Coupled cytoplasmic transcription-and-translation--a method of choice for heterologous gene expression in Xenopus oocytes

Construction and Application of Orthogonal T7 Expression System in Eukaryote: An Overview

The T7 system is an orthogonal transcription-system, which is characterized by simplicity, higher efficiency, and higher processivity, and it is used for protein or mRNA synthesis in various biological-systems. In comparison with prokaryotes, the construction of the T7 expression system is still on-going in eukaryotes, but it shows greatly applicable prospects. In the present paper, development of T7 expression system construction in eukaryotes is reviewed, including its construction in animal (mammalian cells, trypanosomatid protozoa, Xenopus oocytes, zebrafish), plant, and microorganism and its application in vaccine production and gene therapy. In addition, the innate challenges of T7 expression system construction in eukaryote and its potential application in vaccine production and gene therapy are discussed.

Directed Evolution of a Panel of Orthogonal T7 RNA Polymerase Variants for in Vivo or in Vitro Synthetic Circuitry

T7 RNA polymerase is the foundation of synthetic biological circuitry both in vivo and in vitro due to its robust and specific control of transcription from its cognate promoter. Here we present the directed evolution of a panel of orthogonal T7 RNA polymerase:promoter pairs that each specifically recognizes a synthetic promoter. These newly described pairs can be used to independently control up to six circuits in parallel.

Production of membrane proteins using cell–free expression systems

Different overexpression systems are widely used in the laboratory to produce proteins in a reasonable amount for functional and structural studies. However, to optimize these systems without modifying the cellular functions of the living organism remains a challenging task. Cell-free expression systems have become a convenient method for the high-throughput expression of recombinant proteins, and great effort has been focused on generating high yields of proteins. Furthermore, these systems represent an attractive alternative for producing difficult-to-express proteins, such as membrane proteins. In this review, we highlight the recent improvements of these cell-free expression systems and their direct applications in the fields of membrane proteins production, protein therapy and modern proteomics.

Monitoring gene expression in a single Xenopus oocyte using multiple cytoplasmic collections and quantitative RT-PCR

Oocytes and eggs of the African clawed frog, Xenopus laevis, are commonly used in gene expression studies. However, monitoring transcript levels in the individual living oocytes remains challenging. To address this challenge, we used a technique based on multiple repeated collections of nanoliter volumes of cytoplasmic material from a single oocyte. Transcript quantification was performed by quantitative RT-PCR. The technique allowed monitoring of heterologous gene expression in a single oocyte without affecting its viability. We also used this approach to profile the expression of endogenous genes in living Xenopus oocytes. Although frog oocytes are traditionally viewed as a homogenous cell population, a significant degree of gene expression variation was observed among the individual oocytes. A lognormal distribution of transcript levels was revealed in the oocyte population. Finally, using this technique, we observed a dramatic decrease in the content of various cytoplasmic mRNAs in aging unfertilized eggs but not in oocytes, suggesting a link between mRNA degradation and egg apoptosis.

Activation of T7 RNA polymerase in Xenopus oocytes and cell-free extracts

Single-subunit bacteriophage T7 RNA polymerase (T7 RNAP) is universally employed for in vivo and in vitro transcription of genes put under control of the T7 promoter. The enzyme is capable of transcribing a complete gene without additional proteins. In this study, we reveal the presence of a low molecular weight factor, which induces several-fold activation of T7 RNAP in the cytoplasm of oocytes and eggs from Xenopus laevis. Cell-free reconstitution of the T7 RNAP activation allowed us to investigate the molecular properties of the activator, establish its peptide nature and suggest T7 RNAP activation mechanism. In contrast to the previously described nonspecific transcriptional activators, which interact with scattered ionic sites on nucleic acids, the peptide activator associates with T7 RNAP molecule, thus being a bona fide activator of the polymerase. To our knowledge, this is the first report concerning the specific activation of T7 RNAP by a factor of peptide or protein origin. Besides rather obvious merits in gaining more efficient transcription with T7 RNAP, this finding can provide additional insights into regulatory mechanisms of transcription. The study also introduces a novel highly sensitive luminescent assay of T7 RNAP activity.

Using fluorometry and ion-sensitive microelectrodes to study the functional expression of heterologously-expressed ion channels and transporters in Xenopus oocytes

The Xenopus laevis oocyte is a model system for the electrophysiological study of exogenous ion transporters. Three main reasons make the oocyte suitable for this purpose: (a) it has a large cell size (approximately 1mm diameter), (b) it has an established capacity to produce-from microinjected mRNAs or cRNAs-exogenous ion transporters with close-to-physiological post-translational modifications and actions, and (c) its membranes contain endogenous ion-transport activities which are usually smaller in magnitude than the activities of exogenously-expressed ion transporters. The expression of ion transporters as green fluorescent protein fusions allows the fluorometric assay of transporter yield in living oocytes. Monitoring of transporter-mediated movement of ions such as Cl(-), H(+) (and hence base equivalents like OH(-) and HCO(3)(-)), K(+), and Na(+) is achieved by positioning the tips of ion-sensitive microelectrodes inside the oocyte and/or at the surface of the oocyte plasma membrane. The use of ion-sensitive electrodes is critical for studying net ion-movements mediated by electroneutral transporters. The combined use of fluorometry and electrophysiology expedites transporter study by allowing measurement of transporter yield prior to electrophysiological study and correlation of relative transporter yield with transport rates.

The use of the Xenopus oocyte as a model system to analyze the expression and function of eukaryotic heat shock proteins

The analysis of the expression and function of heat shock protein (hsp) genes, a class of molecular chaperones, has been greatly aided by studies carried out with Xenopus oocytes. The large size of the oocyte facilitates microinjection of DNA, mRNA or protein, permits manual dissection of nuclei, and allows certain assays to be performed with single oocytes. These and other characteristics were useful in identifying the cis- and trans-acting factors involved in hsp gene transcription as well as the role of chaperones and co-chaperones in the repression and activation of heat shock factor. Xenopus oocytes were used to examine heat shock protein (HSP) molecular chaperone function as well as their involvement in intracellular trafficking, maturation, and secretion of protein. Possible new areas of research with this system include the role of membranes in the heat shock response, involvement of HSPs in viral replication and maturation, and in vivo NMR spectroscopy of microinjected HSPs.

Coupled transcription-and-translation in Xenopus oocyte and egg extracts

Coinjection of T7 promoter-driven plasmids and T7 RNA polymerase (T7 RNAP) into Xenopus oocytes results in robust protein synthesis, due to simultaneous gene transcription-and-translation (TnT) in the oocyte cytoplasm [Geib, S., Sandoz, G., Carlier, E., V. Cornet, Cheynet-Sauvion, V., De Waard, M., 2001. A novel Xenopus oocyte expression system based on cytoplasmic coinjection of T7-driven plasmids and purified T7-RNA polymerase. Receptors Channels 7, 331-343; Tokmakov, A.A., Matsumoto, E., Shirouzu, M., Yokoyama, S., 2006. Coupled cytoplasmic transcription-and-translation--a method of choice for heterologous gene experession in Xenopus oocytes. J. Biotechnol. 122, 5-15]. In the present study, we demonstrate that the TnT reaction of protein synthesis can be reconstituted in cell-free extracts of Xenopus oocytes and eggs. Similar to the reaction in oocytes, the effective coupling of bacteriophage T7 RNAP-mediated transcription with the eukaryotic translation machinery takes place in the Xenopus oocyte and egg extracts. However, the kinetics of protein and RNA production in the extracts are quite different from those observed in oocytes. Potent RNA synthesis in the extracts starts immediately after the addition of T7 promoter-driven DNA and T7 RNAP and continues for about 30 min, followed by RNA degradation. The protein product is detectable in the extracts in 15 min after the initiation of the TnT reaction. Efficient protein synthesis in the extracts continues for about 1h. The productivity of this expression system can be boosted by the additions of an RNase inhibitor and an ATP-regeneration system, and by extract dilution. Kinetic analyses suggested that extending the lifetime of the extracts would further increase their productivity.