Codon—Anticodon interaction studied with trinucleoside diphosphates containing 2-thiouridine, 4-thiouridine, 2,4-dithiouridine, or 2-thiocytidine

Photochemical and Photodynamical Properties of Sulfur-Substituted Nucleic Acid Bases

Sulfur-substituted nucleobases (a.k.a., thiobases) are among the world's leading prescriptions for chemotherapy and immunosuppression. Long-term treatment with azathioprine, 6-mercaptopurine and 6-thioguanine has been correlated with the photoinduced formation of carcinomas. Establishing an in-depth understanding of the photochemical properties of these prodrugs may provide a route to overcoming these carcinogenic side effects, or, alternatively, a basis for developing effective compounds for targeted phototherapy. In this review, a broad examination is undertaken, surveying the basic photochemical properties and excited-state dynamics of sulfur-substituted analogs of the canonical DNA and RNA nucleobases. A molecular-level understanding of how sulfur substitution so remarkably perturbs the photochemical properties of the nucleobases is presented by combining experimental results with quantum-chemical calculations. Structure-property relationships demonstrate the impact of site-specific sulfur substitution on the photochemical properties, particularly on the population of the reactive triplet state. The value of fundamental photochemical investigations for driving the development of ultraviolet-A chemotherapeutics is showcased. The most promising photodynamic agents identified thus far have been investigated in various carcinoma cell lines and shown to decrease cell proliferation upon exposure to ultraviolet-A radiation. Overarching principles have been elucidated for the impact that sulfur substitution of the carbonyl oxygen has on the photochemical properties of the nucleobases.

Thiolated uridine substrates and templates improve the rate and fidelity of ribozyme-catalyzed RNA copying

Ribozyme-catalyzed RNA polymerization is inefficient and error prone. Here we demonstrate that two alternative bases, 2-thio-uridine (s(2)U) and 2-thio-ribo-thymidine (s(2)T), improve the rate and fidelity of ribozyme catalyzed nucleotide addition as NTP substrates and as template bases. We also demonstrate the functionality of s(2)U and s(2)T-containing ribozymes.

Photophysics and Photochemistry of Canonical Nucleobases’ Thioanalogs: From Quantum Mechanical Studies to Time Resolved Experiments

Interest in understanding the photophysics and photochemistry of thiated nucleobases has been awakened because of their possible involvement in primordial RNA or their potential use as photosensitizers in medicinal chemistry. The interpretation of the photodynamics of these systems, conditioned by their intricate potential energy surfaces, requires the powerful interplay between experimental measurements and state of the art molecular simulations. In this review, we provide an overview on the photophysics of natural nucleobases’ thioanalogs, which covers the last 30 years and both experimental and computational contributions. For all the canonical nucleobase’s thioanalogs, we have compiled the main steady state absorption and emission features and their interpretation in terms of theoretical calculations. Then, we revise the main topographical features, including stationary points and interstate crossings, of their potential energy surfaces based on quantum mechanical calculations and we conclude, by combining the outcome of different spectroscopic techniques and molecular dynamics simulations, with the mechanism by which these nucleobase analogs populate their triplet excited states, which are at the origin of their photosensitizing properties.

Increase in the photoreactivity of uracil derivatives by doubling thionation

The ability of 4-thiouracil to strongly absorb UVA radiation and to populate a reactive triplet state in high yield has enabled its use as a versatile photocrosslinker for nearly 50 years. In this contribution, we present a detailed spectroscopic and photochemical investigation of the 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil series in an effort to further advance this chemistry and to scrutinize the photoreactivity of 2,4-dithiouracil. Our results reveal that excitation of 2,4-dithiouracil leads to intersystem crossing to the triplet manifold in 220 ± 40 fs, which enables the population of the reactive triplet state with near unity yield (ΦT = 0.90 ± 0.15) and ultimately leads to a ca. 50% singlet oxygen generation (ΦΔ = 0.49 ± 0.02)-one of the highest singlet oxygen yields reported to date for a photoexcited thiobase. In addition, the long-lived triplet state of 2,4-dithiouracil reacts efficiently with the nucleic acid base adenine 5'-monophosphate through a direct, oxygen-independent photocycloaddition mechanism and at a rate that is at least 3-fold faster than that of 4-thiouracil under equal conditions. The new physico-chemical insights reported for these RNA-thiobase derivatives are compared to those of the DNA and RNA bases and the DNA-thiobase derivatives. Furthermore, the strong near-visible absorption and increased photoreactivity measured for 2,4-dithiouracil lays a solid foundation for developing RNA-targeted photocrosslinking and phototherapeutic agents that are more effective than those currently available.

Crystal structure studies of RNA duplexes containing s(2)U:A and s(2)U:U base pairs

Structural studies of modified nucleobases in RNA duplexes are critical for developing a full understanding of the stability and specificity of RNA base pairing. 2-Thio-uridine (s²U) is a modified nucleobase found in certain tRNAs. Thermodynamic studies have evaluated the effects of s²U on base pairing in RNA, where it has been shown to stabilize U:A pairs and destabilize U:G wobble pairs. Surprisingly, no high-resolution crystal structures of s²U-containing RNA duplexes have yet been reported. We present here two high-resolution crystal structures of heptamer RNA duplexes (5′-uagcs²Ucc-3′ paired with 3′-aucgAgg-5′ and with 3′-aucgUgg-5′) containing s²U:A and s²U:U pairs, respectively. For comparison, we also present the structures of their native counterparts solved under identical conditions. We found that replacing O2 with S2 stabilizes the U:A base pair without any detectable structural perturbation. In contrast, an s²U:U base pair is strongly stabilized in one specific U:U pairing conformation out of four observed for the native U:U base pair. This s²U:U stabilization appears to be due at least in part to an unexpected sulfur-mediated hydrogen bond. This work provides additional insights into the effects of 2-thio-uridine on RNA base pairing.

Unrestricted accessibility of short oligonucleotides to RNA

The propensity of RNA to fold into higher-order structures poses a major barrier to the use of short probes (<15 nucleotides) by preventing their accessibility. Introduction of the pseudo-complementary bases 2-aminoadenine (nA) and 2-thiouracil (sU) and the destabilizing base 7-deazaguanine (cG) into RNA provides a partial solution to this problem. While complementary in hydrogen bonding groups, nA and sU cannot form a stable base pair due to steric hindrance, and are thus pseudo-complementary. Each, however, recognizes the regular T/U and A complements, allowing pairing with oligonucleotides. Short pseudo-complementary RNAs can be prepared by in vitro transcription. Relative to standard transcripts, the modified transcripts possess reduced secondary structure and increased accessibility to short (8-mer) probes in the locked nucleic acid (LNA) configuration. They also hybridize to complementary probes with increased specificity and thermostability. Practical application of this strategy to oligonucleotide-based hybridization assays will require engineering of RNA polymerase for more efficient utilization of pseudo-complementary nucleoside triphosphates.

Modified bases in RNA reduce secondary structure and enhance hybridization

Secondary structure in RNA targets is a significant barrier to short DNA probes. However, when such targets are the end product of an in vitro amplification scheme, it is possible to carry out transcription in the presence of nucleoside triphosphate analogues that reduce secondary structure of the RNA without impairing subsequent hybridization. Here we show that nucleoside triphosphates of 2-aminoadenine (nA) and 2-thiouracil (sU) are taken up by T7 RNA polymerase and that the resulting RNA possesses reduced secondary structure and improved accessibility to DNA probes. The hybridization properties of short RNA transcripts were studied using a new gel mobility shift assay from which melting temperatures were determined. RNA hairpins that contained nA and sU were able to hybridize to DNA probes under conditions where the unmodified hairpins did not. DNA-RNA hybrids that contained nA and sU in the RNA strand exhibited enhanced specificity, increased stability, and greater equality of base pairing strength than the same hybrids without modifications. Substitution of guanine (G) with inosine (I) further reduced secondary structure, but RNA with this base hybridized nonselectively. The high stability of nA-T and A-sU base pairs in DNA-RNA hybrids, combined with the destabilizing effect of the nA-sU couple in RNA targets, accounts for the improved hybridization properties. These results suggest that incorporation of nA and sU during in vitro transcription is a promising strategy for enhancing the performance of oligomeric DNA probes with an RNA target.

Thermodynamics of RNA-RNA duplexes with 2- or 4-thiouridines: implications for antisense design and targeting a group I intron

Antisense compounds are designed to optimize selective hybridization of an exogenous oligonucleotide to a cellular target. Typically, Watson-Crick base pairing between the antisense compound and target provides the key recognition element. Uridine (U), however, not only stably base pairs with adenosine (A) but also with guanosine (G), thus reducing specificity. Studies of duplex formation by oligonucleotides with either an internal or a terminal 2- or 4-thiouridine (s(2)U or s(4)U) show that s(2)U can increase the stability of base pairing with A more than with G, while s(4)U can increase the stability of base pairing with G more than with A. The latter may be useful when binding can be enhanced by tertiary interactions with a s(4)U-G pair. To test the effects of s(2)U and s(4)U substitutions on tertiary interactions, binding to a group I intron ribozyme from mouse-derived Pneumocystis carinii was measured for the hexamers, r(AUGACU), r(AUGACs(2)U), and r(AUGACs(4)U), which mimic the 3' end of the 5' exon. The results suggest that at least one of the carbonyl groups of the 3' terminal U of r(AUGACU) is involved in tertiary interactions with the catalytic core of the ribozyme and/or thio groups change the orientation of a terminal U-G base pair. Thus thio substitutions may affect tertiary interactions. Studies of trans-splicing of 5' exon mimics to a truncated rRNA precursor, however, indicate that thio substitutions have negligible effects on overall reactivity. Therefore, modified bases can enhance the specificity of base pairing while retaining other activities and, thus, increase the specificity of antisense compounds targeting cellular RNA.

The modification of the wobble base of tRNAGlu modulates the translation rate of glutamic acid codons in vivo

In Escherichia coli, uridine in the wobble position of tRNAGlu and tRNALys is modified to mnm5s2U34. This modification is believed to restrict the base-pairing capability, i.e. to prevent misreading of near-cognate codons and reduce the efficiency of cognate codon reading, especially of codons ending in G. We have determined the influence of the 5-methylaminomethyl and the 2-thio modifications of mnm5s2U34 in tRNAGlu on the translation rate of the glutamate codons GAA and GAG in vivo. In wild-type cells, GAG is translated slower (7. 7 codons/second) and GAA faster (18 codons/second) than the average codon (13 codons/second). Surprisingly, tRNAGlu lacking the 5-methylaminomethyl group, thus containing s2U34, translated GAA twofold faster (47 codons/second) and GAG fourfold slower (1.9 codons/second) than fully modified tRNAGlu. In contrast, tRNAGlu that contains mnm5U34 instead of mnm5s2U34 translated GAA fourfold slower (4.5 codons/second) and GAG only 20% slower (6.2 codons/second). Clearly, the 5-methylaminomethyl group of mnm5s2U34 facilitates base-pairing with G while decreasing base-pairing with A, resulting in rates of translation of GAG and GAA that approach that of the average codon. The 2-thio group increases the recognition of GAA and has only a minor effect on the decoding of GAG. Furthermore, the 2-thio group is important for aminoacylation (see the accompanying paper). These data imply that the function of mnm5s2U34 may be different from what has been suggested previously.

Poly(4-thiouridylic acid) as messenger RNA and its application for photoaffinity labelling of the ribosomal mRNA binding site

Poly(4-thiouridylic acid) [poly(s4U)] synthesized by polymerization of 4-thiouridine 5'-diphosphate with Escherichia coli polynucleotide phosphorylase (EC 2.7.7.8) acts as messenger RNA in vitro in a protein-synthesizing system from E. coli. It stimulates binding of Phe-tRNA to ribosomes both in the presence of EF-Tu-Ts at 5 mM Mg2+ concentration and nonenzymatically at 20 mM Mg2+ concentration. It codes for the synthesis of polyphenylalanine. Poly(s4U) competes with poly(U) for binding to E. coli ribosomes. Light of 330 nm photoactivates poly(s4U) thus making it a useful photoaffinity label for the ribosomal mRNA binding site. Upon irradiation of 70-S ribosomal complexes, photoreaction occurs with ribosomal proteins as well as 16-S RNA. Ribosomes pre-incubated with R17 RNA are protected against the photoaffinity reaction. The labelling of 16-S RNA can be reduced by treatment of ribosomes with colicin E3.

The stereochemical basis of template function

The behavior of nucelotides with thioketo-substituted pyrimidine bases (4-thiouracil, 2-thiouracil and 2-thiocytosine) or amino-analogue purine bases (2-aminopurine and 2,6-diaminopurine) in transcription and translation was investigated. The experimental results obtained led to the following conclusions. 1. The stereochemical basis of substrate selection in transcription is the geometry of Watson-Crick base pairs A-U (or A-T) and G-C between substrate and template bases. 2. The topology of the active site of Escherichia coli RNA polymerase is precisely adopted to the geometry of Watson-Crick base pairs. 3. The enzyme active site discriminates between A-U (A-T) and G-C base pairs. An essential feature in this discrimination is the 6-NH2 group of the A-U (A-T) base pair and the 2-keto group of cytosine in the G-C base pair. 4. The codon properties of a nucleic acid base in messenger RNA can be predicted on the basis of its specificity in polynucleotide interactions. There seems to be no evidence for the participation of protein topological sites in the control of the specificity of codon-anticodon interactions in translation.

Physical and coding properties of poly(5-aminouridylic acid) and of 5-aminouridine-containing trinucleotides

This report concerns the synthesis of poly(5-aminouridylic acid) and of 5-aminouridine-containing trinucleotides. Starting from 5-aminouridine the nucleoside 5'-phosphate was prepared enzymatically with carrot phosphotransferase whereas the nucleoside 5'-diphosphate was prepared chemically and polymerised with polynucleotide phosphorylase. The aminouridine-containing trinucleotides were prepared by known enzymatic procedures. Besides an increase of stability in the secondary structure poly(nh25U) forms a triple-stranded complex with poly(A) and stimulates the poly(Phe) synthesis like poly(U). In contrast to U-nh25U-U, the triplet containing the 3'-terminal aminouridine does not stimulate the binding of Phe-tRNA to 70-S ribosomes. This behavior is discussed with respect to the influence of a modification on the stacking geometry of a codon and the base pairing scheme between the 5'-nucleotide of the anticodon and the 3'-nucleotide of the condon.