T4 DNA ligase-mediated RAA coupled with RNA aptamer-driven cascade signal amplification for ultra-sensitive monkeypox virus detection.

Chlorella virus DNA ligase (ChVLig) is an instructive model for mechanistic studies of the ATP-dependent DNA ligase family. ChVLig seals 3'-OH and 5'-PO(4) termini via three chemical steps: 1) ligase attacks the ATP α phosphorus to release PP(i) and form a covalent ligase-adenylate intermediate; 2) AMP is transferred to the nick 5'-phosphate to form DNA-adenylate; 3) the 3'-OH of the nick attacks DNA-adenylate to join the polynucleotides and release AMP. Each chemical step requires Mg(2+). Kinetic analysis of nick sealing by ChVLig-AMP revealed that the rate constant for phosphodiester synthesis (k(step3) = 25 s(-1)) exceeds that for DNA adenylylation (k(step2) = 2.4 s(-1)) and that Mg(2+) binds with similar affinity during step 2 (K(d) = 0.77 mM) and step 3 (K(d) = 0.87 mM). The rates of DNA adenylylation and phosphodiester synthesis respond differently to pH, such that step 3 becomes rate-limiting at pH ≤ 6.5. The pH profiles suggest involvement of one and two protonation-sensitive functional groups in catalysis of steps 2 and 3, respectively. We suggest that the 5'-phosphate of the nick is the relevant protonation-sensitive moiety and that a dianionic 5'-phosphate is necessary for productive step 2 catalysis. Motif VI, located at the C terminus of the OB-fold domain of ChVLig, is a conserved feature of ATP-dependent DNA ligases and GTP-dependent mRNA capping enzymes. Presteady state and burst kinetic analysis of the effects of deletion and missense mutations highlight the catalytic contributions of ChVLig motif VI, especially the Asp-297 carboxylate, exclusively during the ligase adenylylation step.

The spread of the monkeypox virus (MPXV) from Central and West Africa to previously non-endemic regions has caused a global panic. In this context, the rapid, specific, and ultrasensitive detection of MPXV is crucial to contain its spread, though such technology has seldom been reported. Herein, we proposed an MPXV assay combining recombinase-aided amplification (RAA) and CRISPR/Cas12a. This assay targeted the highly conserved MPXV F3L gene and demonstrates a low detection limit (LOD) of 101 copies per μL. By leveraging the high specificity nature of RAA and CRISPR/Cas12a, we rationally optimized probes and conditions to achieve high selectivity that differentiates MPXV from other orthopox viruses and current high-profile viruses. To facilitate on-site screening of potential MPXV carriers, a kit integrating lateral flow strips was developed, enabling naked-eye MPXV detection with a LOD of 104 copies per μL. Our RAA-Cas12a-MPXV assay was able to detect MPXV without the need for sophisticated operation and expensive equipment. We believe that this assay can be rapidly deployed in emerging viral outbreaks for on-site surveillance of MPXV.

Monkeypox: A global threat to domestic and wild animals – Correspondence

The joining of duplex DNA at base-paired ends by bacteriophage T4 DNA ligase was confirmed using either a synthetic duplex decamer or restriction endonuclease fragments of ColE1 DNA as substrates. The reaction was not linearly dependent on enzyme concentration but increased markedly at high enzyme concentrations. Although T4 RNA ligase did not catalyze this blunt end joining, it makedly stimulated the DNA ligase reaction particularly at low DNA ligase concentrations. The apparent Km for the decamer was 50 micronM in the presence or absence of RNA ligase. In the presence of RNA ligase, T4 DNA ligase had about the same turnover number for blunt end and cohesive end joining. The joining of duplex DNA at base-paired ends was proven by several techniques including restriction endonuclease cleavage of the products. The products of the ligation reaction using restriction enzyme fragments were mostly linear oligomers but included some circular duplexes. Escherichia coli DNA ligase in the presence or absence of RNA ligase did not catalyze blunt end joining. RNA ligase only moderately affected the joining of cohesive ends by T4 DNA ligase or E. coli DNA ligase and did not itself catalyze this reaction.

Binding of Nucleotides by T4 DNA Ligase and T4 RNA Ligase: Optical Absorbance and Fluorescence Studies

DNA ligases seal 5'-PO4 and 3'-OH polynucleotide ends via three nucleotidyl transfer steps involving ligase-adenylate and DNA-adenylate intermediates. DNA ligases are essential guardians of genomic integrity, and ligase dysfunction underlies human genetic disease syndromes. Crystal structures of DNA ligases bound to nucleotide and nucleic acid substrates have illuminated how ligase reaction chemistry is catalyzed, how ligases recognize damaged DNA ends, and how protein domain movements and active-site remodeling are used to choreograph the end-joining pathway. Although a shared feature of DNA ligases is their envelopment of the nicked duplex as a C-shaped protein clamp, they accomplish this feat by using remarkably different accessory structural modules and domain topologies. As structural, biochemical, and phylogenetic insights coalesce, we can expect advances on several fronts, including (i) pharmacological targeting of ligases for antibacterial and anticancer therapies and (ii) the discovery and design of new strand-sealing enzymes with unique substrate specificities.

The lack of antiviral drugs for the treatment of orthopoxvirus disease represents an unmet medical need, particularly due to the threat of variola virus (the causative agent of smallpox) as an agent of biowarfare or bioterrorism (Henderson, 283:1279-1282, 1999). In addition to variola, monkeypox, cowpox, and vaccinia viruses are orthopoxviruses of concern to human health (Lewis-Jones, 17:81-89, 2004). Smallpox vaccination, using the closely related vaccinia virus, is no longer provided to the general public leading to a worldwide population increasingly susceptible not only to variola but to monkeypox, cowpox, and vaccinia viruses as well. Orthopoxviruses share similar life cycles (Fenner et al., WHO, Geneva, 1988), and significant nucleotide and protein homology, and are immunologically cross-protective against other species within the genus, which was the basis of the highly successful vaccinia virus vaccine. These similarities also serve as the basis for screening for antivirals for dangerous pathogens such as variola and monkeypox virus using generally safer viruses such as cowpox and vaccinia. Methods for preliminary screening and initial characterization of potential orthopoxvirus antivirals in vitro, using vaccinia virus as a relatively safe surrogate for more pathogenic orthopoxviruses, are described herein. They include candidate identification in a viral cytopathic effect (CPE) assay as well as evaluation of the antiviral activity in inhibition assays to determine mean effective (or inhibitory) concentrations (EC50 or IC50). These assays were utilized in the identification and early characterization of tecovirimat (ST-246) (Yang et al., 79:13,139-13,149, 2005). These initial steps in identifying and characterizing the antiviral activity should be followed up with additional in vitro studies including specificity testing (for other orthopoxviruses and against other viruses), single-cycle growth curves, time of addition assays, cytotoxicity testing, and identification of the drug target.

Whole Genome Amplification of Plasma-Circulating DNA Enables Expanded Screening for Allelic Imbalance in Plasma

DNA ligases catalyze the repair of phosphate backbone breaks in DNA, acting with highest activity on breaks in one strand of duplex DNA. Some DNA ligases have also been observed to ligate two DNA fragments with short complementary overhangs or blunt-ended termini. In this study, several wild-type DNA ligases (phage T3, T4, and T7 DNA ligases, Paramecium bursaria chlorella virus 1 (PBCV1) DNA ligase, human DNA ligase 3, and Escherichia coli DNA ligase) were tested for their ability to ligate DNA fragments with several difficult to ligate end structures (blunt-ended termini, 3′- and 5′- single base overhangs, and 5′-two base overhangs). This analysis revealed that T4 DNA ligase, the most common enzyme utilized for in vitro ligation, had its greatest activity on blunt- and 2-base overhangs, and poorest on 5′-single base overhangs. Other ligases had different substrate specificity: T3 DNA ligase ligated only blunt ends well; PBCV1 DNA ligase joined 3′-single base overhangs and 2-base overhangs effectively with little blunt or 5′- single base overhang activity; and human ligase 3 had highest activity on blunt ends and 5′-single base overhangs. There is no correlation of activity among ligases on blunt DNA ends with their activity on single base overhangs. In addition, DNA binding domains (Sso7d, hLig3 zinc finger, and T4 DNA ligase N-terminal domain) were fused to PBCV1 DNA ligase to explore whether modified binding to DNA would lead to greater activity on these difficult to ligate substrates. These engineered ligases showed both an increased binding affinity for DNA and increased activity, but did not alter the relative substrate preferences of PBCV1 DNA ligase, indicating active site structure plays a role in determining substrate preference.

Monkeypox virus (MPXV) outbreak in non-endemic countries is a worldwide public health emergency. An enveloped double-stranded DNA virus belongs to the genus Orth poxvirus. A viral zoonotic infection known as monkeypox has been a serious risk to public health, especially in Africa. However, it has recently spread to other continents, so it might soon become a worldwide problem. There is an increased risk of transmission of the virus because there is a lack of effective treatment that cures the disease. To stop the multi-country outbreak from spreading, it is important to discover effective medications urgently. The objective of the current study is to swiftly find new treatments for the monkeypox virus using advanced computational approaches. By investigating five potential MPXV targets (DNA ligase, Palmytilated Extracellular Enveloped Virus (EEV) membrane protein, Scaffold protein D13, Thymidylate Kinase, and Viral core cysteine proteinase), this research was carried out using cutting-edge computational techniques against human monkeypox virus infection. Here we present the accurate 3D structures and their binding cavities of the selected targets with higher confidence using AlphaFold 2 and SiteMap analysis. Molecular docking and MD simulation analysis revealed the top five potential lead compounds with higher binding affinity and stability toward selected targets. Binding free energy calculations and other essential dynamics analysis supports the finding. The selected lead compounds utilizing virtual screening and drug repurposing approach reported in this study are beneficial for medical scientists and experimental biologists in drug development for the treatment of human MPXV.Communicated by Ramaswamy H. Sarma

The ability of DNA repair enzymes to carry out excision repair of pyrimidine dimers in SV40 minichromosomes irradiated with 16 to 64 J/m2 of UV light was examined. Half of the dimers were substrate for the DNA glycosylase activity of phage T4 UV endonuclease immediately after irradiation, but this limit decreased to 27% after 2 h at 0 degrees C. Moreover, the apyrimidinic (AP) endonuclease activity of the enzyme did not incise all of the AP sites created by glycosylase activity, although all AP sites were substrate for HeLa AP endonuclease II. The initial rate of the glycosylase was 40% that upon DNA. After incision by the T4 enzyme, excision was mediated by HeLa DNase V (acting with an exonuclease present in the chromatin preparation). Under physiological salt conditions, excision did not proceed appreciably beyond the damaged nucleotides in DNA or chromatin. With chromatin, about 70% of the accessible dimers were removed, but at a rate slower than for DNA. Finally, HeLa DNA polymerase beta was able to fill the short gaps created after dimer excision, and these patches were sealed by T4 DNA ligase. Overall, roughly 30% of the sites incised by the endonuclease were ultimately sealed by the ligase. The resistance of some sites was due to interference with the ligase by the chromatin structure, as only 30-40% of the nicks created in chromatin by pancreatic DNase could be sealed by T4 or HeLa DNA ligases. The overall excision repair process did not detectably disrupt the chromatin structure, since the repair label was recovered in Form I DNA present in 75 S condensed minichromosomes. Although other factors might stimulate the rate of this repair process, it appears that the enzymes utilized could carry out excision repair of chromatin to a limit near that observed at the initial rate in mammalian cells in vivo.

The Mg(2+)-dependent adenylylation of the T4 DNA and RNA ligases was studied in the absence of a DNA substrate using transient optical absorbance and fluorescence spectroscopy. The concentrations of Mg(2+), ATP, and pyrophosphate were systematically varied, and the results led to the conclusion that the nucleotidyl transfer proceeds according to a two-metal ion mechanism. According to this mechanism, only the di-magnesium-coordinated form Mg(2)ATP(0) reacts with the enzyme forming the covalent complex E.AMP. The reverse reaction (ATP synthesis) occurs between the mono-magnesium-coordinated pyrophosphate form MgP(2)O(7)(2-) and the enzyme.MgAMP complex. The nucleotide binding rate decreases in the sequence ATP(4-) > MgATP(2-) > Mg(2)ATP(0), indicating that the formation of the non-covalent enzyme.nucleotide complex is driven by electrostatic interactions. T4 DNA ligase shows notably higher rates of ATP binding and of subsequent adenylylation compared with RNA ligase, in part because it decreases the K(d) of Mg(2+) for the enzyme-bound Mg(2)ATP(0) more than 10-fold. To elucidate the role of Mg(2+) in the nucleotidyl transfer catalyzed by T4 DNA and RNA ligases, we propose a transition state configuration, in which the catalytic Mg(2+) ion coordinates to both reacting nucleophiles: the lysyl moiety of the enzyme that forms the phosphoramidate bond and the alpha-beta-bridging oxygen of ATP.

In the bacteriophage T4 DNA replication system, T4 RNase H removes the RNA primers and some adjacent DNA before the lagging strand fragments are ligated. This 5'-nuclease has strong structural and functional similarity to the FEN1 nuclease family. We have shown previously that T4 32 protein binds DNA behind the nuclease and increases its processivity. Here we show that T4 RNase H with a C-terminal deletion (residues 278-305) retains its exonuclease activity but is no longer affected by 32 protein. T4 gene 45 replication clamp stimulates T4 RNase H on nicked or gapped substrates, where it can be loaded behind the nuclease, but does not increase its processivity. An N-terminal deletion (residues 2-10) of a conserved clamp interaction motif eliminates stimulation by the clamp. In the crystal structure of T4 RNase H, the binding sites for the clamp at the N terminus and for 32 protein at the C terminus are located close together, away from the catalytic site of the enzyme. By using mutant T4 RNase H with deletions in the binding site for either the clamp or 32 protein, we show that it is the interaction of T4 RNase H with 32 protein, rather than the clamp, that most affects the maturation of lagging strand fragments in the T4 replication system in vitro and T4 phage production in vivo.

Repair of mtDNA in Vertebrates

DNA ligase is required for encapsidation of bacteriophage T4 DNA