes the recognition event for this pathway and that subsequent participation by Tha4 leads to translocation.     

Involvement of<Emphasis Type="Italic">Leishmania donovani</Emphasis> major surface glycoprotein gp63 in promastigote multiplication

The major surface glycoprotein gp63 of the kinetoplastid protozoal parasiteLeishmania is implicated as a ligand mediating uptake of the parasite into, and survival within, the host macrophage. By expressing gp63 antisense RNA from an episomal vector inL. donovani promastigotes, gp63-deficient transfectants were obtained. Reduction of the gp63 level resulted in increased generation times, altered cell morphology, accumulation of cells in the G2/M phase of the cell cycle, and increased numbers of binucleate cells with one or two kinetoplasts. Growth was stimulated, and the number of binucleate cells reduced, by addition to the culture of a bacterially expressed fusion protein containing the fibronectin-like SRYD motif and the zinc-binding (metalloprotease) domain of gp63. These observations support an additional role of gp63 in promastigote multiplication; the fibronectin-like properties of gp63 may be important in this process     

Development of high oleic and low linoleic acid transgenics in a zero erucic acid Brassica juncea L. (Indian mustard) line by antisense suppression of the fad2 gene

A zero erucic acid (C22:1) line of Brassica juncea (VH486), adapted to the agronomic conditions of Northern India, has been modified for its fatty acid composition in the seed oil with antisense constructs using the sequence of fad2 gene of B. rapa. The full-length B. rapa fad2 cDNA sequence was determined by 5 and 3 RACE of a partial sequence available in the EST database. Construct pASfad2.1 contained 315 to 1251 bp and construct pASfad2.2 contained 1 to 1251 bp fragment of the fad2 gene, both in antisense orientation, driven by a truncated napin promoter. Analysis of the levels of linoleic acid (C18:2) in the BC1 seeds of single-copy transgenics showed that the construct pASfad2.2 gave better suppression of the fad2 gene as compared to the construct pASfad2.1. The BC1 transgenic seeds containing the pASfad2.2 construct segregated into two distinct classes of C18:2>20% (putative null homozygotes) and C18:2<20% (putative heterozygotes) in a 1:1 ratio, while the T1 seeds segregated into three classes, C18:2>20%, C18:2 between 12% and 20%) and C18:2<12% (putative homozygotes) in a 1:2:1 ratio. Putative homozygous T1 seeds (C18:2<12% analyzed by the half-seed method) of four of the transgenic lines were grown to establish T2 homozygous lines. These had ca. 73% C18:1 and 8 to 9% each of C18:2 and C18:3 (-linolenic acid) fractions in comparison to ca. 53% C18:1, 24% C18:2 and 16% C18:3 in the parental line VH486.     

The N- and C-terminal mutations in tryptophan permease Tat2 confer cell growth in<Emphasis Type="Italic"> Saccharomyces cerevisiae</Emphasis> under high-pressure and low-temperature conditions

Tryptophan uptake appears to be the limiting factor in growth of tryptophan auxotrophic Saccharomyces cerevisiae strains under the conditions of high hydrostatic pressure and low temperature. When the cells are subjected to a pressure of 25 MPa, tryptophan permease Tat2 is degraded in a manner dependent on ubiquitination by Rsp5. One of the high-pressure growth-conferring genes, HPG2, was shown to be allelic to TAT2. The HPG2-1 (Tat2^E27F) mutation site is located within the ExKS motif in the N-terminus, and the HPG2-2 (Tat2^D563N) and HPG2-3 (Tat2^E570K) mutation sites are located at the KQEIAE sequence in the C-terminus. The HPG2 mutations enhance the stability of Tat2 during high-pressure or low-temperature incubation, leading to cell growth under these stressful conditions. These results suggest that the cytoplasmic tails are involved in Rsp5-mediated ubiquitination of Tat2 under high-pressure or low-temperature conditions.Communicated by K. Horikoshi