Biomedical subjects

N G Walter

Publications and source records attributed to N G Walter.

24 records · Page 2Linked to original sources

PubMed1996

Fluorescence correlation analysis of probe diffusion simplifies quantitative pathogen detection by PCR.

A sensitive, labor-saving, and easily automatable nonradioactive procedure named APEX-FCS (amplified probe extension detected by fluorescence correlation spectroscopy) has been established to detect specific in vitro amplification of pathogen genomic sequences. As an example, Mycobacterium tuberculosis genomic DNA was subjected to PCR amplification with the Stoffel fragment of Thermus aquaticus DNA polymerase in the presence of nanomolar concentrations of a rhodamine-labeled probe (third primer), binding to the target in between the micromolar amplification primers. The probe becomes extended only when specific amplification occurs. Its low concentration avoids false-positives due to unspecific hybridization under PCR conditions. With increasing portion of extended probe molecules, the probe's average translational diffusion properties gradually change over the course of the reaction, reflecting amplification kinetics. Following PCR, this change from a stage of high to a stage of low mobility can directly be monitored during a 30-s measurement using a fluorescence correlation spectroscopy device. Quantitation down to 10 target molecules in a background of 2.5 micrograms unspecific DNA without post-PCR probe manipulations could be achieved with different primer/ probe combinations. The assay holds the promise to concurrently perform amplification, probe hybridization, and specific detection without opening the reaction chamber, if sealable foils are used.

Automation↗

PubMed1996

Quantitative hybridization kinetics of DNA probes to RNA in solution followed by diffusional fluorescence correlation analysis.

Binding kinetics in solution of six N,N,N',N'-tetramethyl-5-carboxyrhodamine-labeled oligodeoxyribonucleotide probes to a 101mer target RNA comprising the primer binding site for HIV-1 reverse transcriptase were characterized using fluorescence correlation spectroscopy (FCS). FCS allows a sensitive, non-radioactive real time observation of hybridization of probes to the RNA target in the buffer of choice without separation of free and bound probe. The binding process could directly be monitored by the change in translational diffusion time of the 17mer to 37mer DNA probe upon specific hybridization with the larger RNA target. The characteristic diffusion time through a laser-illuminated open volume element with 0.5 micron in diameter increased from 0.13-0.2 ms (free) to 0.37-0.50 ms (bound), depending on the probe. Hybridization was approximated by biphasic irreversible second-order reaction kinetics, yielding first-phase association rate constants between 3 x 10(4) and 1.5 x 10(6) M-1 s-1 for the different probes. These varying initial rates reflected the secondary structures of probes and target sites, being consistent with a hypothetical binding pathway starting from loop-loop interactions in a kissing complex, and completion of hybridization requiring an additional interaction involving single-stranded regions of both probe and target. FCS thus permits rapid screening for suitable antisense nucleic acids directed against an important target like HIV-1 RNA with low consumption of probes and target.

Animals↗

PubMed1995

Modelling viral evolution in vitro using exo- Klenow polymerase: continuous selection of strand displacement amplified DNA that binds an oligodeoxynucleotide to form a triple-helix.

Evolution comprises cycles of amplification, mutagenesis and selection. To study evolutionary phenomena, isothermal strand displacement amplification (SDA) of double-stranded DNA as an in vitro model for rolling-circle replication of viruses has been coupled to a positive selection procedure. First, two subsequent amplification reactions utilizing exo- Klenow polymerase were performed under direct observation using the fluorescent dye thiazole orange. Under the chosen conditions, the mutation rate was 1.5 x 10(-3) and 0.4 x 10(-3) for base substitutions and deletions, respectively. Then, a 16mer oligodeoxynucleotide with an acridine moiety coupled to its 5' end was used to select for double strands that retained their ability to form a triple-helix with the oligodeoxynucleotide. Conditions for triple-helix formation were chosen such that only 10 to 40% of the SDA products were allowed to bind the third strand. Non-denaturing polyacrylamide gel electrophoresis was used to separate triple-helices from unmodified double strands, and only triplex strands were used to initiate a new round of error-prone amplification and selection. Nine such rounds with about 270 molecular generations were performed. The final mutant spectrum was characterized and compared with those of amplification reactions without additional selection pressure. While without selection pressure base substitutions and deletions throughout the initial wild-type rapidly produce a diverse mutant distribution, the consensus after nine selection rounds clearly shows two mutational hotspot positions. Using gel shift assays and a newly developed non-radioactive DNase I footprinting technique, it could be shown that both the initial wild-type and the final consensus do not differ significantly in their triplex formation ability. As opposed to this, they do show different amplification efficiencies. The final consensus sequence is amplified with the highest rate in the exponential reaction phase, while the most abundant clone, which is characterized by two additional point deletions, is the sequence with the highest amplification rate in the linear growth phase.

Base Sequence↗

PubMed1995

Multichannel PCR and serial transfer machine as a future tool in evolutionary biotechnology.

As an improved strategy for producing functional macromolecules by in vitro evolutionary optimization, we propose an automated machine that can process up to 960 samples in parallel. It consists of a 960-well PCR machine with special sealed plastic reaction vessels and appropriate handling devices. We show that the heat-sealing technique does not significantly affect the activity of Taq DNA Polymerase or that of the temperature-sensitive Q beta RNA polymerase but avoids cross-contamination and evaporation of the samples. Initial experiments demonstrate the suitability of the apparatus to uniformly process the samples and to perform the thermocycling. Serial transfer of reaction products into fresh reaction solution was used to initiate further rounds of amplification as a typical experimental setup for an evolutionary biotechnology application.

Allolevivirus↗

PubMed1994

Strand displacement amplification as an in vitro model for rolling-circle replication: deletion formation and evolution during serial transfer.

Strand displacement amplification is an isothermal DNA amplification reaction based on a restriction endonuclease nicking its recognition site and a polymerase extending the nick at its 3' end, displacing the downstream strand. The reaction resembles rolling-circle replication of single-stranded phages and small plasmids. The displaced sense strand serves as target for an antisense reaction and vice versa, resulting in exponential growth and the autocatalytic nature of this in vitro reaction as long as the template is the limiting agent. We describe the optimization of strand displacement amplification for in vitro evolution experiments under serial transfer conditions. The reaction was followed and controlled by use of the fluorescent dye thiazole orange binding to the amplified DNA. We were able to maintain exponential growth conditions with a doubling time of 3.0 min throughout 100 transfers or approximately 350 molecular generations by using an automatic handling device. Homology of in vitro amplification with rolling-circle replication was mirrored by the occurring evolutionary processes. Deletion events most likely caused by a slipped mispairing mechanism as postulated for in vivo replication took place. Under our conditions, the mutation rate was high and a molecular quasi-species formed with a mutant lacking internal hairpin formation ability and thus outgrowing all other species under dGTP/dCTP deficiency.

Base Composition↗

PubMedDate not supplied

In the fluorescent spotlight: global and local conformational changes of small catalytic RNAs.

RNA is a ubiquitous biopolymer that performs a multitude of essential cellular functions involving the maintenance, transfer, and processing of genetic information. RNA is unique in that it can carry both genetic information and catalytic function. Its secondary structure domains, which fold stably and independently, assemble hierarchically into modular tertiary structures. Studies of these folding events are key to understanding how catalytic RNAs (ribozymes) are able to position reaction components for site-specific chemistry. We have made use of fluorescence techniques to monitor the rates and free energies of folding of the small hairpin and hepatitis delta virus (HDV) ribozymes, found in satellite RNAs of plant and the human hepatitis B viruses, respectively. In particular, fluorescence resonance energy transfer (FRET) has been employed to monitor global conformational changes, and 2-aminopurine fluorescence quenching to probe for local structural rearrangements. In this review we illuminate what we have learned about the reaction pathways of the hairpin and HDV ribozymes, and how our results have complemented other biochemical and biophysical investigations. The structural transitions observed in these two small catalytic RNAs are likely to be found in many other biological RNAs, and the described fluorescence techniques promise to be broadly applicable.

2-Aminopurine↗