A new method of DNA sequencing using deoxynucleoside alpha-thiotriphosphates

Nucleoside alpha-thiotriphosphates, polymerases and the exonuclease III analysis of oligonucleotides containing phosphorothioate linkages

The use of DNA polymerases to incorporate phosphorothioate linkages into DNA, and the use of exonuclease III to determine where those linkages have been incorporated, are re-examined in this work. The results presented here show that exonuclease III degrades single-stranded DNA as a substrate and digests through phosphorothioate linkages having one absolute stereochemistry, assigned (assuming inversion in the polymerase reaction) as S, but not the other absolute stereochemistry. This contrasts with a general view in the literature that exonuclease III favors double-stranded nucleic acid as a substrate and stops completely at phosphorothioate linkages. Furthermore, not all DNA polymerases appear to accept exclusively the (R) stereoisomer of nucleoside alpha-thiotriphosphates [and not the (S) diastereomer], a conclusion inferred two decades ago by examination of five Family-A polymerases and a reverse transcriptase. This suggests that caution is appropriate when extrapolating the detailed behavior of one polymerase from the behaviors of other polymerases. Furthermore, these results provide constraints on how exonuclease III–thiotriphosphate–polymerase combinations can be used to analyze the behavior of the components of a synthetic biology.

Reverse Sanger sequencing of RNA by MALDI-TOF mass spectrometry after solid phase purification

Several DNA/RNA sequencing strategies have been developed using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). In the reverse Sanger sequencing approach alpha-thiophosphate-containing NTPs are employed. Sequencing ladders are produced by the subsequent exonuclease cleavage, which is inhibited by the alpha-S-NTP at the 3' terminus. Here the reverse Sanger sequencing of RNA is described. The stability of RNA during the UV-MALDI process is higher relative to DNA, and RNA can be easily synthesized by transcription using bacteriophage RNA polymerase. alpha-S-rNTP was added to the reaction in a ratio of 1:3 to the native rNTPs and was incorporated statistically by the RNA polymerase. Four separate sequence ladders were produced, to avoid the problem of the only 1u mass difference between uridine and cytidine. However, it was shown that RNA transcription does not produce homogeneous transcripts. Therefore isolation of the full-length transcript is required to attain a non-ambiguous interpretation of cleavage spectra. This is achieved by the exclusive immobilization of the full-length transcript on a solid phase. The full-length transcripts were hybridized to magnetic beads, coated with short universal sequences, complementary to the in vitro RNA. After purification and isolation the RNA full-length transcript is cleaved by snake venom phosphodiesterase (SVP) and the obtained sequence ladder is analyzed by MALDI-MS.

Recognition of base-pairing by DNA polymerases during nucleotide incorporation: the properties of the mutagenic nucleotide dPTP alphaS

The highly mutagenic nucleoside dP (6-(2-deoxy-beta-D-erythro-pentofuranosyl)-3,4-dihydro-6H,8H-pyrimido[4,5-c][1,2]oxazin-2-one) is a bicyclic analogue of N4-methoxy-2'-deoxycytidine. It exists as a mixture of its imino and amino tautomers in solution with a ratio of about 10:1 based on its tautomeric constant. The bicyclic nature of the heterocycle P restrains the amino substituent in an anti conformation and permits effective Watson-Crick base-pairing using either tautomer. The specificity of incorporation of dP by the 3'-5'-exonuclease-free Klenow fragment of DNA polymerase I (exo-free Klenow) has been studied using the 5'-(1-thio)triphosphate dPTP alphaS in combination with phosphorothioate-specific sequencing of the DNA products. The method provides a convenient qualitative assay for studying nucleotide incorporation and reveals for the first time a potential role for the minor tautomeric forms of the natural DNA bases in base misinsertion (substitution mutagenesis) during replication.

Direct PCR sequencing with boronated nucleotides

A method is described to simultaneously amplify and sequence DNA using a new class of nucleotides containing boron. During the polymerase chain reaction, boron-modified nucleotides, i.e. 2'-deoxynucleoside 5'-alpha-[P-borano]-triphosphates, are incorporated into the product DNA. The boranophosphate linkages are resistant to nucleases and thus the positions of the boranophosphates can be revealed by exonuclease digestion, thereby generating a set of fragments that defines the DNA sequence. The boranophosphate method offers an alternative to current PCR sequencing methods. Single-sided primer extension with dideoxynucleotide chain terminators is avoided, with the consequence that the sequencing fragments are derived directly from the original PCR products. Boranophosphate sequencing is demonstrated with the Pharmacia and the Applied Biosystems 373A automatic sequencers, producing data that is comparable with cycle sequencing.

A procedure for selective DNA alkylation and detection by mass spectrometry

A method which improves the detectability of DNA by mass spectrometry is presented. By quantitatively alkylating the backbone of phosphorothioate oligonucleotides the problems of gas phase ion generation by matrix assisted laser desorption ionization can be controlled. We have developed a selective alkylating protocol for phosphorothioate oligonucleotides which is a facile way of generating non-ionic nucleic acids. A variety of alkylating agents was studied and their kinetics were monitored in a gel electrophoretic assay and by mass spectrometry.

A method for sequence-specific deletion mutagenesis

We describe a novel procedure for the construction of deletion mutants. Existing exonuclease-based protocols are efficient at producing randomly positioned deletions over large regions of DNA, but are of limited use in targetted mutagenesis due to their inherent sequence-specificity. We have taken advantage of the Exonuclease III-resistant nature of alpha-thio-dNTPs, incorporated into the target DNA template by a primer extension reaction, to generate base-specific alpha-thio-dNTP terminated products. Following removal of the 5' overhanging strands, the products can be cloned to generate a nested set of deletions with single base-pair increments. We demonstrate the utility of this technique by isolating multiple deletions over a 40bp region of the human beta-interferon promoter.

Incomplete primer extension during in vitro DNA amplification catalyzed by Taq polymerase; exploitation for DNA sequencing

Polyacrylamide gel electrophoresis of DNA fragments obtained by the polymerase chain reaction using Taq polymerase revealed the presence of multiple fragments shorter than the expected product. These abortive extension products were observed even when analysis by agarose gel electrophoresis showed only a single band. The production of prematurely terminated fragments can be exploited for the sequencing of PCR products if phosphorothioate groups are incorporated base specifically during the reaction in the presence of two oligonucleotide primers, one of which is 5'-32P-labeled. The addition of snake venom phosphodiesterase to the reaction mixture after completion of the amplification cycles digests each fragment from the 3'-end to a phosphorothioate group so that the sequence can be read by polyacrylamide gel electrophoresis.

Phosphorothioates in molecular biology

The observation that phosphorothioate analogues of the nucleoside triphosphates are substrates for DNA- and RNA-polymerases has proven a boon for the molecular biologist. As these phosphorothioate-containing polymers are stable to degradation by nucleases and the sulfur atom confers many favourable chemical properties, several applications in molecular biology have been developed, including new methods for site-directed mutagenesis and DNA sequencing.