1
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Tor Y. Isomorphic Fluorescent Nucleosides. Acc Chem Res 2024; 57:1325-1335. [PMID: 38613490 PMCID: PMC11079976 DOI: 10.1021/acs.accounts.4c00042] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2024] [Revised: 04/01/2024] [Accepted: 04/02/2024] [Indexed: 04/15/2024]
Abstract
In 1960, Weber prophesied that "There are many ways in which the properties of the excited state can be utilized to study points of ignorance of the structure and function of proteins". This has been realized, illustrating that an intrinsic and highly responsive fluorophore such as tryptophan can alter the course of an entire scientific discipline. But what about RNA and DNA? Adapting Weber's protein photophysics prophecy to nucleic acids requires the development of intrinsically emissive nucleoside surrogates as, unlike Trp, the canonical nucleobases display unusually low emission quantum yields, which render nucleosides, nucleotides, and oligonucleotides practically dark for most fluorescence-based applications.Over the past decades, we have developed emissive nucleoside surrogates that facilitate the monitoring of nucleoside-, nucleotide-, and nucleic acid-based transformations at a nucleobase resolution in real time. The premise underlying our approach is the identification of minimal atomic/structural perturbations that endow the synthetic analogs with favorable photophysical features while maintaining native conformations and pairing. As illuminating probes, the photophysical parameters of such isomorphic nucleosides display sensitivity to microenvironmental factors. Responsive isomorphic analogs that function similarly to their native counterparts in biochemical contexts are defined as isofunctional.Early analogs included pyrimidines substituted with five-membered aromatic heterocycles at their 5 position and have been used to assess the polarity of the major groove in duplexes. Polarized quinazolines have proven useful in assembling FRET pairs with established fluorophores and have been used to study RNA-protein and RNA-small-molecule binding. Completing a fluorescent ribonucleoside alphabet, composed of visibly emissive purine (thA, thG) and pyrimidine (thU, thC) analogs, all derived from thieno[3,4-d]pyrimidine as the heterocyclic nucleus, was a major breakthrough. To further augment functionality, a second-generation emissive RNA alphabet based on an isothiazolo[4,3-d]pyrimidine core (thA, tzG, tzU, and tzC) was fabricated. This single-atom "mutagenesis" restored the basic/coordinating nitrogen corresponding to N7 in the purine skeleton and elevated biological recognition.The isomorphic emissive nucleosides and nucleotides, particularly the purine analogs, serve as substrates for diverse enzymes. Beyond polymerases, we have challenged the emissive analogs with metabolic and catabolic enzymes, opening optical windows into the biochemistry of nucleosides and nucleotides as metabolites as well as coenzymes and second messengers. Real-time fluorescence-based assays for adenosine deaminase, guanine deaminase, and cytidine deaminase have been fabricated and used for inhibitor discovery. Emissive cofactors (e.g., SthAM), coenzymes (e.g., NtzAD+), and second messengers (e.g., c-di-tzGMP) have been enzymatically synthesized, using xyNTPs and native enzymes. Both their biosynthesis and their transformations can be fluorescently monitored in real time.Highly isomorphic and isofunctional emissive surrogates can therefore be fabricated and judiciously implemented. Beyond their utility, side-by-side comparison to established analogs, particularly to 2-aminopurine, the workhorse of nucleic acid biophysics over 5 decades, has proven prudent as they refined the scope and limitations of both the new analogs and their predecessors. Challenges, however, remain. Associated with such small heterocycles are relatively short emission wavelengths and limited brightness. Recent advances in multiphoton spectroscopy and further structural modifications have shown promise for overcoming such barriers.
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Affiliation(s)
- Yitzhak Tor
- Department of Chemistry and
Biochemistry, University of California,
San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States
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2
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Dutta N, Deb I, Sarzynska J, Lahiri A. Inosine and its methyl derivatives: Occurrence, biogenesis, and function in RNA. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2022; 169-170:21-52. [PMID: 35065168 DOI: 10.1016/j.pbiomolbio.2022.01.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/11/2021] [Revised: 12/11/2021] [Accepted: 01/11/2022] [Indexed: 05/21/2023]
Abstract
Inosine is one of the most common post-transcriptional modifications. Since its discovery, it has been noted for its ability to contribute to non-Watson-Crick interactions within RNA. Rapidly accumulating evidence points to the widespread generation of inosine through hydrolytic deamination of adenosine to inosine by different classes of adenosine deaminases. Three naturally occurring methyl derivatives of inosine, i.e., 1-methylinosine, 2'-O-methylinosine and 1,2'-O-dimethylinosine are currently reported in RNA modification databases. These modifications are expected to lead to changes in the structure, folding, dynamics, stability and functions of RNA. The importance of the modifications is indicated by the strong conservation of the modifying enzymes across organisms. The structure, binding and catalytic mechanism of the adenosine deaminases have been well-studied, but the underlying mechanism of the catalytic reaction is not very clear yet. Here we extensively review the existing data on the occurrence, biogenesis and functions of inosine and its methyl derivatives in RNA. We also included the structural and thermodynamic aspects of these modifications in our review to provide a detailed and integrated discussion on the consequences of A-to-I editing in RNA and the contribution of different structural and thermodynamic studies in understanding its role in RNA. We also highlight the importance of further studies for a better understanding of the mechanisms of the different classes of deamination reactions. Further investigation of the structural and thermodynamic consequences and functions of these modifications in RNA should provide more useful information about their role in different diseases.
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Affiliation(s)
- Nivedita Dutta
- Department of Biophysics, Molecular Biology and Bioinformatics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata, 700009, West Bengal, India
| | - Indrajit Deb
- Department of Biophysics, Molecular Biology and Bioinformatics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata, 700009, West Bengal, India
| | - Joanna Sarzynska
- Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704, Poznan, Poland
| | - Ansuman Lahiri
- Department of Biophysics, Molecular Biology and Bioinformatics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata, 700009, West Bengal, India.
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3
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Thuy-Boun AS, Thomas JM, Grajo HL, Palumbo CM, Park S, Nguyen LT, Fisher AJ, Beal PA. Asymmetric dimerization of adenosine deaminase acting on RNA facilitates substrate recognition. Nucleic Acids Res 2020; 48:7958-7972. [PMID: 32597966 PMCID: PMC7641318 DOI: 10.1093/nar/gkaa532] [Citation(s) in RCA: 32] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2020] [Revised: 06/09/2020] [Accepted: 06/24/2020] [Indexed: 12/20/2022] Open
Abstract
Adenosine deaminases acting on RNA (ADARs) are enzymes that convert adenosine to inosine in duplex RNA, a modification that exhibits a multitude of effects on RNA structure and function. Recent studies have identified ADAR1 as a potential cancer therapeutic target. ADARs are also important in the development of directed RNA editing therapeutics. A comprehensive understanding of the molecular mechanism of the ADAR reaction will advance efforts to develop ADAR inhibitors and new tools for directed RNA editing. Here we report the X-ray crystal structure of a fragment of human ADAR2 comprising its deaminase domain and double stranded RNA binding domain 2 (dsRBD2) bound to an RNA duplex as an asymmetric homodimer. We identified a highly conserved ADAR dimerization interface and validated the importance of these sequence elements on dimer formation via gel mobility shift assays and size exclusion chromatography. We also show that mutation in the dimerization interface inhibits editing in an RNA substrate-dependent manner for both ADAR1 and ADAR2.
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Affiliation(s)
| | - Justin M Thomas
- Department of Chemistry, University of California, Davis, CA, USA
| | - Herra L Grajo
- Department of Chemistry, University of California, Davis, CA, USA
| | - Cody M Palumbo
- Department of Chemistry, University of California, Davis, CA, USA
| | - SeHee Park
- Department of Chemistry, University of California, Davis, CA, USA
| | - Luan T Nguyen
- Department of Chemistry, University of California, Davis, CA, USA
| | - Andrew J Fisher
- Department of Chemistry, University of California, Davis, CA, USA
- Department of Molecular and Cellular Biology, University of California, Davis, CA, USA
| | - Peter A Beal
- Department of Chemistry, University of California, Davis, CA, USA
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4
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Monteleone LR, Matthews MM, Palumbo CM, Thomas JM, Zheng Y, Chiang Y, Fisher AJ, Beal PA. A Bump-Hole Approach for Directed RNA Editing. Cell Chem Biol 2019; 26:269-277.e5. [PMID: 30581135 PMCID: PMC6386613 DOI: 10.1016/j.chembiol.2018.10.025] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2018] [Revised: 09/28/2018] [Accepted: 10/26/2018] [Indexed: 12/17/2022]
Abstract
Molecules capable of directing changes to nucleic acid sequences are powerful tools for molecular biology and promising candidates for the therapeutic correction of disease-causing mutations. However, unwanted reactions at off-target sites complicate their use. Here we report selective combinations of mutant editing enzyme and directing oligonucleotide. Mutations in human ADAR2 (adenosine deaminase acting on RNA 2) that introduce aromatic amino acids at position 488 reduce background RNA editing. This residue is juxtaposed to the nucleobase that pairs with the editing site adenine, suggesting a steric clash for the bulky mutants. Replacing this nucleobase with a hydrogen atom removes the clash and restores editing activity. A crystal structure of the E488Y mutant bound to abasic site-containing RNA shows the accommodation of the tyrosine side chain. Finally, we demonstrate directed RNA editing in vitro and in human cells using mutant ADAR2 proteins and modified guide RNAs with reduced off-target activity.
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Affiliation(s)
- Leanna R Monteleone
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Melissa M Matthews
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Cody M Palumbo
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Justin M Thomas
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Yuxuan Zheng
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Yao Chiang
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Andrew J Fisher
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA; Department of Molecular and Cellular Biology, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Peter A Beal
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA.
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5
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Abstract
Inosine is one of the most common modifications found in human RNAs and the Adenosine Deaminases that act on RNA (ADARs) are the main enzymes responsible for its production. ADARs were first discovered in the 1980s and since then our understanding of ADARs has advanced tremendously. For instance, it is now known that defective ADAR function can cause human diseases. Furthermore, recently solved crystal structures of the human ADAR2 deaminase bound to RNA have provided insights regarding the catalytic and substrate recognition mechanisms. In this chapter, we describe the occurrence of inosine in human RNAs and the newest perspective on the ADAR family of enzymes, including their substrate recognition, catalytic mechanism, regulation as well as the consequences of A-to-I editing, and their relation to human diseases.
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6
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Rovira AR, Fin A, Tor Y. Expanding a fluorescent RNA alphabet: synthesis, photophysics and utility of isothiazole-derived purine nucleoside surrogates. Chem Sci 2017; 8:2983-2993. [PMID: 28451365 PMCID: PMC5380116 DOI: 10.1039/c6sc05354h] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2016] [Accepted: 01/26/2017] [Indexed: 01/09/2023] Open
Abstract
A series of emissive ribonucleoside purine mimics, all comprised of an isothiazolo[4,3-d]pyrimidine core, was prepared using a divergent pathway involving a key Thorpe-Ziegler cyclization. In addition to an adenosine and a guanosine mimic, analogues of the noncanonical xanthosine, isoguanosine, and 2-aminoadenosine were also synthesized and found to be emissive. Isothiazolo 2-aminoadenosine, an adenosine surrogate, was found to be particularly emissive and effectively deaminated by adenosine deaminase. Competitive studies with adenosine deaminase with each analogue in combination with native adenosine showed preference for the native substrate while still deaminating the isothiazolo analogues.
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Affiliation(s)
- Alexander R Rovira
- Department of Chemistry and Biochemistry , University of California , San Diego , La Jolla , California 92093-0358 , USA .
| | - Andrea Fin
- Department of Chemistry and Biochemistry , University of California , San Diego , La Jolla , California 92093-0358 , USA .
| | - Yitzhak Tor
- Department of Chemistry and Biochemistry , University of California , San Diego , La Jolla , California 92093-0358 , USA .
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7
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Wang Y, Beal PA. Probing RNA recognition by human ADAR2 using a high-throughput mutagenesis method. Nucleic Acids Res 2016; 44:9872-9880. [PMID: 27614075 PMCID: PMC5175354 DOI: 10.1093/nar/gkw799] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2016] [Revised: 08/30/2016] [Accepted: 08/31/2016] [Indexed: 01/05/2023] Open
Abstract
Adenosine deamination is one of the most prevalent post-transcriptional modifications in mRNA. In humans, ADAR1 and ADAR2 catalyze this modification and their malfunction correlates with disease. Recently our laboratory reported crystal structures of the human ADAR2 deaminase domain bound to duplex RNA revealing a protein loop that binds the RNA on the 5′ side of the modification site. This 5′ binding loop appears to be one contributor to substrate specificity differences between ADAR family members. In this study, we endeavored to reveal detailed structure–activity relationships in this loop to advance our understanding of RNA recognition by ADAR2. To achieve this goal, we established a high-throughput mutagenesis approach which allows rapid screening of ADAR variants in single yeast cells and provides quantitative evaluation for enzymatic activity. Using this approach, we determined the importance of specific amino acids at 19 different positions in the ADAR2 5′ binding loop and revealed six residues that provide essential structural elements supporting the fold of the loop and key RNA-binding functional groups. This work provided new insight into RNA recognition by ADAR2 and established a new tool for defining structure–function relationships in ADAR reactions.
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Affiliation(s)
- Yuru Wang
- Department of Chemistry, University of California, One Shields Ave, Davis, CA 95616, USA
| | - Peter A Beal
- Department of Chemistry, University of California, One Shields Ave, Davis, CA 95616, USA
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8
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Structures of human ADAR2 bound to dsRNA reveal base-flipping mechanism and basis for site selectivity. Nat Struct Mol Biol 2016; 23:426-33. [PMID: 27065196 PMCID: PMC4918759 DOI: 10.1038/nsmb.3203] [Citation(s) in RCA: 138] [Impact Index Per Article: 17.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2016] [Accepted: 03/11/2016] [Indexed: 12/20/2022]
Abstract
ADARs (adenosine deaminases acting on RNA) are editing enzymes that convert adenosine (A) to inosine (I) in duplex RNA, a modification reaction with wide-ranging consequences on RNA function. Our understanding of the ADAR reaction mechanism, origin of editing site selectivity and effect of mutations is limited by the lack of high-resolution structural data for complexes of ADARs bound to substrate RNAs. Here we describe four crystal structures of the deaminase domain of human ADAR2 bound to RNA duplexes bearing a mimic of the deamination reaction intermediate. These structures, together with structure-guided mutagenesis and RNA-modification experiments, explain the basis for ADAR deaminase domain’s dsRNA specificity, its base-flipping mechanism, and nearest neighbor preferences. In addition, an ADAR2-specific RNA-binding loop was identified near the enzyme active site rationalizing differences in selectivity observed between different ADARs. Finally, our results provide a structural framework for understanding the effects of ADAR mutations associated with human disease.
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9
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Wang Y, Havel J, Beal PA. A Phenotypic Screen for Functional Mutants of Human Adenosine Deaminase Acting on RNA 1. ACS Chem Biol 2015; 10:2512-9. [PMID: 26372505 DOI: 10.1021/acschembio.5b00711] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Adenosine deaminases acting on RNA (ADARs) are RNA-editing enzymes responsible for the conversion of adenosine to inosine at specific locations in cellular RNAs. ADAR1 and ADAR2 are two members of the family that have been shown to be catalytically active. Earlier, we reported a phenotypic screen for the study of human ADAR2 using budding yeast S. cerevisiae as the host system. While this screen has been successfully applied to the study of ADAR2, it failed with ADAR1. Here, we report a new reporter that uses a novel editing substrate and is suitable for the study of ADAR1. We screened plasmid libraries with randomized codons for two important residues in human ADAR1 (G1007 and E1008). The screening results combined with in vitro deamination assays led to the identification of mutants that are more active than the wild type protein. Furthermore, a screen of the ADAR1 E1008X library with a reporter construct bearing an A•G mismatch at the editing site suggests one role for the residue at position 1008 is to sense the identity of the base pairing partner for the editing site adenosine. This work has provided a starting point for future in vitro evolution studies of ADAR1 and led to new insight into ADAR's editing site selectivity.
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Affiliation(s)
- Yuru Wang
- Department of Chemistry, University of California, Davis, 1 Shields Ave, Davis, California 95616, United States
| | - Jocelyn Havel
- Department of Chemistry, University of California, Davis, 1 Shields Ave, Davis, California 95616, United States
| | - Peter A. Beal
- Department of Chemistry, University of California, Davis, 1 Shields Ave, Davis, California 95616, United States
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10
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Mizrahi RA, Shin D, Sinkeldam RW, Phelps KJ, Fin A, Tantillo DJ, Tor Y, Beal PA. A Fluorescent Adenosine Analogue as a Substrate for an A-to-I RNA Editing Enzyme. Angew Chem Int Ed Engl 2015; 54:8713-6. [PMID: 26095193 PMCID: PMC4532316 DOI: 10.1002/anie.201502070] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2015] [Revised: 04/21/2015] [Indexed: 12/14/2022]
Abstract
Adenosine to inosine RNA editing catalyzed by ADAR enzymes is common in humans, and altered editing is associated with disease. Experiments using substrate RNAs with adenosine analogues at editing sites are useful for defining features of the ADAR reaction mechanism. The reactivity of ADAR2 was evaluated with RNA containing the emissive adenosine analogue thieno[3,4-d]-6-aminopyrimidine ((th)A). This nucleoside was incorporated into a mimic of the glutamate receptor B (GluR B) mRNA R/G editing site. We found that (th)A is recognized by AMV reverse transcriptase as A, and is deaminated rapidly by human ADAR2 to give (th)I. Importantly, ADAR reaction progress can be monitored by following the deamination-induced change in fluorescence of the (th)A-modified RNA. The observed high (th)A reactivity adds to our understanding of the structural features that are necessary for an efficient hADAR2 reaction. Furthermore, the new fluorescent assay is expected to accelerate mechanistic studies of ADARs.
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Affiliation(s)
- Rena A Mizrahi
- Department of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616 (USA)
| | - Dongwon Shin
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 (USA)
| | - Renatus W Sinkeldam
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 (USA)
| | - Kelly J Phelps
- Department of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616 (USA)
| | - Andrea Fin
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 (USA)
| | - Dean J Tantillo
- Department of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616 (USA)
| | - Yitzhak Tor
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 (USA).
| | - Peter A Beal
- Department of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616 (USA).
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11
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Wierzchowski J, Antosiewicz JM, Shugar D. 8-Azapurines as isosteric purine fluorescent probes for nucleic acid and enzymatic research. MOLECULAR BIOSYSTEMS 2015; 10:2756-74. [PMID: 25124808 DOI: 10.1039/c4mb00233d] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
The 8-azapurines, and their 7-deaza and 9-deaza congeners, represent a unique class of isosteric (isomorphic) analogues of the natural purines, frequently capable of substituting for the latter in many biochemical processes. Particularly interesting is their propensity to exhibit pH-dependent room-temperature fluorescence in aqueous medium, and in non-polar media. We herein review the physico-chemical properties of this class of compounds, with particular emphasis on the fluorescence emission properties of their neutral and/or ionic species, which has led to their widespread use as fluorescent probes in enzymology, including enzymes involved in purine metabolism, agonists/antagonists of adenosine receptors, mechanisms of catalytic RNAs, RNA editing, etc. They are also exceptionally useful fluorescent probes for analytical and clinical applications in crude cell homogenates.
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Affiliation(s)
- Jacek Wierzchowski
- Department of Biophysics, University of Varmia & Masuria, Oczapowskiego 4, 10-719 Olsztyn, Poland.
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12
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Mizrahi RA, Shin D, Sinkeldam RW, Phelps KJ, Fin A, Tantillo DJ, Tor Y, Beal PA. A Fluorescent Adenosine Analogue as a Substrate for an A-to-I RNA Editing Enzyme. Angew Chem Int Ed Engl 2015. [DOI: 10.1002/ange.201502070] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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13
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Phelps KJ, Tran K, Eifler T, Erickson AI, Fisher AJ, Beal PA. Recognition of duplex RNA by the deaminase domain of the RNA editing enzyme ADAR2. Nucleic Acids Res 2015; 43:1123-32. [PMID: 25564529 PMCID: PMC4333395 DOI: 10.1093/nar/gku1345] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Adenosine deaminases acting on RNA (ADARs) hydrolytically deaminate adenosines (A) in a wide variety of duplex RNAs and misregulation of editing is correlated with human disease. However, our understanding of reaction selectivity is limited. ADARs are modular enzymes with multiple double-stranded RNA binding domains (dsRBDs) and a catalytic domain. While dsRBD binding is understood, little is known about ADAR catalytic domain/RNA interactions. Here we use a recently discovered RNA substrate that is rapidly deaminated by the isolated human ADAR2 deaminase domain (hADAR2-D) to probe these interactions. We introduced the nucleoside analog 8-azanebularine (8-azaN) into this RNA (and derived constructs) to mechanistically trap the protein–RNA complex without catalytic turnover for EMSA and ribonuclease footprinting analyses. EMSA showed that hADAR2-D requires duplex RNA and is sensitive to 2′-deoxy substitution at nucleotides opposite the editing site, the local sequence and 8-azaN nucleotide positioning on the duplex. Ribonuclease V1 footprinting shows that hADAR2-D protects ∼23 nt on the edited strand around the editing site in an asymmetric fashion (∼18 nt on the 5′ side and ∼5 nt on the 3′ side). These studies provide a deeper understanding of the ADAR catalytic domain–RNA interaction and new tools for biophysical analysis of ADAR–RNA complexes.
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Affiliation(s)
- Kelly J Phelps
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Kiet Tran
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Tristan Eifler
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Anna I Erickson
- Department of Molecular and Cellular Biology, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Andrew J Fisher
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA Department of Molecular and Cellular Biology, University of California, One Shields Avenue, Davis, CA 95616, USA
| | - Peter A Beal
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
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14
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Phelps KJ, Ibarra-Soza JM, Tran K, Fisher AJ, Beal PA. Click modification of RNA at adenosine: structure and reactivity of 7-ethynyl- and 7-triazolyl-8-aza-7-deazaadenosine in RNA. ACS Chem Biol 2014; 9:1780-7. [PMID: 24896732 PMCID: PMC4136661 DOI: 10.1021/cb500270x] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Ribonucleoside analogues bearing terminal alkynes, including 7-ethynyl-8-aza-7-deazaadenosine (7-EAA), are useful for RNA modification applications. However, although alkyne- and triazole-bearing ribonucleosides are in widespread use, very little information is available on the impact of these modifications on RNA structure. By solving crystal structures for RNA duplexes containing these analogues, we show that, like adenosine, 7-EAA and a triazole derived from 7-EAA base pair with uridine and are well-accommodated within an A-form helix. We show that copper-catalyzed azide/alkyne cycloaddition (CuAAC) reactions with 7-EAA are sensitive to the RNA secondary structure context, with single-stranded sites reacting faster than duplex sites. 7-EAA and its triazole products are recognized in RNA template strands as adenosine by avian myoblastosis virus reverse transcriptase. In addition, 7-EAA in RNA is a substrate for an active site mutant of the RNA editing adenosine deaminase, ADAR2. These studies extend our understanding of the impact of these novel nucleobase analogues and set the stage for their use in probing RNA structure and metabolism.
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Affiliation(s)
- Kelly J. Phelps
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
| | - José M. Ibarra-Soza
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
| | - Kiet Tran
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
| | - Andrew J. Fisher
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
| | - Peter A. Beal
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
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15
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Eifler T, Pokharel S, Beal PA. RNA-Seq analysis identifies a novel set of editing substrates for human ADAR2 present in Saccharomyces cerevisiae. Biochemistry 2013; 52:7857-69. [PMID: 24124932 DOI: 10.1021/bi4006539] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
ADAR2 is a member of a family of RNA editing enzymes found in metazoa that bind double helical RNAs and deaminate select adenosines. We find that when human ADAR2 is overexpressed in the budding yeast Saccharomyces cerevisiae it substantially reduces the rate of cell growth. This effect is dependent on the deaminase activity of the enzyme, suggesting yeast transcripts are edited by ADAR2. Characterization of this novel set of RNA substrates provided a unique opportunity to gain insight into ADAR2's site selectivity. We used RNA-Seq. to identify transcripts present in S. cerevisiae subject to ADAR2-catalyzed editing. From this analysis, we identified 17 adenosines present in yeast RNAs that satisfied our criteria for candidate editing sites. Substrates identified include both coding and noncoding RNAs. Subsequent Sanger sequencing of RT-PCR products from yeast total RNA confirmed efficient editing at a subset of the candidate sites including BDF2 mRNA, RL28 intron RNA, HAC1 3'UTR RNA, 25S rRNA, U1 snRNA, and U2 snRNA. Two adenosines within the U1 snRNA sequence not identified as substrates during the original RNA-Seq. screen were shown to be deaminated by ADAR2 during the follow-up analysis. In addition, examination of the RNA sequence surrounding each edited adenosine in this novel group of ADAR2 sites revealed a previously unrecognized sequence preference. Remarkably, rapid deamination at one of these sites (BDF2 mRNA) does not require ADAR2's dsRNA-binding domains (dsRBDs). Human glioma-associated oncogene 1 (GLI1) mRNA is a known ADAR2 substrate with similar flanking sequence and secondary structure to the yeast BDF2 site discovered here. As observed with the BDF2 site, rapid deamination at the GLI1 site does not require ADAR2's dsRBDs.
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Affiliation(s)
- Tristan Eifler
- Department of Chemistry, University of California , One Shields Avenue, Davis, California 95616, United States
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Mizrahi RA, Schirle NT, Beal PA. Potent and selective inhibition of A-to-I RNA editing with 2'-O-methyl/locked nucleic acid-containing antisense oligoribonucleotides. ACS Chem Biol 2013; 8:832-9. [PMID: 23394403 DOI: 10.1021/cb300692k] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
ADARs (adenosine deaminases acting on RNA) are RNA editing enzymes that bind double helical RNAs and deaminate select adenosines (A). The product inosine (I) is read during translation as guanosine (G), so such changes can alter codon meaning. ADAR-catalyzed A to I changes occur in coding sequences for several proteins of importance to the nervous system. However, these sites constitute only a very small fraction of known A to I sites in the human transcriptome, and the significance of editing at the vast majority sites is unknown at this time. Site-selective inhibitors of RNA editing are needed to advance our understanding of the function of editing at specific sites. Here we show that 2'-O-methyl/locked nucleic acid (LNA) mixmer antisense oligonucleotides are potent and selective inhibitors of RNA editing on two different target RNAs. These reagents are capable of binding with high affinity to RNA editing substrates and remodeling the secondary structure by a strand-invasion mechanism. The potency observed here for 2'-O-methyl/LNA mixmers suggests this backbone structure is superior to the morpholino backbone structure for inhibition of RNA editing. Finally, we demonstrate antisense inhibition of editing of the mRNA for the DNA repair glycosylase NEIL1 in cultured human cells, providing a new approach to exploring the link between RNA editing and the cellular response to oxidative DNA damage.
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Affiliation(s)
- Rena A. Mizrahi
- Department of Chemistry, University of California, Davis, California 95616, United States
| | - Nicole T. Schirle
- Department of Chemistry, University of California, Davis, California 95616, United States
| | - Peter A. Beal
- Department of Chemistry, University of California, Davis, California 95616, United States
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17
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Garncarz W, Tariq A, Handl C, Pusch O, Jantsch MF. A high-throughput screen to identify enhancers of ADAR-mediated RNA-editing. RNA Biol 2013; 10:192-204. [PMID: 23353575 PMCID: PMC3594278 DOI: 10.4161/rna.23208] [Citation(s) in RCA: 61] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Adenosine to inosine deamination of RNA is widespread in metazoa. Inosines are recognized as guanosines and, therefore, this RNA-editing can influence the coding potential, localization and stability of RNAs. Therefore, RNA editing contributes to the diversification of the transcriptome in a flexible manner. The editing reaction is performed by adenosine deaminases that act on RNA (ADARs), which are essential for normal life and development in many organisms. Changes in editing levels are observed during development but also in neurological pathologies like schizophrenia, depression or tumors. Frequently, changes in editing levels are not reflected by changes in ADAR levels suggesting a regulation of enzyme activity. Until now, only a few factors are known that influence the activity of ADARs. Here we present a two-stage in vivo editing screen aimed to isolate enhancers of editing. A primary, high-throughput yeast-screen is combined with a more accurate secondary screen in mammalian cells that uses a fluorescent read-out to detect minor differences in RNA-editing. The screen was successfully employed to identify DSS1/SHFM1, the RNA binding protein hnRNP A2/B1 and a 3′ UTR as enhancers of editing. By varying intracellular DSS1/SHFM1 levels, we can modulate A to I editing by up to 30%. Proteomic analysis indicates an interaction of DSS1/SHFM1 and hnRNP A2/B1 suggesting that both factors may act by altering the cellular RNP landscape. An extension of this screen to cDNAs from different tissues or developmental stages may prove useful for the identification of additional enhancers of RNA-editing.
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Affiliation(s)
- Wojciech Garncarz
- Department of Chromosome Biology, Max F. Perutz Laboratories, University of Vienna, Vienna, Austria
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18
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Eifler T, Chan D, Beal PA. A screening protocol for identification of functional mutants of RNA editing adenosine deaminases. CURRENT PROTOCOLS IN CHEMICAL BIOLOGY 2012; 4:357-69. [PMID: 23788559 PMCID: PMC3690185 DOI: 10.1002/9780470559277.ch120139] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Genetic screens can be used to evaluate a spectrum of mutations and thereby infer the function of particular residues within a protein. The Adenosine Deaminase Acting on RNA (ADAR) family of RNA-editing enzymes selectively deaminate adenosines (A) in double-helical RNA, generating inosine (I). The protocol described here exploits the editing activity of ADAR2 in a yeast-based screen by inserting an editing substrate sequence with a stop codon incorporated at the editing site upstream from the sequence encoding the reporter α-galactosidase. A-to-I editing changes the stop codon to a tryptophan codon, allowing normal expression of the reporter. This technique is particularly well-suited for screening ADAR and ADAR substrate mutant libraries for editing activity. Curr. Protoc. Chem. Biol. 4:357-369 © 2012 by John Wiley & Sons, Inc.
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Affiliation(s)
- Tristan Eifler
- Department of Chemistry, University of California, Davis
| | - Dalen Chan
- Department of Chemistry, University of California, Davis
| | - Peter A. Beal
- Department of Chemistry, University of California, Davis
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Mizrahi RA, Phelps KJ, Ching AY, Beal PA. Nucleoside analog studies indicate mechanistic differences between RNA-editing adenosine deaminases. Nucleic Acids Res 2012; 40:9825-35. [PMID: 22885375 PMCID: PMC3479202 DOI: 10.1093/nar/gks752] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
Adenosine deaminases acting on RNA (ADAR1 and ADAR2) are human RNA-editing adenosine deaminases responsible for the conversion of adenosine to inosine at specific locations in cellular RNAs. Since inosine is recognized during translation as guanosine, this often results in the expression of protein sequences different from those encoded in the genome. While our knowledge of the ADAR2 structure and catalytic mechanism has grown over the years, our knowledge of ADAR1 has lagged. This is due, at least in part, to the lack of well defined, small RNA substrates useful for mechanistic studies of ADAR1. Here, we describe an ADAR1 substrate RNA that can be prepared by a combination of chemical synthesis and enzymatic ligation. Incorporation of adenosine analogs into this RNA and analysis of the rate of ADAR1 catalyzed deamination revealed similarities and differences in the way the ADARs recognize the edited nucleotide. Importantly, ADAR1 is more dependent than ADAR2 on the presence of N7 in the edited base. This difference between ADAR1 and ADAR2 appears to be dependent on the identity of a single amino acid residue near the active site. Thus, this work provides an important starting point in defining mechanistic differences between two functionally distinct human RNA editing ADARs.
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Affiliation(s)
- Rena A Mizrahi
- Department of Chemistry, University of California, Davis, CA 95616, USA
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20
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Ibarra-Soza JM, Morris AA, Jayalath P, Peacock H, Conrad WE, Donald MB, Kurth MJ, Beal PA. 7-Substituted 8-aza-7-deazaadenosines for modification of the siRNA major groove. Org Biomol Chem 2012; 10:6491-7. [PMID: 22766576 DOI: 10.1039/c2ob25647a] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Here we describe the synthesis of new 7-substituted 8-aza-7-deazaadenosine ribonucleoside phosphoramidites and their use in generating major groove-modified duplex RNAs. A 7-ethynyl analog leads to further structural diversification of the RNA via post-automated RNA synthesis azide-alkyne cycloaddition reactions. In addition, we report preliminary studies on the effects of eight different purine 7-position modifications on RNA duplex stability and pairing specificity. Finally, the effect on RNAi activity of this type of modification at eight different positions in an siRNA guide strand has been explored. Analogs were identified with large 7-position substituents that maintain adenosine pairing specificity and are well-tolerated at specific positions in an siRNA guide strand.
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Affiliation(s)
- José M Ibarra-Soza
- Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, USA
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21
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Abstract
The past several years have seen numerous reports of new chemical modifications for use in RNA. In addition, in that time period, we have seen the discovery of several previously unknown naturally occurring modifications that impart novel properties on the parent RNAs. In this review, we describe recent discoveries in these areas with a focus on RNA modifications that introduce spectroscopic tags, reactive handles, or new recognition properties.
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Affiliation(s)
- Kelly Phelps
- Department
of Chemistry, University of California, Davis, California 95616, United States
| | - Alexi Morris
- Department
of Chemistry, University of California, Davis, California 95616, United States
| | - Peter A. Beal
- Department
of Chemistry, University of California, Davis, California 95616, United States
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Hernández AR, Kool ET. The components of xRNA: synthesis and fluorescence of a full genetic set of size-expanded ribonucleosides. Org Lett 2011; 13:676-9. [PMID: 21214222 PMCID: PMC3039074 DOI: 10.1021/ol102915f] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The synthesis and properties of a full set of four benzo-expanded ribonucleosides (xRNA), analogous to A, G, C, and U RNA monomers, are described. The nucleosides are efficient fluorophores with emission maxima of 369-411 nm. The compounds are expected to be useful as RNA pathway probes and as components of an unnatural ribopolymer.
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Affiliation(s)
| | - Eric T. Kool
- Department of Chemistry, Stanford University, Stanford, California 94305-5080
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Abstract
Since the discovery of the adenosine deaminase (ADA) acting on RNA (ADAR) family of proteins in 1988 (Bass and Weintraub, Cell 55:1089-1098, 1988) (Wagner et al. Proc Natl Acad Sci U S A 86:2647-2651, 1989), we have learned much about their structure and catalytic mechanism. However, much about these enzymes is still unknown, particularly regarding the selective recognition and processing of specific adenosines within substrate RNAs. While a crystal structure of the catalytic domain of human ADAR2 has been solved, we still lack structural data for an ADAR catalytic domain bound to RNA, and we lack any structural data for other ADARs. However, by analyzing the structural data that is available along with similarities to other deaminases, mutagenesis and other biochemical experiments, we have been able to advance the understanding of how these fascinating enzymes function.
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Balakrishnan L, Polaczek P, Pokharel S, Campbell JL, Bambara RA. Dna2 exhibits a unique strand end-dependent helicase function. J Biol Chem 2010; 285:38861-8. [PMID: 20929864 DOI: 10.1074/jbc.m110.165191] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Dna2 endonuclease/helicase participates in eukaryotic DNA transactions including cleavage of long flaps generated during Okazaki fragment processing. Its unusual substrate interaction consists of recognition and binding of the flap base, then threading over the 5'-end of the flap, and cleaving periodically to produce a terminal product ∼5 nt in length. Blocking the 5'-end prevents cleavage. The Dna2 ATP-driven 5' to 3' DNA helicase function promotes motion of Dna2 on the flap, presumably aiding its nuclease function. Here we demonstrate using two different nuclease-dead Dna2 mutants that on substrates simulating Okazaki fragments, Dna2 must thread onto an unblocked 5' flap to display helicase activity. This requirement is maintained on substrates with single-stranded regions thousands of nucleotides in length. To our knowledge this is the first description of a eukaryotic helicase that cannot load onto its tracking strand internally but instead must enter from the end. Biologically, the loading requirement likely helps the helicase to coordinate with the Dna2 nuclease function to prevent creation of undesirably long flaps during DNA transactions.
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Affiliation(s)
- Lata Balakrishnan
- Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA
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