Stabilization of mRNA in an Escherichia coli cell-free translation system

Genetic alphabet expansion technology by creating unnatural base pairs

Recent advancements in the creation of artificial extra base pairs (unnatural base pairs, UBPs) are opening the door to a new research area, xenobiology, and genetic alphabet expansion technologies. UBPs that function as third base pairs in replication, transcription, and/or translation enable the site-specific incorporation of novel components into DNA, RNA, and proteins. Here, we describe the UBPs developed by three research teams and their application in PCR-based diagnostics, high-affinity DNA aptamer generation, site-specific labeling of RNAs, semi-synthetic organism creation, and unnatural-amino-acid-containing protein synthesis.

DNA aptamer generation by ExSELEX using genetic alphabet expansion with a mini-hairpin DNA stabilization method

A novel aptamer generation method to greatly augment the affinity and stability of DNA aptamers was developed by genetic alphabet expansion combined with mini-hairpin DNA technology. The genetic alphabet expansion increases the physicochemical and structural diversities of DNA aptamers by introducing extra components, unnatural bases, as a fifth base, allowing for the enhancement of DNA aptamer affinities. Furthermore, the mini-hairpin DNA technology stabilizes DNA aptamers against nuclease digestion and thermal denaturation, by introducing an extraordinarily stable mini-hairpin DNA containing a GCGAAGC sequence. This novel method provides stabilized high-affinity DNA aptamers for diagnostic and therapeutic applications.

Cell-free expression as an emerging technique for the large scale production of integral membrane protein

Membrane proteins are highly underrepresented in structural data banks due to tremendous difficulties that occur upon approaching their structural analysis. Inefficient sample preparation from conventional cellular expression systems is in many cases the first major bottleneck. Preparative scale cell-free expression has now become an emerging alternative tool for the high level production of integral membrane proteins. Many toxic effects attributed to the overproduction of recombinant proteins are eliminated by cell-free expression as viable host cells are no longer required. A unique characteristic is the open nature of cell-free systems that offers a variety of options to manipulate the reaction conditions in order to protect or to stabilize the synthesized recombinant proteins. Detergents or lipids can easily be supplemented and membrane proteins can therefore be synthesized directly into a defined hydrophobic environment of choice that permits solubility and allows the functional folding of the proteins. Alternatively, cell-free produced precipitates of membrane proteins can efficiently be solubilized in mild detergents after expression. Highly valuable for structural approaches is the fast and efficient cell-free production of uniformly or specifically labeled proteins. A considerable number of membrane proteins from diverse families like prokaryotic small multidrug transporters or eukaryotic G-protein coupled receptors have been produced in cell-free systems in high amounts and in functionally active forms. We will give an overview about the current state of the art of this new approach with special emphasis on technical aspects as well as on the functional and structural characterization of cell-free produced membrane proteins.

Cell-free synthesis of recombinant proteins from PCR-amplified genes at a comparable productivity to that of plasmid-based reactions

The functional stability of mRNA is one of the crucial factors affecting the efficiency of cell-free protein synthesis. The importance of the stability of mRNA in the prolonged synthesis of protein molecules becomes even greater when the cell-free protein synthesis is directed by PCR-amplified DNAs, because the linear DNAs are rapidly degraded by the endogenous nucleases and, thus, the continuous generation of mRNA molecules is limited. With the aim of developing a highly efficient cell-free protein synthesis system directed by PCR products, in this study, we describe a systematic approach to enhance the stability of mRNA in cell-free extracts. First, exonuclease-mediated degradation was substantially reduced by introducing a stem-loop structure at the 3'-end of the mRNA. The endonucleolytic cleavage of the mRNA was minimized by using an S30 extract prepared from an Escherichia coli strain that is deficient in a major endonuclease (RNase E). Taken together, through the retardation of the endonucleolytic and exonucleolytic degradations of the mRNA molecules, the level of protein expression from the PCR-amplified DNA templates becomes comparable to that of conventional plasmid-based reactions. The enhanced productivity of the PCR-based cell-free protein synthesis enables the high-throughput generation of protein molecules required for many post-genomic applications.

Ricin A-chain activity on stem-loop and unstructured DNA substrates

Ricin toxin A-chain (RTA) depurinates a single adenylate on a GAGA stem-loop region of eukaryotic 28S RNA, making it a potent toxin. Steady state rate analysis is used to establish the kinetic parameters for depurination of short RNA, DNA, and RNA-DNA hybrids of GAGA linear segments and stem-loop regions as substrates for RTA. Both stem and tetraloop structures are essential for action on RNA. For DNA stem-loop substrates, stem stability plays a small role in enhancing catalytic turnover but can enhance binding by up to 3 orders of magnitude. DNA sequences of d[GAGA] without stem-loop structures are found to be slow substrates for RTA. In contrast, equivalent RNA sequences exhibit no activity with RTA. Introduction of a deoxyadenosine at the depurination site of short RNA oligonucleotides restores catalytic function. NMR analysis indicates that the short, nonsubstrate GAGA is converted to substrate in GdAGA by the presence of a more flexible ribosyl group at the deoxyadenosine site. Conversion between C2'-endo and C2'-exo conformations at the deoxyadenosine site moves the 3'- and 5'-phosphorus atoms by 1.1 A, and the former is proposed to place them in a catalytically favorable configuration. The ability to use short RNA-DNA hybrids as substrates for RTA permits exploration of related structures to function as substrates and inhibitors.

Hybridization kinetics of oligodeoxyribonucleotides with a d(GCGAAGC) hairpin at the 3'-end

In order to protect them against enzymatic attack of serum in the antisense strategy, oligodeoxyribonucleotides can be protected on their 3'-side by the sequence d(GCGAAGC) which spontaneously forms a hairpin which is known for its extraordinary stability with regard to thermal denaturation or nuclease degradation (I. Hirao, G. Kawai, S. Yoshizawa, Y. Nishimura, Y. Ishido, K. Watanabe and K. Miura, Nucleic Acids Res. 22, 576-582 (1994)). By contrast, the hairpin does not prevent hybridization of the 5'-stem part of the oligonucleotide to a target DNA strand. As soon as this pairing occurs, the stability of the hairpin is disrupted. Its opening rate, followed by its pairing if possible, is of the same order than that of hybridization of the stem part.

Opening of the extraordinarily stable mini-hairpin d(GCGAAGC)

For the purposes of the antisense strategy oligodeoxyribonucleotides can be protected against serum and cell nuclease digestion by tagging at their 3'-end with a sequence naturally forming a very stable hairpin, d(GCGAAGC). This nuclease-resistant hairpin is also known for its high thermostability. We demonstrate in this study that attachment of d(GCGAAGC) at the 3'-end of an oligodeoxyribonucleotide does not hinder hybridization of the 5'-part of this oligonucleotide to a complementary DNA strand. Moreover, the hairpin is in equilibrium between a folded and an open structure, with an energy minimum in favor of pairing if it is possible, even with mismatches.

GNA trinucleotide loop sequences producing extraordinarily stable DNA minihairpins

d(GCGAAAGC) and d(GCGAAGC) fragments form extraordinarily stable DNA minihairpins containing only two G-C base pairs and a GAAA or GAA loop, respectively, with a Tm of 76 degrees C. These sequences are frequently found in some important regions such as replication origins and promoter regions for transcription. We examined all 64 possible DNA fragments, d(GCNNNGC), in which the triloop region of the d(GCGAAGC) minihairpin was randomized and found that only four fragments, d(GCGNAGC) (N = A, G, C, or T), formed extraordinarily stable minihairpins as shown by their gel mobility and resistance to a single-stranded DNA-specific exonuclease. Structural and thermodynamic analyses suggest that the extraordinary stability is caused by a unique structural property of the trinucleotide sequences corresponding to the GNA loop.

Nuclease resistance of an extraordinarily thermostable mini-hairpin DNA fragment, d(GCGAAGC) and its application to in vitro protein synthesis

The nuclease resistance of a short, thermostable mini-hairpin, d(GCGAAGC), and other related hairpins was examined. Hairpins possessing a purine-rich (GAA) or (GAAA) loop appeared to be more resistant against nucleases than those with a pyrimidine-rich loop or single-stranded oligomers. Among 8 kinds of oligodeoxyribonucleotides examined, the fragment most resistant against nucleases was a hairpin with the sequence of d(CGCGAAGCG). This hairpin was then utilized for the stabilization of mRNA in an in vitro translation system; the 3'-terminal region of an mRNA was hybridized with an oligodeoxyribonucleotide including the sequence complementary to the 3'-terminus of the mRNA tagged with the nuclease-resistant d(CGCGAAGCG) hairpin sequence. By using this method, dihydrofolate reductase (DHFR) mRNA was stabilized against nucleases contaminating a cell-free translation system of E.coli, with a consequent increase in protein synthesis efficiency of 200%.

Expression and stability of recombinant RQ-mRNAs in cell-free translation systems

Expression of dihydrofolate reductase (DHFR) and chloramphenicol acetyltransferase (CAT) mRNAs in cell-free Escherichia coli translation systems is greatly enhanced as a result of their insertion into RQ135 RNA, a naturally occurring satellite of phage Q beta. The enhancement is due to protection of the recombinant mRNAs against endogenous ribonucleases and to an increased initial rate of translation in the case of the RQ-CAT mRNA.

Most compact hairpin-turn structure exerted by a short DNA fragment, d(GCGAAGC) in solution: an extraordinarily stable structure resistant to nucleases and heat

The three-dimensional structure of a short DNA fragment, d(GCGAAGC) exhibiting an extraordinarily stable hairpin structure was determined by nuclear magnetic resonance spectroscopy. Two possible models were obtained by molecular modelling using distance and torsion constraints. Only one of these two models is the correct structure, which can clearly explain all the 1H chemical shifts. d(GCGAAGC) is folded back on itself between A4 and A5, and all the sugars in the fragment adopt the C2'-endo conformation. This compact molecule is stabilized by regular extensive base-stacking interaction within each B-form helical strand of G1C2G3A4 and A5G6C7, and by two G-C and one G3-A5 base pairs. The molecule is hard to differentiate into stem and loop regions, so that we classify it as a turn (hairpin-turn) structure experted by a single-stranded DNA. This highly stacked structure shows high thermostability and strong resistance against nucleases contained in E. coli extracts and in human serum.