Neuropathogenesis-on-chips for neurodegenerative diseases.

Developing diagnostics and treatments for neurodegenerative diseases (NDs) is challenging due to multifactorial pathogenesis that progresses gradually. Advanced in vitro systems that recapitulate patient-like pathophysiology are emerging as alternatives to conventional animal-based models. In this review, we explore the interconnected pathogenic features of different types of ND, discuss the general strategy to modelling NDs using a microfluidic chip, and introduce the organoid-on-a-chip as the next advanced relevant model. Lastly, we overview how these models are being applied in academic and industrial drug development. The integration of microfluidic chips, stem cells, and biotechnological devices promises to provide valuable insights for biomedical research and developing diagnostic and therapeutic solutions for NDs.

Cryo-electron microscopy-based drug design.

Structure-based drug design (SBDD) has gained popularity owing to its ability to develop more potent drugs compared to conventional drug-discovery methods. The success of SBDD relies heavily on obtaining the three-dimensional structures of drug targets. X-ray crystallography is the primary method used for solving structures and aiding the SBDD workflow; however, it is not suitable for all targets. With the resolution revolution, enabling routine high-resolution reconstruction of structures, cryogenic electron microscopy (cryo-EM) has emerged as a promising alternative and has attracted increasing attention in SBDD. Cryo-EM offers various advantages over X-ray crystallography and can potentially replace X-ray crystallography in SBDD. To fully utilize cryo-EM in drug discovery, understanding the strengths and weaknesses of this technique and noting the key advancements in the field are crucial. This review provides an overview of the general workflow of cryo-EM in SBDD and highlights technical innovations that enable its application in drug design. Furthermore, the most recent achievements in the cryo-EM methodology for drug discovery are discussed, demonstrating the potential of this technique for advancing drug development. By understanding the capabilities and advancements of cryo-EM, researchers can leverage the benefits of designing more effective drugs. This review concludes with a discussion of the future perspectives of cryo-EM-based SBDD, emphasizing the role of this technique in driving innovations in drug discovery and development. The integration of cryo-EM into the drug design process holds great promise for accelerating the discovery of new and improved therapeutic agents to combat various diseases.

Utilizing peptide-anchored DNA templates for novel programmable nanoparticle assemblies in biological macromolecules: A review.

Biological macromolecules such as proteins and DNA are known to self-assemble into various structural moieties with distinct functions. While nucleic acids are the structural building blocks, peptides exemplify diversity as tailorable biochemical units. Thus, combining the scaffold properties of the biomacromolecule DNA and the functionality of peptides could evolve into a powerful method to obtain tailorable nano assemblies. In this review, we discuss the assembly of non-DNA-coated colloidal NPs on DNA/peptide templates using functional anchors. We begin with strategies for directly attaching metallic NPs to DNA templates to ascertain the functional role of DNA as a scaffold. Followed by methods to assemble peptides onto DNA templates to emphasize the functional versatility of biologically abundant DNA-binding peptides. Next, we focus on studies corroborating peptide self-assembling into macromolecular templates onto which NPs can attach to emphasize the properties of NP-binding peptides. Finally, we discuss the assembly of NPs on a DNA template with a focus on the bifunctional DNA-binding peptides with NP-binding affinity (peptide anchors). This review aims to highlight the immense potential of combining the functional power of DNA scaffolds and tailorable functionalities of peptides for NP assembly and the need to utilize them effectively to obtain tailorable hierarchical NP assemblies.

Cryo-EM structure of a 16.5-kDa small heat-shock protein from Methanocaldococcus jannaschii.

The small heat-shock protein (sHSP) from the archaea Methanocaldococcus jannaschii, MjsHSP16.5, functions as a broad substrate ATP-independent holding chaperone protecting misfolded proteins from aggregation under stress conditions. This protein is the first sHSP characterized by X-ray crystallography, thereby contributing significantly to our understanding of sHSPs. However, despite numerous studies assessing its functions and structures, the precise arrangement of the N-terminal domains (NTDs) within this sHSP cage remains elusive. Here we present the cryo-electron microscopy (cryo-EM) structure of MjsHSP16.5 at 2.49-Å resolution. The subunits of MjsHSP16.5 in the cryo-EM structure exhibit lesser compaction compared to their counterparts in the crystal structure. This structural feature holds particular significance in relation to the biophysical properties of MjsHSP16.5, suggesting a close resemblance to this sHSP native state. Additionally, our cryo-EM structure unveils the density of residues 24-33 within the NTD of MjsHSP16.5, a feature that typically remains invisible in the majority of its crystal structures. Notably, these residues show a propensity to adopt a ß-strand conformation and engage in antiparallel interactions with strand ß1, both intra- and inter-subunit modes. These structural insights are corroborated by structural predictions, disulfide bond cross-linking studies of Cys-substitution mutants, and protein disaggregation assays. A comprehensive understanding of the structural features of MjsHSP16.5 expectedly holds the potential to inspire a wide range of interdisciplinary applications, owing to the renowned versatility of this sHSP as a nanoscale protein platform.

Identification of an antivirulence agent targeting the master regulator of virulence genes in Staphylococcus aureus.

The emergence of bactericidal antibiotic-resistant strains has increased the demand for alternative therapeutic agents, such as antivirulence agents targeting the virulence regulators of pathogens. Staphylococcus aureus exoprotein expression (sae) locus, the master regulator of virulence gene expression in multiple drug-resistant S. aureus, is a promising therapeutic target. In this study, we screened a small-molecule library using a SaeRS green fluorescent protein (GFP)-reporter that responded to transcription controlled by the sae locus. We identified the compound, N-(2-methylcyclohexyl)-11-oxo-10,11-dihydrodibenzo[b,f][1,4]thiazepine-8-carboxamide (SKKUCS), as an efficient repressor of sae-regulated GFP activity. SKKUCS inhibited hemolysin production and reduced α-hemolysin-mediated cell lysis. Moreover, SKKUCS substantially reduced the expression levels of various virulence genes controlled by the master regulators, sae, and the accessory gene regulator (agr), demonstrating its potential as an antivirulence reagent targeting the key virulence regulators. Furthermore, autokinase inhibition assay and molecular docking suggest that SKKUCS inhibits the kinase activity of SaeS and potentially targets the active site of SaeS kinase, possibly inhibiting ATP binding. Next, we evaluated the efficacy and toxicity of SKKUCS in vivo using murine models of staphylococcal intraperitoneal and skin infections. Treatment with SKKUCS markedly increased animal survival and significantly decreased the bacterial burden in organs and skin lesion sizes. These findings highlight SKKUCS as a potential antivirulence drug for drug-resistant staphylococcal infections.

Structural understanding of SARS-CoV-2 virus entry to host cells.

Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a major global health concern associated with millions of fatalities worldwide. Mutant variants of the virus have further exacerbated COVID-19 mortality and infection rates, emphasizing the urgent need for effective preventive strategies. Understanding the viral infection mechanism is crucial for developing therapeutics and vaccines. The entry of SARS-CoV-2 into host cells is a key step in the infection pathway and has been targeted for drug development. Despite numerous reviews of COVID-19 and the virus, there is a lack of comprehensive reviews focusing on the structural aspects of viral entry. In this review, we analyze structural changes in Spike proteins during the entry process, dividing the entry process into prebinding, receptor binding, proteolytic cleavage, and membrane fusion steps. By understanding the atomic-scale details of viral entry, we can better target the entry step for intervention strategies. We also examine the impacts of mutations in Spike proteins, including the Omicron variant, on viral entry. Structural information provides insights into the effects of mutations and can guide the development of therapeutics and vaccines. Finally, we discuss available structure-based approaches for the development of therapeutics and vaccines. Overall, this review provides a detailed analysis of the structural aspects of SARS-CoV-2 viral entry, highlighting its significance in the development of therapeutics and vaccines against COVID-19. Therefore, our review emphasizes the importance of structural information in combating SARS-CoV-2 infection.

Molecular basis for SOX2-dependent regulation of super-enhancer activity.

Pioneer transcription factors (TFs) like SOX2 are vital for stemness and cancer through enhancing gene expression within transcriptional condensates formed with coactivators, RNAs and mediators on super-enhancers (SEs). Despite their importance, how these factors work together for transcriptional condensation and activation remains unclear. SOX2, a pioneer TF found in SEs of pluripotent and cancer stem cells, initiates SE-mediated transcription by binding to nucleosomes, though the mechanism isn't fully understood. To address SOX2's role in SEs, we identified mSE078 as a model SOX2-enriched SE and p300 as a coactivator through bioinformatic analysis. In vitro and cell assays showed SOX2 forms condensates with p300 and SOX2-binding motifs in mSE078. We further proved that SOX2 condensation is highly correlated with mSE078's enhancer activity in cells. Moreover, we successfully demonstrated that p300 not only elevated transcriptional activity but also triggered chromatin acetylation via its direct interaction with SOX2 within these transcriptional condensates. Finally, our validation of SOX2-enriched SEs showcased their contribution to target gene expression in both stem cells and cancer cells. In its entirety, this study imparts valuable mechanistic insights into the collaborative interplay of SOX2 and its coactivator p300, shedding light on the regulation of transcriptional condensation and activation within SOX2-enriched SEs.

Sensor histidine kinases kdpD and aauS regulate biofilm and virulence in Pseudomonas aeruginosa PA14.

Pseudomonas aeruginosa is a multidrug-resistant opportunistic human pathogen that utilizes two-component systems (TCSs) to sense pathophysiological signals and coordinate virulence. P. aeruginosa contains 64 sensor histidine kinases (HKs) and 72 response regulators (RRs) that play important roles in metabolism, bacterial physiology, and virulence. However, the role of some TCSs in virulence remains uncharacterized. In this study, we evaluated the virulence potential of some uncharacterized sensor HK and RR knockouts in P. aeruginosa using a Galleria mellonella infection model. Furthermore, we demonstrated that KdpD and AauS HKs regulate virulence by affecting P. aeruginosa biofilm formation and motility. Both ΔkdpD and ΔaauS showed reduced biofilm and motility which were confirmed by restored phenotypes upon complementation. Moreover, ΔkdpD and ΔaauS exhibited increased survival of HeLa cells and G. mellonella during in vivo infection. Altered expression of the transcriptional regulators anR and lasR, along with the virulence genes lasA, pelA, cupA, pqsA, pqsB, pqsC, and pqsD in the mutant strains elucidated the mechanism by which ΔkdpD and ΔaauS affect virulence. These findings confirm that kdpD and aauS play important roles in P. aeruginosa pathogenesis by regulating biofilm formation and motility.

Stabilization of RNA G-quadruplexes in the SARS-CoV-2 genome inhibits viral infection via translational suppression.

The G-quadruplex (G4) formed in single-stranded DNAs or RNAs plays a key role in diverse biological processes and is considered as a potential antiviral target. In the genome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), 25 putative G4-forming sequences are predicted; however, the effects of G4-binding ligands on SARS-CoV-2 replication have not been studied in the context of viral infection. In this study, we investigated whether G4-ligands suppressed SARS-CoV-2 replication and whether their antiviral activity involved stabilization of viral RNA G4s and suppression of viral gene expression. We found that pyridostatin (PDS) suppressed viral gene expression and genome replication as effectively as the RNA polymerase inhibitor remdesivir. Biophysical analyses revealed that the 25 predicted G4s in the SARS-CoV-2 genome formed a parallel G4 structure. In particular, G4-644 and G4-3467 located in the 5' region of ORF1a, formed a G4 structure that could be effectively stabilized by PDS. We also showed that PDS significantly suppressed translation of the reporter genes containing these G4s. Taken together, our results demonstrate that stabilization of RNA G4s by PDS in the SARS-CoV-2 genome inhibits viral infection via translational suppression, highlighting the therapeutic potential of G4-ligands in SARS-CoV-2 infection.

Structural and Biochemical Insights into Bis(2-hydroxyethyl) Terephthalate Degrading Carboxylesterase Isolated from Psychrotrophic Bacterium Exiguobacterium antarcticum.

This study aimed to elucidate the crystal structure and biochemically characterize the carboxylesterase EaEst2, a thermotolerant biocatalyst derived from Exiguobacterium antarcticum, a psychrotrophic bacterium. Sequence and phylogenetic analyses showed that EaEst2 belongs to the Family XIII group of carboxylesterases. EaEst2 has a broad range of substrate specificities for short-chain p-nitrophenyl (pNP) esters, 1-naphthyl acetate (1-NA), and 1-naphthyl butyrate (1-NB). Its optimal pH is 7.0, losing its enzymatic activity at temperatures above 50 °C. EaEst2 showed degradation activity toward bis(2-hydroxyethyl) terephthalate (BHET), a polyethylene terephthalate degradation intermediate. We determined the crystal structure of EaEst2 at a 1.74 Å resolution in the ligand-free form to investigate BHET degradation at a molecular level. Finally, the biochemical stability and immobilization of a crosslinked enzyme aggregate (CLEA) were assessed to examine its potential for industrial application. Overall, the structural and biochemical characterization of EaEst2 demonstrates its industrial potency as a biocatalyst.

Feruloyl Esterase (LaFae) from Lactobacillus acidophilus: Structural Insights and Functional Characterization for Application in Ferulic Acid Production.

Ferulic acid and related hydroxycinnamic acids, used as antioxidants and preservatives in the food, cosmetic, pharmaceutical and biotechnology industries, are among the most abundant phenolic compounds present in plant biomass. Identification of novel compounds that can produce ferulic acid and hydroxycinnamic acids, that are safe and can be mass-produced, is critical for the sustainability of these industries. In this study, we aimed to obtain and characterize a feruloyl esterase (LaFae) from Lactobacillus acidophilus. Our results demonstrated that LaFae reacts with ethyl ferulate and can be used to effectively produce ferulic acid from wheat bran, rice bran and corn stalks. In addition, xylanase supplementation was found to enhance LaFae enzymatic hydrolysis, thereby augmenting ferulic acid production. To further investigate the active site configuration of LaFae, crystal structures of unliganded and ethyl ferulate-bound LaFae were determined at 2.3 and 2.19 Å resolutions, respectively. Structural analysis shows that a Phe34 residue, located at the active site entrance, acts as a gatekeeper residue and controls substrate binding. Mutating this Phe34 to Ala produced an approximately 1.6-fold increase in LaFae activity against p-nitrophenyl butyrate. Our results highlight the considerable application potential of LaFae to produce ferulic acid from plant biomass and agricultural by-products.

Binding of USP4 to cortactin enhances cell migration in HCT116 human colon cancer cells.

Ubiquitin-specific protease 4 (USP4) is highly overexpressed in colon cancer and acts as a potent protooncogenic protein by deubiquitinating ß-catenin. However, its prominent roles in tumor formation and migration in cancer cells are not fully understood by its deubiquitinating enzyme (DUB) activity on ß-catenin. Thus, we investigated an additional role of USP4 in cancer. In this study, we identified cortactin (CTTN), an actin-binding protein involved in the regulation of cytoskeleton dynamics and a potential prognostic marker for cancers, as a new cellular interacting partner of USP4 from proximal labeling of HCT116 cells. Additionally, the role of USP4 in CTTN activation and promotion of cell dynamics and migration was investigated in HCT116 cells. We confirmed that interacting of USP4 with CTTN increased cell movement. This finding was supported by the fact that USP4 overexpression in HCT116 cells with reduced expression of CTTN was insufficient to promote cell migration. Additionally, we observed that USP4 overexpression led to a significant increase in CTTN phosphorylation, which is a requisite mechanism for cell migration, by regulating Src/focal adhesion kinase (FAK) binding to CTTN and its activation. Our results suggest that USP4 plays a dual role in cancer progression, including stabilization of ß-catenin as a DUB and interaction with CTTN to promote cell dynamics by inducing CTTN phosphorylation. Therefore, this study demonstrates that USP4 is important for cancer progression and is a good target for treating or preventing cancer.

Characterization of Z-DNA Using Circular Dichroism.

The B-DNA to Z-DNA transition is a remarkable conformational change in DNA, which was originally observed in poly-GC DNA in the presence of high salt concentration. This eventually prompted the observation of the crystal structure of Z-DNA, a left-handed double-helical DNA, at atomic resolution. Despite advances in Z-DNA research, the application of circular dichroism (CD) spectroscopy as the fundamental technique to characterize this unique DNA conformation has remained constant. In this chapter, we describe a CD spectroscopic method for characterizing the B-DNA to Z-DNA transition of a CG-repeat double-stranded DNA fragment formed from a protein or chemical inducer.

Crystal structure and biochemical analysis of acetylesterase (LgEstI) from Lactococcus garvieae.

Esterase, a member of the serine hydrolase family, catalyzes the cleavage and formation of ester bonds with high regio- and stereospecificity, making them attractive biocatalysts for the synthesis of optically pure molecules. In this study, we performed an in-depth biochemical and structural characterization of a novel microbial acetylesterase, LgEstI, from the bacterial fish pathogen Lactococcus garvieae. The dimeric LgEstI displayed substrate preference for the short acyl chain of p-nitrophenyl esters and exhibited increased activity with F207A mutation. Comparative analysis with other esterases indicated that LgEstI has a narrow and shallow active site that may exhibit substrate specificity to short acyl chains. Unlike other esterases, LgEstI contains bulky residues such as Trp89, Phe194, and Trp217, which block the acyl chain channel. Furthermore, immobilized LgEstI retained approximately 90% of its initial activity, indicating its potential in industrial applications. This study expands our understanding of LgEstI and proposes novel ideas for improving its catalytic efficiency and substrate specificity for various applications.

Strategic Application of Epigenetic Regulators for Efficient Neuronal Reprogramming of Human Fibroblasts.

Background and Objectives: Cellular reprogramming in regenerative medicine holds great promise for treating patients with neurological disorders. In this regard, small molecule-mediated cellular conversion has attracted special attention because of its ease of reproducibility, applicability, and fewer safety concerns. However, currently available protocols for the direct conversion of somatic cells to neurons are limited in clinical application due of their complex nature, lengthy process, and low conversion efficiency. Methods and Results: Here, we report a new protocol involving chemical-based direct conversion of human fibroblasts (HF) to matured neuron-like cells with a short duration and high conversion efficiency using temporal and strategic dual epigenetic regulation. In this protocol, epigenetic modulation by inhibition of histone deacetylase and bromodomain enabled to overcome "recalcitrant" nature of adult fibroblasts and shorten the duration of neuronal reprogramming. We further observed that an extended epigenetic regulation is necessary to maintain the induced neuronal program to generate a homogenous population of neuron-like cells. Conclusions: Therefore, our study provides a new protocol to produce neurons-like cells and highlights the need of proper epigenetic resetting to establish and maintain neuronal program in HF.

G-quadruplexes formed by Varicella-Zoster virus reiteration sequences suppress expression of glycoprotein C and regulate viral cell-to-cell spread.

G-quadruplex (G4) formed by repetitive guanosine-rich sequences plays important roles in diverse cellular processes; however, its roles in viral infection are not fully understood. In this study, we investigated the genome-wide distribution of G4-forming sequences (G4 motifs) in Varicella-Zoster virus (VZV) and found that G4 motifs are enriched in the internal repeat short and the terminal repeat short regions flanking the unique short region and also in some reiteration (R) sequence regions. A high density of G4 motifs in the R2 region was found on the template strand of ORF14, which encodes glycoprotein C (gC), a virulent factor for viral growth in skin. Analyses such as circular dichroism spectroscopy, thermal difference spectra, and native polyacrylamide gel electrophoresis with oligodeoxynucleotides demonstrated that several G4 motifs in ORF14 form stable G4 structures. In transfection assays, gC expression from the G4-disrupted ORF14 gene was increased at the transcriptional level and became more resistant to suppression by G4-ligand treatment. The recombinant virus containing the G4-disrupted ORF14 gene expressed a higher level of gC mRNA, while it showed a slightly reduced growth. This G4-disrupted ORF14 virus produced smaller plaques than the wild-type virus. Our results demonstrate that G4 formation via reiteration sequences suppresses gC expression during VZV infection and regulates viral cell-to-cell spread.

Functional and Structural Changes in the Membrane-Bound O-Acyltransferase Family Member 7 (MBOAT7) Protein: The Pathomechanism of a Novel MBOAT7 Variant in Patients With Intellectual Disability.

The membrane-bound O-acyltransferase domain-containing 7 (MBOAT7) gene is associated with intellectual disability, early onset seizures, and autism spectrum disorders. This study aimed to determine the pathogenetic mechanism of the MBOAT7 missense variant via molecular modeling. Three patients from a consanguineous family were found to have a homozygous c.757G>A (p.Glu253Lys) variant of MBOAT7. The patients showed prominent dysfunction in gait, swallowing, vocalization, and fine motor function and had intellectual disabilities. Brain magnetic resonance imaging showed signal changes in the bilateral globus pallidi and cerebellar dentate nucleus, which differed with age. In the molecular model of human MBOAT7, Glu253 in the wild-type protein is located close to the backbone carbonyl oxygens in the loop near the helix, suggesting that the ionic interaction could contribute to the conformational stability of the funnel. Molecular modeling showed that Lys253 in the mutant protein was expected to alter the surface charge distribution, thereby potentially affecting substrate specificity. Changes in conformational stability and substrate specificity through varied ionic interactions are the suggested pathophysiological mechanisms of the MBOAT7 variant found in patients with intellectual disabilities.

A quorum-based fluorescent probe for imaging pathogenic bacteria.

Imaging of bacterial infections can be used for a wide range of investigations, including diagnosis and pathogenesis of infections, and molecular probes targeting biological processes during infection have been used extensively. However, in vivo visualization of bacteria using molecular probes is still challenging due to the difficulty of directly targeting specific bacteria. Here, we propose a new fluorescent nano-bio probe based on a quorum-sensing (QS) antagonist for imaging drug-resistant bacteria. Because most bacteria use QS-based communication to synchronize the regulation of virulence gene expression during infection, QS-based probes can be used to assess the in vivo localization and distribution of pathogenic bacteria. Our developed fluorescent, quorum-based nano-bio probes (QNBPs) target agr-activation in multiple-drug resistant Staphylococcus aureus (MRSA) and were successfully used for in vivo localization of MRSA in sepsis and dermonecrotic animal infection models. Interestingly, we found that QNBPs can diffuse through biofilms and thus provide a new strategy for detecting MRSA embedded within a biofilm. Our findings suggest that our QNBPs have great potential for directly imaging pathogenic bacteria.

Generation of a Pure Culture of Neuron-like Cells with a Glutamatergic Phenotype from Mouse Astrocytes.

Astrocyte-to-neuron reprogramming is a promising therapeutic approach for treatment of neurodegenerative diseases. The use of small molecules as an alternative to the virus-mediated ectopic expression of lineage-specific transcription factors negates the tumorigenic risk associated with viral genetic manipulation and uncontrolled differentiation of stem cells. However, because previously developed methods for small-molecule reprogramming of astrocytes to neurons are multistep, complex, and lengthy, their applications in biomedicine, including clinical treatment, are limited. Therefore, our objective in this study was to develop a novel chemical-based approach to the cellular reprogramming of astrocytes into neurons with high efficiency and low complexity. To accomplish that, we used C8-D1a, a mouse astrocyte cell line, to assess the role of small molecules in reprogramming protocols that otherwise suffer from inconsistencies caused by variations in donor of the primary cell. We developed a new protocol by which a chemical mixture formulated with Y26732, DAPT, RepSox, CHIR99021, ruxolitinib, and SAG rapidly and efficiently induced the neural reprogramming of astrocytes in four days, with a conversion efficiency of 82 ± 6%. Upon exposure to the maturation medium, those reprogrammed cells acquired a glutaminergic phenotype over the next eleven days. We also demonstrated the neuronal functionality of the induced cells by confirming KCL-induced calcium flux.

Analysis of Novel Drug-Resistant Human Cytomegalovirus DNA Polymerase Mutations Reveals the Role of a DNA-Binding Loop in Phosphonoformic Acid Resistance.

The appearance of drug-resistant mutations in UL54 DNA polymerase and UL97 kinase genes is problematic for the treatment of human cytomegalovirus (HCMV) diseases. During treatment of HCMV infection in a pediatric hematopoietic cell transplant recipient, H600L and T700A mutations and E576G mutation were independently found in the UL54 gene. Foscarnet (FOS; phosphonoformic acid) resistance by T700A mutation is reported. Here, we investigated the role of novel mutations in drug resistance by producing recombinant viruses and a model polymerase structure. The H600L mutant virus showed an increase in resistance to ganciclovir (GCV) by 11-fold and to FOS and cidofovir (CDV) by 5-fold, compared to the wild type, while the E756G mutant virus showed an increase in resistance to FOS by 9-fold and modestly to CDV by 2-fold. With the FOS-resistant T700A mutation, only H600L produced increased FOS resistance up to 37-fold, indicating an additive effect of these mutations on FOS resistance. To gain insight into drug resistance mechanisms, a model structure for UL54 polymerase was constructed using the yeast DNA polymerase as a template. In this model, HCMV DNA polymerase contains a long palm loop domain of which H600 and T700 are located on each end and T700 interacts with the FOS binding pocket. Our results demonstrate that H600L and E756G mutations in UL54 polymerase are novel drug-resistant mutations and that the acquisition of both H600L and T700A mutations in the DNA-binding loop confers increased resistance to FOS treatment, providing novel insights for the mechanism acquiring foscarnet resistance.