The FANCI/FANCD2 complex links DNA damage response to R-loop regulation through SRSF1-mediated mRNA export.

Fanconi anemia (FA) is characterized by congenital abnormalities, bone marrow failure, and cancer susceptibility. The central FA protein complex FANCI/FANCD2 (ID2) is activated by monoubiquitination and recruits DNA repair proteins for interstrand crosslink (ICL) repair and replication fork protection. Defects in the FA pathway lead to R-loop accumulation, which contributes to genomic instability. Here, we report that the splicing factor SRSF1 and FANCD2 interact physically and act together to suppress R-loop formation via mRNA export regulation. We show that SRSF1 stimulates FANCD2 monoubiquitination in an RNA-dependent fashion. In turn, FANCD2 monoubiquitination proves crucial for the assembly of the SRSF1-NXF1 nuclear export complex and mRNA export. Importantly, several SRSF1 cancer-associated mutants fail to interact with FANCD2, leading to inefficient FANCD2 monoubiquitination, decreased mRNA export, and R-loop accumulation. We propose a model wherein SRSF1 and FANCD2 interaction links DNA damage response to the avoidance of pathogenic R-loops via regulation of mRNA export.

Metagenomics harvested genus-specific single-stranded DNA-annealing proteins improve and expand recombineering in Pseudomonas species.

The widespread Pseudomonas genus comprises a collection of related species with remarkable abilities to degrade plastics and polluted wastes and to produce a broad set of valuable compounds, ranging from bulk chemicals to pharmaceuticals. Pseudomonas possess characteristics of tolerance and stress resistance making them valuable hosts for industrial and environmental biotechnology. However, efficient and high-throughput genetic engineering tools have limited metabolic engineering efforts and applications. To improve their genome editing capabilities, we first employed a computational biology workflow to generate a genus-specific library of potential single-stranded DNA-annealing proteins (SSAPs). Assessment of the library was performed in different Pseudomonas using a high-throughput pooled recombinase screen followed by Oxford Nanopore NGS analysis. Among different active variants with variable levels of allelic replacement frequency (ARF), efficient SSAPs were found and characterized for mediating recombineering in the four tested species. New variants yielded higher ARFs than existing ones in Pseudomonas putida and Pseudomonas aeruginosa, and expanded the field of recombineering in Pseudomonas taiwanensisand Pseudomonas fluorescens. These findings will enhance the mutagenesis capabilities of these members of the Pseudomonas genus, increasing the possibilities for biotransformation and enhancing their potential for synthetic biology applications. .

Extracellular vesicles biogenesis, isolation, manipulation and genetic engineering for potential in vitro and in vivo therapeutics: An overview.

The main goals of medicine consist of early detection and effective treatment of different diseases. In this regard, the rise of exosomes as carriers of natural biomarkers has recently attracted a lot of attention and managed to shed more light on the future of early disease diagnosis methods. Here, exosome biogenesis, its role as a biomarker in metabolic disorders, and recent advances in state-of-art technologies for exosome detection and isolation will be reviewed along with future research directions and challenges regarding the manipulation and genetic engineering of exosomes for potential in vitro and in vivo disease diagnosis approaches.

Optimization of Triarylpyridinone Inhibitors of the Main Protease of SARS-CoV-2 to Low-Nanomolar Antiviral Potency.

Non-covalent inhibitors of the main protease (Mpro) of SARS-CoV-2 having a pyridinone core were previously reported with IC50 values as low as 0.018 µM for inhibition of enzymatic activity and EC50 values as low as 0.8 µM for inhibition of viral replication in Vero E6 cells. The series has now been further advanced by consideration of placement of substituted five-membered-ring heterocycles in the S4 pocket of Mpro and N-methylation of a uracil ring. Free energy perturbation calculations provided guidance on the choice of the heterocycles, and protein crystallography confirmed the desired S4 placement. Here we report inhibitors with EC50 values as low as 0.080 µM, while remdesivir yields values of 0.5-2 µM in side-by-side testing with infectious SARS-CoV-2. A key factor in the improvement is enhanced cell permeability, as reflected in PAMPA measurements. Compounds 19 and 21 are particularly promising as potential therapies for COVID-19, featuring IC50 values of 0.044-0.061 µM, EC50 values of ca. 0.1 µM, good aqueous solubility, and no cytotoxicity.

Potent Noncovalent Inhibitors of the Main Protease of SARS-CoV-2 from Molecular Sculpting of the Drug Perampanel Guided by Free Energy Perturbation Calculations.

Starting from our previous finding of 14 known drugs as inhibitors of the main protease (Mpro) of SARS-CoV-2, the virus responsible for COVID-19, we have redesigned the weak hit perampanel to yield multiple noncovalent, nonpeptidic inhibitors with ca. 20 nM IC50 values in a kinetic assay. Free-energy perturbation (FEP) calculations for Mpro-ligand complexes provided valuable guidance on beneficial modifications that rapidly delivered the potent analogues. The design efforts were confirmed and augmented by determination of high-resolution X-ray crystal structures for five analogues bound to Mpro. Results of cell-based antiviral assays further demonstrated the potential of the compounds for treatment of COVID-19. In addition to the possible therapeutic significance, the work clearly demonstrates the power of computational chemistry for drug discovery, especially FEP-guided lead optimization.

Binding of FANCI-FANCD2 Complex to RNA and R-Loops Stimulates Robust FANCD2 Monoubiquitination.

Fanconi anemia (FA) is characterized by developmental abnormalities, bone marrow failure, and cancer predisposition. FA cells are hypersensitive to DNA replicative stress and accumulate co-transcriptional R-loops. Here, we use the Damage At RNA Transcription assay to reveal colocalization of FANCD2 with R-loops in a highly transcribed genomic locus upon DNA damage. We further demonstrate that highly purified human FANCI-FANCD2 (ID2) complex binds synthetic single-stranded RNA (ssRNA) and R-loop substrates with high affinity, preferring guanine-rich sequences. Importantly, we elucidate that human ID2 binds an R-loop structure via recognition of the displaced ssDNA and ssRNA but not the RNA:DNA hybrids. Finally, a series of RNA and R-loop substrates are found to strongly stimulate ID2 monoubiquitination, with activity corresponding to their binding affinity. In summary, our results support a mechanism whereby the ID2 complex suppresses the formation of pathogenic R-loops by binding ssRNA and ssDNA species, thereby activating the FA pathway.

ROS-induced R loops trigger a transcription-coupled but BRCA1/2-independent homologous recombination pathway through CSB.

Actively transcribed regions of the genome are protected by transcription-coupled DNA repair mechanisms, including transcription-coupled homologous recombination (TC-HR). Here we used reactive oxygen species (ROS) to induce and characterize TC-HR at a transcribed locus in human cells. As canonical HR, TC-HR requires RAD51. However, the localization of RAD51 to damage sites during TC-HR does not require BRCA1 and BRCA2, but relies on RAD52 and Cockayne Syndrome Protein B (CSB). During TC-HR, RAD52 is recruited by CSB through an acidic domain. CSB in turn is recruited by R loops, which are strongly induced by ROS in transcribed regions. Notably, CSB displays a strong affinity for DNA:RNA hybrids in vitro, suggesting that it is a sensor of ROS-induced R loops. Thus, TC-HR is triggered by R loops, initiated by CSB, and carried out by the CSB-RAD52-RAD51 axis, establishing a BRCA1/2-independent alternative HR pathway protecting the transcribed genome.

RAD52 is required for RNA-templated recombination repair in post-mitotic neurons.

It has been long assumed that post-mitotic neurons only utilize the error-prone non-homologous end-joining pathway to repair double-strand breaks (DSBs) associated with oxidative damage to DNA, given the inability of non-replicating neuronal DNA to utilize a sister chromatid template in the less error-prone homologous recombination (HR) repair pathway. However, we and others have found recently that active transcription triggers a replication-independent recombinational repair mechanism in G0/G1 phase of the cell cycle. Here we observed that the HR repair protein RAD52 is recruited to sites of DNA DSBs in terminally differentiated, post-mitotic neurons. This recruitment is dependent on the presence of a nascent mRNA generated during active transcription, providing evidence that an RNA-templated HR repair mechanism exists in non-dividing, terminally differentiated neurons. This recruitment of RAD52 in neurons is decreased by transcription inhibition. Importantly, we found that high concentrations of amyloid ß, a toxic protein associated with Alzheimer's disease, inhibits the expression and DNA damage response of RAD52, potentially leading to a defect in the error-free, RNA-templated HR repair mechanism. This study shows a novel RNA-dependent repair mechanism of DSBs in post-mitotic neurons and demonstrates that defects in this pathway may contribute to neuronal genomic instability and consequent neurodegenerative phenotypes such as those seen in Alzheimer's disease.

Overhang polarity of chromosomal double-strand breaks impacts kinetics and fidelity of yeast non-homologous end joining.

Non-homologous end joining (NHEJ) is the main repair pathway for DNA double-strand breaks (DSBs) in cells with limited 5' resection. To better understand how overhang polarity of chromosomal DSBs affects NHEJ, we made site-specific 5'-overhanging DSBs (5' DSBs) in yeast using an optimized zinc finger nuclease at an efficiency that approached HO-induced 3' DSB formation. When controlled for the extent of DSB formation, repair monitoring suggested that chromosomal 5' DSBs were rejoined more efficiently than 3' DSBs, consistent with a robust recruitment of NHEJ proteins to 5' DSBs. Ligation-mediated qPCR revealed that Mre11-Rad50-Xrs2 rapidly modified 5' DSBs and facilitated protection of 3' DSBs, likely through recognition of overhang polarity by the Mre11 nuclease. Next-generation sequencing revealed that NHEJ at 5' DSBs had a higher mutation frequency, and validated the differential requirement of Pol4 polymerase at 3' and 5' DSBs. The end processing enzyme Tdp1 did not impact joining fidelity at chromosomal 5' DSBs as in previous plasmid studies, although Tdp1 was recruited to only 5' DSBs in a Ku-independent manner. These results suggest distinct DSB handling based on overhang polarity that impacts NHEJ kinetics and fidelity through differential recruitment and action of DSB modifying enzymes.

Yeast DNA ligase IV mutations reveal a nonhomologous end joining function of BRCT1 distinct from XRCC4/Lif1 binding.

LIG4/Dnl4 is the DNA ligase that (re)joins DNA double-strand breaks (DSBs) via nonhomologous end joining (NHEJ), an activity supported by binding of its tandem BRCT domains to the ligase accessory protein XRCC4/Lif1. We screened a panel of 88 distinct ligase mutants to explore the structure­function relationships of the yeast Dnl4 BRCT domains and inter-BRCT linker in NHEJ. Screen results suggested two distinct classes of BRCT mutations with differential effects on Lif1 interaction as compared to NHEJ completion. Validated constructs confirmed that D800K and GG(868:869)AA mutations, which target the Lif1 binding interface, showed a severely defective Dnl4­Lif1 interaction but a less consistent and often small decrease in NHEJ activity in some assays, as well as nearly normal levels of Dnl4 accumulation at DSBs. In contrast, mutants K742A and KTT(742:744)ATA, which target the ß3-α2 region of the first BRCT domain, substantially decreased NHEJ function commensurate with a large defect in Dnl4 recruitment to DSBs, despite a comparatively greater preservation of the Lif1 interaction. Together, these separation-of-function mutants indicate that Dnl4 BRCT1 supports DSB recruitment and NHEJ in a manner distinct from Lif1 binding and reveal a complexity of Dnl4 BRCT domain functions in support of stable DSB association.

In vivo formation of double-stranded T-DNA molecules by T-strand priming.

During plant genetic transformation, Agrobacterium transfers a single-stranded DNA (T-strand) into the host cell. Increasing evidence suggests that double-stranded (ds) T-DNA, converted from T-strands, are potent substrates for integration. Nevertheless, the molecular mechanism governing T-strand conversion to dsT-DNA is unknown. Integrated T-DNA molecules typically exhibit deletions at their 3' end as compared with their 5' end. We hypothesize that this may result from asymmetric polymerization of T-DNA's ends. Here we show that ß-glucuronidase (GUS) expression from sense T-strands is more efficient than from antisense T-strands, supporting asymmetric conversion. Co-transfection with two partially complementary, truncated GUS-encoding T-strands results in GUS expression, which suggests functional hybridization of the T-strands via complementary annealing and supports the notion that T-strands can anneal with primers. Indeed, red fluorescent protein (RFP) expression from mutated T-strand can be restored by delivery of synthetic DNA and RNA oligonucleotides with partial wild-type RFP sequence, implying the involvement of plant DNA repair machinery.

Saccharomyces cerevisiae DNA ligase IV supports imprecise end joining independently of its catalytic activity.

DNA ligase IV (Dnl4 in budding yeast) is a specialized ligase used in non-homologous end joining (NHEJ) of DNA double-strand breaks (DSBs). Although point and truncation mutations arise in the human ligase IV syndrome, the roles of Dnl4 in DSB repair have mainly been examined using gene deletions. Here, Dnl4 catalytic point mutants were generated that were severely defective in auto-adenylation in vitro and NHEJ activity in vivo, despite being hyper-recruited to DSBs and supporting wild-type levels of Lif1 interaction and assembly of a Ku- and Lif1-containing complex at DSBs. Interestingly, residual levels of especially imprecise NHEJ were markedly higher in a deletion-based assay with Dnl4 catalytic mutants than with a gene deletion strain, suggesting a role of DSB-bound Dnl4 in supporting a mode of NHEJ catalyzed by a different ligase. Similarly, next generation sequencing of repair joints in a distinct single-DSB assay showed that dnl4-K466A mutation conferred a significantly different imprecise joining profile than wild-type Dnl4 and that such repair was rarely observed in the absence of Dnl4. Enrichment of DNA ligase I (Cdc9 in yeast) at DSBs was observed in wild-type as well as dnl4 point mutant strains, with both Dnl4 and Cdc9 disappearing from DSBs upon 5' resection that was unimpeded by the presence of catalytically inactive Dnl4. These findings indicate that Dnl4 can promote mutagenic end joining independently of its catalytic activity, likely by a mechanism that involves Cdc9.

Repair of double-strand breaks by end joining.

Nonhomologous end joining (NHEJ) refers to a set of genome maintenance pathways in which two DNA double-strand break (DSB) ends are (re)joined by apposition, processing, and ligation without the use of extended homology to guide repair. Canonical NHEJ (c-NHEJ) is a well-defined pathway with clear roles in protecting the integrity of chromosomes when DSBs arise. Recent advances have revealed much about the identity, structure, and function of c-NHEJ proteins, but many questions exist regarding their concerted action in the context of chromatin. Alternative NHEJ (alt-NHEJ) refers to more recently described mechanism(s) that repair DSBs in less-efficient backup reactions. There is great interest in defining alt-NHEJ more precisely, including its regulation relative to c-NHEJ, in light of evidence that alt-NHEJ can execute chromosome rearrangements. Progress toward these goals is reviewed.

Zinc finger nuclease and homing endonuclease-mediated assembly of multigene plant transformation vectors.

Binary vectors are an indispensable component of modern Agrobacterium tumefaciens-mediated plant genetic transformation systems. A remarkable variety of binary plasmids have been developed to support the cloning and transfer of foreign genes into plant cells. The majority of these systems, however, are limited to the cloning and transfer of just a single gene of interest. Thus, plant biologists and biotechnologists face a major obstacle when planning the introduction of multigene traits into transgenic plants. Here, we describe the assembly of multitransgene binary vectors by using a combination of engineered zinc finger nucleases (ZFNs) and homing endonucleases. Our system is composed of a modified binary vector that has been engineered to carry an array of unique recognition sites for ZFNs and homing endonucleases and a family of modular satellite vectors. By combining the use of designed ZFNs and commercial restriction enzymes, multiple plant expression cassettes were sequentially cloned into the acceptor binary vector. Using this system, we produced binary vectors that carried up to nine genes. Arabidopsis (Arabidopsis thaliana) protoplasts and plants were transiently and stably transformed, respectively, by several multigene constructs, and the expression of the transformed genes was monitored across several generations. Because ZFNs can potentially be engineered to digest a wide variety of target sequences, our system allows overcoming the problem of the very limited number of commercial homing endonucleases. Thus, users of our system can enjoy a rich resource of plasmids that can be easily adapted to their various needs, and since our cloning system is based on ZFN and homing endonucleases, it may be possible to reconstruct other types of binary vectors and adapt our vectors for cloning on multigene vector systems in various binary plasmids.

Molecular evolution of the E8 promoter in tomato and some of its relative wild species.

The E8 gene is related to ethylene biosynthesis in plants. To explore the effect of the expression pattern of the E8 gene on different E8 promoters, the molecular evolution of E8 promoters was investigated. A total of 16 E8 promoters were cloned from 16 accessions of seven tomato species,and were further analysed. The results from 19 E8 promoters including three previously cloned E8 promoters (X13437,DQ317599 and AF515784) showed that the size of the E8 promoters varied from 2101 bp (LA2150) to 2256 bp (LA2192); their sequences shared 69.9% homology and the average A/T content was 74.9%. Slide-window analysis divided E8 promoters into three regions -A,B and C - and the sequence identity in these regions was 72.5%, 41.2% and 70.8%, respectively. By searching the cis -elements of E8 promoters in the PLACE database, mutant nucleotides were found in some functional elements,and deletions or insertions were also found in regions responsible for ethylene biosysnthesis (-1702 to -1274) and the negative effect region (-1253 to -936). Our results indicate that the size of the functional region for ethylene biosynthesis in the E8 promoter could be shortened from 429 bp to 113 bp (-1612 to -1500). The results of molecular evolution analysis showed that the 19 E8 promoters could be classified into four clade groups, which is basically consistent with evolution of the tomato genome. Southern blot analysis results showed that the copy number of E8 promoters in tomato and some other wild species changed from 1 to 4. Taken together, our study provides important information for further elucidating the E8 gene expression pattern in tomato, analysing functional elements in the E8 promoter and reconstructing the potent E8 promoter.