Structures of African swine fever virus topoisomerase complex and their implications.

African swine fever virus (ASFV) is the causal agent of African swine fever (ASF), which is contagious and highly lethal to domestic pigs and wild boars. The genome of ASFV encodes many proteins important for ASFV life cycle. The functional importance of topoisomerase AsfvTopII has been confirmed by in vivo and in vitro assays, but the structure of AsfvTopII is poorly studied. Here, we report four AsfvTopII complex structures. The ATPase domain structures reveal the detailed basis for ATP binding and hydrolysis, which is shared by AsfvTopII and eukaryotic TopIIs. The DNA-bound structures show that AsfvTopII follows conserved mechanism in G-DNA binding and cleavage. Besides G-DNA, a T-DNA fragment is also captured in one AsfvTopII structure. Mutagenesis and in vitro assays confirm that Pro852 and the T-DNA-binding residue Tyr744 are important for the function of AsfvTopII. Our study not only advances the understanding on the biological function of AsfvTopII, but also provides a solid basis for the development of AsfvTopII-specific inhibitors.

COVID-19-Associated Cerebellar Ataxia: A Case Series.

COVID-19-associated cerebellar ataxia has rarely been reported and its clinical characteristics remain understudied. This study aims to report patients with COVID-19-associated cerebellar ataxia from our institution. COVID-19-associated cerebellar ataxia was diagnosed based on the prodromal COVID-19 infection and the exclusion of other causes. This study provides a summary of the patients' clinical presentations, neuroimaging features, and the results of anti-cerebellar antibody examinations. Our study included 11 patients and 4 were male. The median onset age was 38 years. Five patients also demonstrated signs of encephalopathy. Brain magnetic resonance imaging (MRI) was either unremarkable (n = 6) or showed bilateral cerebellar lesions (n = 5), which were typically transient, although brain atrophy could be observed later in the disease course. Anti-Homer-3 and anti-Yo antibodies were each detected in one patient, respectively. All patients received immunotherapy and nine improved. Compared with the late-onset group, individuals who exhibited ataxia earlier following COVID-19 onset (interval<5 days) were significantly younger [median age 18 (15.5-31) vs. 53.5 (44-64.8) years, p = 0.009] and more likely to present with encephalopathy (5/5 vs. 0/6, p = 0.002).They also experienced more severe symptoms [median modified Rankin scale (mRS) score at zenith 5 (5-5) vs. 2 (1.75-2.75), p = 0.017] and had a less favorable prognosis [median mRS score at the last follow-up 4 (2-5) vs. 1 (0-1.25), p = 0.009]. COVID-19-associated cerebellar ataxia can appear with encephalopathy. Brain MRI may show transient bilateral cerebellar lesions and brain atrophy later. Patients who exhibited ataxia earlier following COVID-19 were younger, had more severe symptoms and poorer outcomes.

A Peculiar Tau Accumulation Pattern Identified Via 18F-Florzolotau PET Imaging in a Patient With Frontotemporal Dementia Caused by a Mutation in the MAPT Gene.

ABSTRACT: We reported imaging findings with a peculiar tau accumulation pattern in a 56 year-old woman with frontotemporal dementia caused by Q351R mutation in the microtubule-associated protein tau (MAPT) gene. She had a 10-year history of gradually worsening memory loss. 18F-FDG PET demonstrated hypometabolism in medial temporal lobes, but 18F-florbetapir PET manifested no abnormal amyloid beta deposition, which ruled out Alzheimer disease. 18F-florzolotau PET showed tau proteins accumulated in medial temporal lobes, basal ganglia brainstem, and cerebellum. The pattern is different from other known MAPT gene mutation. Multitracer imaging can help differentiate between Alzheimer disease and frontotemporal dementia caused by MAPT mutation.

Structural and functional insights into the helicase protein E5 of Mpox virus.

Mpox virus (MPXV) can cause mpox in humans. Due to its quick and wide spread in the past two years, mpox has turned into a significant public health concern. Helicase E5 is a multi-domain protein; its primer synthesis and DNA unwinding activity are required for genome uncoating and DNA replication of MPXV. However, the in vitro DNA unwinding activity has never been demonstrated. Here, we report the structural and biochemical studies of MPXV E5, showing that the full-length protein adopts an auto-inhibited conformation. Truncation of the N-terminus can recover the in vitro unwinding activity of E5 towards the forked DNA. Further structural analysis reveals that MPXV E5 shares a conserved mechanism in DNA unwinding and primer synthesis with the homologous proteins. These findings not only advance our understanding on the function of MPXV E5, but also provide a solid basis for the development of anti-poxvirus drugs.

A novel MAPT variant (E342K) as a cause of familial progressive supranuclear palsy.

Background: MAPT variants are a known cause of frontotemporal dementia and Parkinsonian syndrome, of which progressive supranuclear palsy syndrome (PSP) is a rare manifestation. Objective: To report a novel MAPT variant in a PSP pedigree with autosomal dominant inheritance pattern, and to produce a literature review of PSP patients with MAPT variants. Methods: A comprehensive clinical, genetic, and molecular neuroimaging investigation was conducted on a 61 years-old female proband diagnosed with PSP. We also collected the clinical presentation data and history of the patient's pedigree, and performed further genetic analysis of 4 relatives, from two generations, with and without symptoms. Results: The proband exhibited typical clinical manifestation of PSP. A cranial MRI revealed midbrain atrophy, and an FDG-PET scan suggested hypo-metabolic changes in caudate nucleus, left prefrontal lobe, both temporal poles, and midbrain. 18F-florzolo-tau-PET revealed tau-protein deposits in the thalamus and brainstem bilaterally. A gene test by whole-exome sequencing identified a novel MAPT variant [NM_005910.6, exon 11, c.1024G > A (p.E342K)], and the same variant was also identified in one affected relative and one asymptomatic relative, a probable pre-symptomatic carrier. Conclusion: The PSP pedigree caused by the novel MAPT (E342K) variant, expanded the mutational spectrum of MAPT.

Phase 1 study of safety and preliminary efficacy of intranasal transplantation of human neural stem cells (ANGE-S003) in Parkinson's disease.

BACKGROUND: Intranasal transplantation of ANGE-S003 human neural stem cells showed therapeutic effects and were safe in preclinical models of Parkinson's disease (PD). We investigated the safety and tolerability of this treatment in patients with PD and whether these effects would be apparent in a clinical trial. METHODS: This was a 12-month, single-centre, open-label, dose-escalation phase 1 study of 18 patients with advanced PD assigned to four-time intranasal transplantation of 1 of 3 doses: 1.5 million, 5 million or 15 million of ANGE-S003 human neural stem cells to evaluate their safety and efficacy. RESULTS: 7 patients experienced a total of 14 adverse events in the 12 months of follow-up after treatment. There were no serious adverse events related to ANGE-S003. Safety testing disclosed no safety concerns. Brain MRI revealed no mass formation. In 16 patients who had 12-month Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) data, significant improvement of MDS-UPDRS total score was observed at all time points (p<0.001), starting with month 3 and sustained till month 12. The most substantial improvement was seen at month 6 with a mean reduction of 19.9 points (95% CI, 9.6 to 30.3; p<0.001). There was no association between improvement in clinical outcome measures and cell dose levels. CONCLUSIONS: Treatment with ANGE-S003 is feasible, generally safe and well tolerated, associated with functional improvement in clinical outcomes with peak efficacy achieved at month 6. Intranasal transplantation of neural stem cells represents a new avenue for the treatment of PD, and a larger, longer-term, randomised, controlled phase 2 trial is warranted for further investigation.

Identifying Mixed Dementia With Lewy Bodies and Alzheimer Disease Using Multitracer PET Imaging: A Case Study.

ABSTRACT: We reported imaging findings with complex signs that were corresponded with both dementia with Lewy bodies (DLB) and Alzheimer disease (AD) in the case of a 78-year-old woman. Initially suspected as DLB due to cognitive and movement issues, diagnostic support included the cingulate island sign on 18 F-FDG PET, positive 131 I-MIBG cardiac scintigraphy, and DAT PET. However, MRI indicated hippocampal atrophy, and 18 F-FDG PET showed hypometabolism in the medial temporal lobe, suggesting the possibility of concomitant AD. Subsequent detection of ß-amyloid pathology and tau accumulation in the brain further supported the concurrent presence of AD pathology.

Diagnostic Superiority of 18 F-FDG PET Over MRI in Detecting Anti-LGI1 Autoimmune Encephalitis : A Comparative Study With Insights Into Faciobrachial Dystonic Seizures Mechanisms and Recurrence Identification.

OBJECTIVE: Our study aimed to investigate the utility of 18 F-FDG PET imaging in diagnosing and monitoring patients with anti-leucine-rich glioma-inactivated 1 antibody autoimmune encephalitis (anti-LGI1 AE). We also sought to understand the mechanisms of faciobrachial dystonic seizures (FBDSs). PATIENTS AND METHODS: We analyzed 18 F-FDG PET scans from 50 patients with anti-LGI1 AE, using visual and semiquantitative methods, and compared these with 24 healthy controls. All patients tested positive for anti-LGI1 antibodies in serum or cerebrospinal fluid before PET imaging. The patients were divided into FBDS and non-FBDS groups to compare metabolic differences using voxel-based semiquantitative analysis. Finally, we separately analyzed PET images of patients with symptom recurrence. RESULTS: The sensitivity of 18 F-FDG PET was superior to MRI (97.9% vs 63.8%, respectively; P < 0.001). Semiquantitative analysis revealed hypermetabolism in the basal ganglia, medial temporal lobe, and brainstem, and hypometabolism in most neocortical regions compared with healthy controls. The FBDS group exhibited hypometabolism in the frontal and temporal lobes compared with the non-FBDS group. Among 7 recurrent patients, 3 were confirmed as recurrence and 3 as sequelae by PET. One patient relapsed shortly after discontinuing corticosteroids when PET indicated active lesions. CONCLUSIONS: 18 F-FDG PET scans were more sensitive than MRI in detecting anti-LGI1 AE, which displayed a pattern of hypermetabolism in the basal ganglia and medial temporal lobe, as well as neocortex hypometabolism. Hypometabolism in the frontal and temporal lobes was associated with FBDS. Furthermore, 18 F-FDG PET scans can differentiate recurrence from sequelae and guide the timing of immunotherapy cessation.

Feasibility of 18F-florzolotau quantification in patients with Alzheimer's disease based on an MRI-free tau PET template.

OBJECTIVES: Quantification of tau accumulation using positron emission tomography (PET) is critical for the diagnosis of Alzheimer's disease (AD). This study aimed to evaluate the feasibility of 18F-florzolotau quantification in patients with AD using a magnetic resonance imaging (MRI)-free tau PET template, since individual high-resolution MRI is costly and not always available in practice. METHODS: 18F-florzolotau PET and MRI scans were obtained in a discovery cohort including (1) patients within the AD continuum (n = 87), (2) cognitively impaired patients with non-AD (n = 32), and (3) cognitively unimpaired subjects (n = 26). The validation cohort comprised 24 patients with AD. Following MRI-dependent spatial normalization (standard approach) in randomly selected subjects (n = 40) to cover the entire spectrum of cognitive function, selected PET images were averaged to create the 18F-florzolotau-specific template. Standardized uptake value ratios (SUVRs) were calculated in five predefined regions of interest (ROIs). MRI-free and MRI-dependent methods were compared in terms of continuous and dichotomous agreement, diagnostic performances, and associations with specific cognitive domains. RESULTS: MRI-free SUVRs had a high continuous and dichotomous agreement with MRI-dependent measures for all ROIs (intraclass correlation coefficient ≥ 0.980; agreement ≥ 94.5%). Similar findings were observed for AD-related effect sizes, diagnostic performances with respect to categorization across the cognitive spectrum, and associations with cognitive domains. The robustness of the MRI-free approach was confirmed in the validation cohort. CONCLUSIONS: The use of an 18F-florzolotau-specific template is a valid alternative to MRI-dependent spatial normalization, improving the clinical generalizability of this second-generation tau tracer. KEY POINTS: • Regional 18F-florzolotau SUVRs reflecting tau accumulation in the living brains are reliable biomarkers for the diagnosis, differential diagnosis, and assessment of disease severity in patients with AD. • The 18F-florzolotau-specific template is a valid alternative to MRI-dependent spatial normalization, improving the clinical generalizability of this second-generation tau tracer.

Deep learning for predicting epidermal growth factor receptor mutations of non-small cell lung cancer on PET/CT images.

Background: Predicting the mutation status of the epidermal growth factor receptor (EGFR) gene based on an integrated positron emission tomography/computed tomography (PET/CT) image of non-small cell lung cancer (NSCLC) is a noninvasive, low-cost method which is valuable for targeted therapy. Although deep learning has been very successful in robotic vision, it is still challenging to predict gene mutations in PET/CT-derived studies because of the small amount of medical data and the different parameters of PET/CT devices. Methods: We used the advanced EfficientNet-V2 model to predict the EGFR mutation based on fused PET/CT images. First, we extracted 3-dimensional (3D) pulmonary nodules from PET and CT as regions of interest (ROIs). We then fused each single PET and CT image. The network model was used to predict the mutation status of lung nodules by the new data after fusion, and the model was weighted adaptively. The EfficientNet-V2 model used multiple channels to represent nodules comprehensively. Results: We trained the EfficientNet-V2 model through our PET/CT fusion algorithm using a dataset of 150 patients. The prediction accuracy of EGFR and non-EGFR mutations was 86.25% in the training dataset, and the accuracy rate was 81.92% in the validation set. Conclusions: Combined with experiments, the demonstrated PET/CT fusion algorithm outperformed radiomics methods in predicting EGFR and non-EGFR mutations in NSCLC.

A deep learning model based on the attention mechanism for automatic segmentation of abdominal muscle and fat for body composition assessment.

Background: Quantitative muscle and fat data obtained through body composition analysis are expected to be a new stable biomarker for the early and accurate prediction of treatment-related toxicity, treatment response, and prognosis in patients with lung cancer. The use of these biomarkers can enable the adjustment of individualized treatment regimens in a timely manner, which is critical to further improving patient prognosis and quality of life. We aimed to develop a deep learning model based on attention for fully automated segmentation of the abdomen from computed tomography (CT) to quantify body composition. Methods: A fully automatic segmentation deep learning model was designed based on the attention mechanism and using U-Net as the framework. Subcutaneous fat, skeletal muscle, and visceral fat were manually segmented by two experts to serve as ground truth labels. The performance of the model was evaluated using Dice similarity coefficients (DSCs) and Hausdorff distance at 95th percentile (HD95). Results: The mean DSC for subcutaneous fat and skeletal muscle were high for both the enhanced CT test set (0.93±0.06 and 0.96±0.02, respectively) and the plain CT test set (0.90±0.09 and 0.95±0.01, respectively). Nevertheless, the model did not perform well in the segmentation performance of visceral fat, especially for the enhanced CT test set. The mean DSC for the enhanced CT test set was 0.87±0.11, while the mean DSC for the plain CT test set was 0.92±0.03. We discuss the reasons for this result. Conclusions: This work demonstrates a method for the automatic outlining of subcutaneous fat, skeletal muscle, and visceral fat areas at L3.

Analysis of positron emission tomography hypometabolic patterns and neuropsychiatric symptoms in patients with dementia syndromes.

AIMS: To estimate the proportions of specific hypometabolic patterns and their association with neuropsychiatric symptoms (NPS) in patients with cognitive impairment (CI). METHODS: This multicenter study with 1037 consecutive patients was conducted from December 2012 to December 2019. 18 F-FDG PET and clinical/demographic information, NPS assessments were recorded and analyzed to explore the associations between hypometabolic patterns and clinical features by correlation analysis and multivariable logistic regression models. RESULTS: Patients with clinical Alzheimer's disease (AD, 81.6%, 605/741) and dementia with Lewy bodies (67.9%, 19/28) mostly had AD-pattern hypometabolism, and 76/137 (55.5%) of patients with frontotemporal lobar degeneration showed frontal and anterior temporal pattern (FT-P) hypometabolism. Besides corticobasal degeneration, patients with behavioral variant frontotemporal dementia (36/58), semantic dementia (7/10), progressive non-fluent aphasia (6/9), frontotemporal lobar degeneration and amyotrophic lateral sclerosis (3/5), and progressive supranuclear palsy (21/37) also mostly showed FT-P hypometabolism. The proportion of FT-P hypometabolism was associated with the presence of hallucinations (R = 0.171, p = 0.04), anxiety (R = 0.182, p = 0.03), and appetite and eating abnormalities (R = 0.200, p = 0.01) in AD. CONCLUSION: Specific hypometabolic patterns in FDG-PET are associated with NPS and beneficial for the early identification and management of NPS in patients with CI.

Characterizing Rosai-Dorfman disease with [18F]FDG PET/CT: a retrospective analysis of a single-center study.

OBJECTIVES: Rosai-Dorfman disease (RDD) is a rare non-Langerhans cell histiocytic proliferative disorder. We performed a retrospective study to characterize the clinical and [18F]FDG PET/CT features of RDD. METHODS: We retrospectively recruited 38 RDD patients with [18F]FDG PET/CT scan in our center. [18F]FDG PET/CT features were assessed, and clinical and follow-up data were recorded. RESULTS: In the recruited patients, 20/38 (52.6%) patients had single-system disease, while others (18/38, 47.4%) had disease affecting multiple system. RDD most commonly involved the upper respiratory tract (47.4%), followed by cutaneous/subcutaneous lesion (39.5%), lymph node (36.8%), bone (31.6%), central nervous system (28.9%), and cardiovascular system (13.2%) in the recruited patients. In PET/CT, the RDD lesions were FDG-avid, and the SUVmax of the hottest lesion in an individual patient was positively correlated with C-reactive protein levels (r = 0.418, p = 0.014), and negatively correlated with hemoglobin levels (r = -0.359, p = 0.036). The overall response rate of the first-line treatment was 80.8% in newly diagnosed RDD patients, and for patients with relapsed/progressive RDD, the overall response rate was 72.7%. CONCLUSION: [18F]FDG PET/CT could be a useful tool for evaluating RDD. KEY POINTS: • About half of the patients with Rosai-Dorfman disease had single-system disease, while others had disease affecting multiple system. Rosai-Dorfman disease most commonly involved the upper respiratory tract, followed by cutaneous/subcutaneous lesion, lymph node, bone, central nervous system, and cardiovascular system. • In [18F]FDG PET/CT, Rosai-Dorfman disease was usually hypermetabolic, and the SUVmax of the hottest lesion in an individual patient was positively correlated with C-reactive protein levels. • Rosai-Dorfman disease usually has a high overall response rate after treatment.

Phase separation of SGS3 drives siRNA body formation and promotes endogenous gene silencing.

The generation of small interfering RNA (siRNA) involves many RNA processing components, including SUPPRESSOR OF GENE SILENCING 3 (SGS3), RNA-DEPENDENT RNA POLYMERASE 6 (RDR6), and DICER-LIKE proteins (DCLs). Nonetheless, how these components are coordinated to produce siRNAs is unclear. Here, we show that SGS3 forms condensates via phase separation in vivo and in vitro. SGS3 interacts with RDR6 and drives it to form siRNA bodies in cytoplasm, which is promoted by SGS3-targeted RNAs. Disrupting SGS3 phase separation abrogates siRNA body assembly and siRNA biogenesis, whereas coexpression of SGS3 and RDR6 induces siRNA body formation in tobacco and yeast cells. Dysfunction in translation and mRNA decay increases the number of siRNA bodies, whereas DCL2/4 mutations enhance their size. Purification of SGS3 condensates identifies numerous RNA-binding proteins and siRNA processing components. Together, our findings reveal that SGS3 phase separation-mediated formation of siRNA bodies is essential for siRNA production and gene silencing.

Generalized Muscular Hypermetabolism Caused by Mitochondrial Myopathy Shown on 18 F-FDG PET/CT.

ABSTRACT: An 18-year-old man presented with progressive exercise intolerance and muscle weakness for 1 year with recent acute exacerbation. Laboratory test demonstrated lactic acidosis. 18 F-FDG PET/CT was performed to exclude malignancy and showed generalized muscular hypermetabolism. Muscle biopsy combined with patient's history suggested mitochondrial myopathy. This report illustrates that mitochondrial myopathy may present as generalized muscular hypermetabolism on 18 F-FDG PET/CT and thus should be added to the differential diagnoses.

The characteristics of glucose metabolism and functional connectivity in posterior default network during nondemented aging: relationship with executive function performance.

BACKGROUND: Understanding the characteristics of intrinsic connectivity networks (ICNs) in terms of both glucose metabolism and functional connectivity (FC) is important for revealing cognitive aging and neurodegeneration, but the relationships between these two aspects during aging has not been well established in older adults. OBJECTIVE: This study is to assess the relationship between age-related glucose metabolism and FC in key ICNs, and their direct or indirect effects on cognitive deficits in older adults. METHODS: We estimated the individual-level standard uptake value ratio (SUVr) and FC of eleven ICNs in 59 cognitively unimpaired older adults, then analyzed the associations of SUVr and FC of each ICN and their relationships with cognitive performance. RESULTS: The results showed both the SUVr and FC in the posterior default mode network (pDMN) had a significant decline with age, and the association between them was also significant. Moreover, both decline of metabolism and FC in the pDMN were significantly correlated with executive function decline. Finally, mediation analysis revealed the glucose metabolism mediated the FC decline with age and FC mediated the executive function deficits. CONCLUSIONS: Our findings indicated that covariance between glucose metabolism and FC in the pDMN is one of the main routes that contributes to age-related executive function decline.

Neurological disorders associated with glutamic acid decarboxylase 65 antibodies: Clinical spectrum and prognosis of a cohort from China.

Objective: To describe clinical phenotypes and prognosis of neurological autoimmunity related to glutamic acid decarboxylase 65 (GAD65) antibodies in China. Method: In this retrospective observational study from Peking Union Medical College Hospital, we identified patients with neurological disorders related to GAD65 antibodies (cell-based assay) from May 2015 to September 2021. Clinical manifestations, immunotherapy responsiveness, and outcomes were collected after obtaining informed consent from all patients. Results: Fifty-five patients were included: 40 (72.73%) were women and initial neurological symptoms developed at 42(34-55) years of age. The median time to the nadir of the disease was 5 months (range from 1 day to 48 months). The clinical syndromes included limbic encephalitis (LE) or epilepsy (Ep) (n = 34, 61.82%), stiff-person syndromes (SPS) (n = 18, 32.73%), autoimmune cerebellar ataxia (ACA) (n = 11, 20%), and overlap syndrome in eight (14.55%) patients. Thirty-two (58.2%) patients had comorbidities of other autoimmune diseases, including Hashimoto thyroiditis (n = 17, 53.13%), T1DM (n = 11, 34.78%), vitiligo (n = 6, 18.75%), and others (n=5, 15.63%). Two (3.64%) patients had tumors, including thymoma and small cell lung cancer. Fifty-one (92.7%) patients received first-line immunotherapy (glucocorticoids and/or IV immunoglobulin), and 4 (7.3%) received second-line immunotherapy (rituximab). Long-term immunotherapy (mycophenolate mofetil) was administered to 23 (41.8%) patients. At the median time of 15 months (IQR 6-33.75 month, range 3-96 month) of follow-up, the patients' median modified Rankin Score (mRS) had declined from 2 to 1. Thirty-eight (70.4%) patients experienced clinical improvement (mRS declined ≥1), 47 (87%) had favorable clinical outcomes (mRS ≤2), and nine were symptom-free (16.7%). The sustained response to immunotherapy ranged from 7/15 (63.63%) in ACA patients and 22/34 (64.7%) in LE/Ep patients to 14/17 (82.35%) in SPS patients. Conclusions: LE/Ep was the most common neurological phenotype of GAD65 antibody neurological autoimmunity in our cohort. Most patients had comorbidities of other autoimmune diseases, but underlying tumors were rare. Most patients responded to immunotherapy. However, the long-term prognosis varied among different clinical phenotypes.

Structural insights into the dual activities of the two-barrel RNA polymerase QDE-1.

Neurospora crassa protein QDE-1, a member of the two-barrel polymerase superfamily, possesses both DNA- and RNA-dependent RNA polymerase (DdRP and RdRP) activities. The dual activities are essential for the production of double-stranded RNAs (dsRNAs), the precursors of small interfering RNAs (siRNAs) in N. crassa. Here, we report five complex structures of N-terminal truncated QDE-1 (QDE-1ΔN), representing four different reaction states: DNA/RNA-templated elongation, the de novo initiation of RNA synthesis, the first step of nucleotide condensation during de novo initiation and initial NTP loading. The template strand is aligned by a bridge-helix and double-psi beta-barrels 2 (DPBB2), the RNA product is held by DPBB1 and the slab domain. The DNA template unpairs with the RNA product at position -7, but the RNA template remains paired. The NTP analog coordinates with cations and is precisely positioned at the addition site by a rigid trigger loop and a proline-containing loop in the active center. The unique C-terminal tail from the QDE-1 dimer partner inserts into the substrate-binding cleft and plays regulatory roles in RNA synthesis. Collectively, this work elucidates the conserved mechanisms for DNA/RNA-dependent dual activities by QDE-1 and other two-barrel polymerase superfamily members.