Asymmetry of 13C labeled 3-pyruvate affords improved site specific labeling of RNA for NMR spectroscopy.

Selective isotopic labeling provides an unparalleled window within which to study the structure and dynamics of RNAs by high resolution NMR spectroscopy. Unlike commonly used carbon sources, the asymmetry of (13)C-labeled pyruvate provides selective labeling in both the ribose and base moieties of nucleotides using Escherichia coli variants, that until now were not feasible. Here we show that an E. coli mutant strain that lacks succinate and malate dehydrogenases (DL323) and grown on [3-(13)C]-pyruvate affords ribonucleotides with site specific labeling at C5' (~95%) and C1' (~42%) and minimal enrichment elsewhere in the ribose ring. Enrichment is also achieved at purine C2 and C8 (~95%) and pyrimidine C5 (~100%) positions with minimal labeling at pyrimidine C6 and purine C5 positions. These labeling patterns contrast with those obtained with DL323 E. coli grown on [1, 3-(13)C]-glycerol for which the ribose ring is labeled in all but the C4' carbon position, leading to multiplet splitting of the C1', C2' and C3' carbon atoms. The usefulness of these labeling patterns is demonstrated with a 27-nt RNA fragment derived from the 30S ribosomal subunit. Removal of the strong magnetic coupling within the ribose and base leads to increased sensitivity, substantial simplification of NMR spectra, and more precise and accurate dynamic parameters derived from NMR relaxation measurements. Thus these new labels offer valuable probes for characterizing the structure and dynamics of RNA that were previously limited by the constraint of uniformly labeled nucleotides.

Biomass production of site selective 13C/15N nucleotides using wild type and a transketolase E. coli mutant for labeling RNA for high resolution NMR.

Characterization of the structure and dynamics of nucleic acids by NMR benefits significantly from position specifically labeled nucleotides. Here an E. coli strain deficient in the transketolase gene (tktA) and grown on glucose that is labeled at different carbon sites is shown to facilitate cost-effective and large scale production of useful nucleotides. These nucleotides are site specifically labeled in C1' and C5' with minimal scrambling within the ribose ring. To demonstrate the utility of this labeling approach, the new site-specific labeled and the uniformly labeled nucleotides were used to synthesize a 36-nt RNA containing the catalytically essential domain 5 (D5) of the brown algae group II intron self-splicing ribozyme. The D5 RNA was used in binding and relaxation studies probed by NMR spectroscopy. Key nucleotides in the D5 RNA that are implicated in binding Mg(2+) ions are well resolved. As a result, spectra obtained using selectively labeled nucleotides have higher signal-to-noise ratio compared to those obtained using uniformly labeled nucleotides. Thus, compared to the uniformly (13)C/(15)N-labeled nucleotides, these specifically labeled nucleotides eliminate the extensive (13)C-(13)C coupling within the nitrogenous base and ribose ring, give rise to less crowded and more resolved NMR spectra, and accurate relaxation rates without the need for constant-time or band-selective decoupled NMR experiments. These position selective labeled nucleotides should, therefore, find wide use in NMR analysis of biologically interesting RNA molecules.

Asymmetry of (13)C labeled 3-pyruvate affords improved site specific labeling of RNA for NMR spectroscopy.

Selective isotopic labeling provides an unparalleled window within which to study the structure and dynamics of RNAs by high resolution NMR spectroscopy. Unlike commonly used carbon sources, the asymmetry of (13)C-labeled pyruvate provides selective labeling in both the ribose and base moieties of nucleotides using E. coli variants, that until now were not feasible. Here we show that an E. coli mutant strain that lacks succinate and malate dehydrogenases (DL323) and grown on [3-(13)C]-pyruvate affords ribonucleotides with site specific labeling at C5' (~95%) and C1' (~42%) and minimal enrichment elsewhere in the ribose ring. Enrichment is also achieved at purine C2 and C8 (~95%) and pyrimidine C5 (~100%) positions with minimal labeling at pyrimidine C6 and purine C5 positions. These labeling patterns contrast with those obtained with DL323 E. coli grown on [1, 3-(13)C]-glycerol for which the ribose ring is labeled in all but the C4' carbon position, leading to multiplet splitting of the C1', C2' and C3' carbon atoms. The usefulness of these labeling patterns is demonstrated with a 27-nt RNA fragment derived from the 30S ribosomal subunit. Removal of the strong magnetic coupling within the ribose and base leads to increased sensitivity, substantial simplification of NMR spectra, and more precise and accurate dynamic parameters derived from NMR relaxation measurements. Thus these new labels offer valuable probes for characterizing the structure and dynamics of RNA that were previously limited by the constraint of uniformly labeled nucleotides.

Selective 13C labeling of nucleotides for large RNA NMR spectroscopy using an E. coli strain disabled in the TCA cycle.

Escherichia coli (E. coli) is an ideal organism to tailor-make labeled nucleotides for biophysical studies of RNA. Recently, we showed that adding labeled formate enhanced the isotopic enrichment at protonated carbon sites in nucleotides. In this paper, we show that growth of a mutant E. coli strain DL323 (lacking succinate and malate dehydrogenases) on (13)C-2-glycerol and (13)C-1,3-glycerol enables selective labeling at many useful sites for RNA NMR spectroscopy. For DL323 E. coli grown in (13)C-2-glycerol without labeled formate, all the ribose carbon atoms are labeled except the C3' and C5' carbon positions. Consequently the C1', C2' and C4' positions remain singlet. In addition, only the pyrimidine base C6 atoms are substantially labeled to ~96% whereas the C2 and C8 atoms of purine are labeled to ~5%. Supplementing the growth media with (13)C-formate increases the labeling at C8 to ~88%, but not C2. Not unexpectedly, addition of exogenous formate is unnecessary for attaining the high enrichment levels of ~88% for the C2 and C8 purine positions in a (13)C-1,3-glycerol based growth. Furthermore, the ribose ring is labeled in all but the C4' carbon position, such that the C2' and C3' positions suffer from multiplet splitting but the C5' position remains singlet and the C1' position shows a small amount of residual C1'-C2' coupling. As expected, all the protonated base atoms, except C6, are labeled to ~90%. In addition, labeling with (13)C-1,3-glycerol affords an isolated methylene ribose with high enrichment at the C5' position (~90%) that makes it particularly attractive for NMR applications involving CH(2)-TROSY modules without the need for decoupling the C4' carbon. To simulate the tumbling of large RNA molecules, perdeuterated glycerol was added to a mixture of the four nucleotides, and the methylene TROSY experiment recorded at various temperatures. Even under conditions of slow tumbling, all the expected carbon correlations were observed, which indicates this approach of using nucleotides obtained from DL323 E. coli will be applicable to high molecular weight RNA systems.

Growth of wildtype and mutant E. coli strains in minimal media for optimal production of nucleic acids for preparing labeled nucleotides.

Since RNAs lie at the center of most cellular processes, there is a need for synthesizing large amounts of RNAs made from stable isotope-labeled nucleotides to advance the study of their structure and dynamics by nuclear magnetic resonance (NMR) spectroscopy. A particularly effective means of obtaining labeled nucleotides is to harvest these nucleotides from bacteria grown in defined minimal media supplemented with 15NH4Cl and various carbon sources. Given the high cost of carbon precursors required for labeling nucleic acids for NMR studies, it becomes important to evaluate the optimal growth for commonly used strains under standard minimal media conditions. Such information is lacking. In this study, we characterize the growth for Escherichia coli strains K12, K10zwf, and DL323 in three minimal media with isotopic-labeled carbon sources of acetate, glycerol, and glycerol combined with formate. Of the three media, the LeMaster-Richards and the Studier media outperform the commonly used M9 media and both support optimal growth of E. coli for the production of nucleotides. However, the growth of all three E. coli strains in acetate is reduced almost twofold compared to growth in glycerol. Analysis of the metabolic pathway and previous gene array studies help to explain this differential growth in glycerol and acetate. These studies should benefit efforts to make selective 13C-15N isotopic-labeled nucleotides for synthesizing biologically important RNAs.

Site-specific labeling of nucleotides for making RNA for high resolution NMR studies using an E. coli strain disabled in the oxidative pentose phosphate pathway.

Escherichia coli (E. coli) is a versatile organism for making nucleotides labeled with stable isotopes ((13)C, (15)N, and/or (2)H) for structural and molecular dynamics characterizations. Growth of a mutant E. coli strain deficient in the pentose phosphate pathway enzyme glucose-6-phosphate dehydrogenase (K10-1516) on 2-(13)C-glycerol and (15)N-ammonium sulfate in Studier minimal medium enables labeling at sites useful for NMR spectroscopy. However, (13)C-sodium formate combined with (13)C-2-glycerol in the growth media adds labels to new positions. In the absence of labeled formate, both C5 and C6 positions of the pyrimidine rings are labeled with minimal multiplet splitting due to (1)J(C5C6) scalar coupling. However, the C2/C8 sites within purine rings and the C1'/C3'/C5' positions within the ribose rings have reduced labeling. Addition of (13)C-labeled formate leads to increased labeling at the base C2/C8 and the ribose C1'/C3'/C5' positions; these new specific labels result in two- to three-fold increase in the number of resolved resonances. This use of formate and (15)N-ammonium sulfate promises to extend further the utility of these alternate site specific labels to make labeled RNA for downstream biophysical applications such as structural, dynamics and functional studies of interesting biologically relevant RNAs.

Synthesis of antiproliferative Cephalotaxus esters and their evaluation against several human hematopoietic and solid tumor cell lines: uncovering differential susceptibilities to multidrug resistance.

Deoxyharringtonine (2), homoharringtonine (3), homodeoxyharringtonine (4), and anhydroharringtonine (5) are reported to be among the most potent members of the antileukemia alkaloids isolated from the Cephalotaxus genus. Convergent syntheses of these four natural products are described, each involving novel synthetic methods and strategies. These syntheses enabled evaluation of several advanced natural and non-natural compounds against an array of human hematopoietic and solid tumor cells. Potent cytotoxicity was observed in several cell lines previously not challenged with these alkaloids. Variations in the structure of the ester chain within this family of alkaloids confer differing activity profiles against vincristine-resistant HL-60/RV+, signalling new avenues for molecular design of these natural products to combat multi-drug resistance.

Inhibition of human parainfluenza virus type 3 infection by novel small molecules.

Human parainfluenza virus type 3 (HPIV3) is an important respiratory tract pathogen of infants and children. There are no vaccines or antivirals currently approved for prevention or treatment of HPIV3 infection. Towards developing an antiviral therapy to combat HPIV3 infection, we have established a green fluorescent protein (GFP)-tagged HPIV3 infected-cell assay and used it for screening of a small molecule library obtained from ChemBridge Diver. Two novel small molecules (C5 and C7) which shared structural similarities were identified and their inhibitory effects on HPIV3 were confirmed in CV-1 and human lung epithelium A549 cells by plaque assay, Western blot and Northern blot analyses. C5 and C7 effectively prevented the cytopathic effect in cells infected with HPIV3, achieving IC(50) values of 2.36 microM and 0.08 microM, respectively, for infectious virus production. The inhibition appears to be at the primary transcriptional level of HPIV3 life cycle based on sequential time course test, binding and internalization assays, and finally by a minigenome transcription assay in cells as well as measuring viral transcripts in cells in the presence of anisomycin. Interestingly, vesicular stomatitis virus (VSV), another member of mononegavirales order, was also inhibited by these compounds, whereas poliovirus-a picornavirus was not. Use of these inhibitors has a strong potential to develop novel antiviral agents against this important human pathogen.

Small-molecule activators of RNase L with broad-spectrum antiviral activity.

RNase L, a principal mediator of innate immunity to viral infections in higher vertebrates, is required for a complete IFN antiviral response against certain RNA stranded viruses. dsRNA produced during viral infections activates IFN-inducible synthetases that produce 5'-phosphorylated, 2',5'-oligoadenylates (2-5A) from ATP. 2-5A activates RNase L in a wide range of different mammalian cell types, thus blocking viral replication. However, 2-5A has unfavorable pharmacologic properties; it is rapidly degraded, does not transit cell membranes, and leads to apoptosis. To obtain activators of RNase L with improved drug-like properties, high-throughput screening was performed on chemical libraries by using fluorescence resonance energy transfer. Seven compounds were obtained that activated RNase L at micromolar concentrations, and structure-activity relationship studies resulted in identification of an additional four active compounds. Two lead compounds were shown to have a similar mechanistic path toward RNase L activation as the natural activator 2-5A. The compounds bound to the 2-5A-binding domain of RNase L (as determined by surface plasmon resonance and confirmed by computational docking), and the compounds induced RNase L dimerization and activation. Interestingly, the low-molecular-weight activators of RNase L had broad-spectrum antiviral activity against diverse types of RNA viruses, including the human pathogen human parainfluenza virus type 3, yet these compounds by themselves were not cytotoxic at the effective concentrations. Therefore, these RNase L activators are prototypes for a previously uncharacterized class of broad-spectrum antiviral agents.

Delivery of 2-5A cargo into living cells using the Tat cell penetrating peptide: 2-5A-tat.

2',5'-Oligoadenylate tetramer (2-5A) has been chemically conjugated to short HIV-1 Tat peptides to provide 2-5A-tat chimeras. Two different convergent synthetic approaches have been employed to provide such 2-5A-tat bioconjugates. One involved generation of a bioconjugate through reaction of a cysteine terminated Tat peptide with a alpha-chloroacetyl derivative of 2-5A. The second synthetic strategy was based upon a cycloaddition reaction of an azide derivative of 2-5A with a Tat peptide bearing an alkyne function. Either bioconjugate of 2-5A-tat was able to activate human RNase L. The union of 2-5A and Tat peptide provided an RNase L-active chimeric nucleopeptide with the ability to be taken up by cells by virtue of the Tat peptide and to activate RNase L in intact cells. This strategy provides a valuable vehicle for the entry of the charged 2-5A molecule into cells and may provide a means for targeted destruction of HIV RNA in vivo.

A convenient and sensitive fluorescence resonance energy transfer assay for RNase L and 2',5' oligoadenylates.

Interferon action against viruses is mediated in part through a ribonucleic acid (RNA) decay pathway known as the 2-5A system. Unusual 5'-triphosphorylated, 2',5'-linked oligoadenylates (2-5A) are produced in mammalian cells by interferon-inducible 2-5A synthetases (OAS) in response to viral double-stranded RNA. 2-5A activates a uniquely regulated endoribonuclease, RNase L, resulting in the cleavage of single-stranded viral and cellular RNAs, thus suppressing viral replication. In addition, RNase L was recently identified as a strong candidate for the hereditary prostate cancer 1 susceptibility allele. RNase L is ubiquitously expressed at basal levels in a wide range of mammalian cell types. Conventional RNase L assays, which can be inconvenient and cumbersome, typically involve cleavage of radioactively labeled RNA species or of endogenous ribosomal RNA. Here we describe a convenient, rapid, nonradioactive, and relatively inexpensive fluorescence resonance energy transfer (FRET) that may be used to accurately measure levels of either 2-5A or RNase L activity with a high degree of specificity and sensitivity. The RNA probe used in the FRET assay was designed based on a region of respiratory syncytial genomic RNA. We demonstrate the utility of our FRET assay with several novel biostable analogs of 2-5A.