UPF1: a potential biomarker in human cancers.

Recently, Up-frameshift protein 1 (UPF1) is reported to be downregulated in various cancers and its low expression is closely correlated with poor prognosis. UPF1 is well known as a master regulator of nonsense-mediated mRNA decay (NMD), which serves as a highly conserved mRNA surveillance process protecting cells from aberrant toxic transcripts. Due to dysfunction of UPF1, NMD fails to proceed, which contributes to tumor initiation and progression. This review shows a brief summary of the aberrant expression, functional roles and molecular mechanisms of UPF1 during tumorigenesis. Increasing evidence has indicated that UPF1 could serve as a potential biomarker for cancer diagnosis and treatment for future clinical applications in cancer.

G3BP1 promotes human breast cancer cell proliferation through coordinating with GSK-3ß and stabilizing ß-catenin.

Ras-GTPase activating SH3 domain-binding protein 1 (G3BP1) is a multifunctional binding protein involved in the development of a variety of human cancers. However, the role of G3BP1 in breast cancer progression remains largely unknown. In this study, we report that G3BP1 is upregulated and correlated with poor prognosis in breast cancer. Overexpression of G3BP1 promotes breast cancer cell proliferation by stimulating ß-catenin signaling, which upregulates a number of proliferation-related genes. We further show that G3BP1 improves the stability of ß-catenin by inhibiting its ubiquitin-proteasome degradation rather than affecting the transcription of ß-catenin. Mechanistically, elevated G3BP1 interacts with and inactivates GSK-3ß to suppress ß-catenin phosphorylation and degradation. Disturbing the G3BP1-GSK-3ß interaction accelerates the degradation of ß-catenin, impairing the proliferative capacity of breast cancer cells. Our study demonstrates that the regulatory mechanism of the G3BP1/GSK-3ß/ß-catenin axis may be a potential therapeutic target for breast cancer.

MAPK4 overexpression promotes tumor progression via noncanonical activation of AKT/mTOR signaling.

MAPK4 is an atypical MAPK. Currently, little is known about its physiological function and involvement in diseases, including cancer. A comprehensive analysis of 8887 gene expression profiles in The Cancer Genome Atlas (TCGA) revealed that MAPK4 overexpression correlates with decreased overall survival, with particularly marked survival effects in patients with lung adenocarcinoma, bladder cancer, low-grade glioma, and thyroid carcinoma. Interestingly, human tumor MAPK4 overexpression also correlated with phosphorylation of AKT, 4E-BP1, and p70S6K, independent of the loss of PTEN or mutation of PIK3CA. This led us to examine whether MAPK4 activates the key metabolic, prosurvival, and proliferative kinase AKT and mTORC1 signaling, independent of the canonical PI3K pathway. We found that MAPK4 activated AKT via a novel, concerted mechanism independent of PI3K. Mechanistically, MAPK4 directly bound and activated AKT by phosphorylation of the activation loop at threonine 308. It also activated mTORC2 to phosphorylate AKT at serine 473 for full activation. MAPK4 overexpression induced oncogenic outcomes, including transforming prostate epithelial cells into anchorage-independent growth, and MAPK4 knockdown inhibited cancer cell proliferation, anchorage-independent growth, and xenograft growth. We concluded that MAPK4 can promote cancer by activating the AKT/mTOR signaling pathway and that targeting MAPK4 may provide a novel therapeutic approach for cancer.

Overexpression and clinical relevance of the RNA helicase DHX15 in hepatocellular carcinoma.

DHX15 is an outstanding member of the DEAH-box RNA helicase family. A few studies suggest that DHX15 contributes to carcinogenesis in several tumor cell lines. However, whether DHX15 acts as an oncogene or tumor suppressor and its association with hepatocellular carcinoma (HCC) prognosis are still poorly understood. To address this question, we used immunohistochemistry to evaluate DHX15 expression patterns and their association with clinicopathological factors and the prognosis of patients with HCC. Our results showed that DHX15 expression was significantly higher in cancerous tissues than that in nontumor tissues (P < .0001). DHX15 expression in HCC patients was associated with differentiation status (P = .018), tumor number (P = .048), intrahepatic or extrahepatic metastasis (P = .001), serum α-fetoprotein (P = .006), hepatitis B virus level (P = .018), and recurrence (P < .001). In addition, the survival analysis revealed that the DHX15-high group had significantly decreased overall survival time (P = .004) and lower 1-year survival rates (P = .002) compared with the DHX15-low group. Furthermore, multivariate analysis identified DHX15 expression as an independent factor associated with poor prognosis in HCC (P = .036). In summary, these findings demonstrate, for the first time, that DHX15 is significantly upregulated in HCC and its high expression was correlated with poor prognosis, suggesting its pivotal role in the progression of HCC. The present results suggest that DHX15 may serve as a potential prognostic biomarker for HCC patients.

The human RNA surveillance factor Up-frameshift 1 inhibits hepatic cancer progression by targeting MRP2/ABCC2.

Although the roles of Up-frameshift 1 (UPF1) in hepatocellular carcinoma (HCC) have been partly revealed, the detailed mechanisms remain poorly understood. Here, quantitative real-time PCR (qRT-PCR) and immunohistochemistry assays indicated that UPF1 expression was decreased in HCC tissues compared to the corresponding adjacent tissues, and was negatively correlated with MRP2/ABCC2 expression. Cell viability and apoptosis analyses showed that overexpression of UPF1 enhanced HCC cell sensitivity to sorafenib treatment, while knockdown of UPF1 decreased the sensitivity. Additionally, ectopic expression of UPF1 suppressed the epithelial-mesenchymal transition (EMT) process and the generation of cells with stem cell properties. Mechanistically, UPF1 directly bound with ABCC2, increased nonsense-mediated mRNA decay (NMD) efficiency and thus led to downregualtion of ABCC2. Collectively, UPF1 functions as a tumor suppressor by preventing cancer stem cell (CSC)-like characteristics, inhibiting EMT process and enhancing chemotherapeutic sensitivity via inhibiting ABCC2 expression in HCC cells. These findings establish UPF1 as a potential therapeutic target for HCC patients.

Targeted inactivation of murine Ddx3x: essential roles of Ddx3x in placentation and embryogenesis.

The X-linked DEAD-box RNA helicase DDX3 (DDX3X) is a multifunctional protein that has been implicated in gene regulation, cell cycle control, apoptosis, and tumorigenesis. However, the precise physiological function of Ddx3x during development remains unknown. Here, we show that loss of Ddx3x results in an early post-implantation lethality in male mice. The size of the epiblast marked by Oct3/4 is dramatically reduced in embryonic day 6.5 (E6.5) Ddx3x-/Y embryos. Preferential paternal X chromosome inactivation (XCI) in extraembryonic tissues of Ddx3x heterozygous (Ddx3x-/+) female mice with a maternally inherited null allele leads to placental abnormalities and embryonic lethality during development. In the embryonic tissues, Ddx3x exhibits developmental- and tissue-specific differences in escape from XCI. Targeted Ddx3x ablation in the epiblast leads to widespread apoptosis and abnormal growth, which causes embryonic lethality in the Sox2-cre/+;Ddx3xflox/Y mutant around E11.5. The observation of significant increases in γH2AX and p-p53Ser15 indicates DNA damage, which suggests that loss of Ddx3x leads to higher levels of genome damage. Significant upregulation of p21WAF1/Cip1 and p15Ink4b results in cell cycle arrest and apoptosis in Ddx3x-deficient cells. These results have uncovered that mouse Ddx3x is essential for both embryo and extraembryonic development.

Gene expression profiling reveals activation of the FA/BRCA pathway in advanced squamous cervical cancer with intrinsic resistance and therapy failure.

BACKGROUND: Advanced squamous cervical cancer, one of the most commonly diagnosed cancers in women, still remains a major problem in oncology due to treatment failure and distant metastasis. Antitumor therapy failure is due to both intrinsic and acquired resistance; intrinsic resistance is often decisive for treatment response. In this study, we investigated the specific pathways and molecules responsible for baseline therapy failure in locally advanced squamous cervical cancer. METHODS: Twenty-one patients with locally advanced squamous cell carcinoma were enrolled in this study. Primary biopsies harvested prior to therapy were analyzed for whole human gene expression (Agilent) based on the patient's 6 months clinical response. Ingenuity Pathway Analysis was used to investigate the altered molecular function and canonical pathways between the responding and non-responding patients. The microarray results were validated by qRT-PCR and immunohistochemistry. An additional set of 24 formalin-fixed paraffin-embedded cervical cancer samples was used for independent validation of the proteins of interest. RESULTS: A 2859-gene signature was identified to distinguish between responder and non-responder patients. 'DNA Replication, Recombination and Repair' represented one of the most important mechanisms activated in non-responsive cervical tumors, and the 'Role of BRCA1 in DNA Damage Response' was predicted to be the most significantly altered canonical pathway involved in intrinsic resistance (p = 1.86E-04, ratio = 0.262). Immunohistological staining confirmed increased expression of BRCA1, BRIP1, FANCD2 and RAD51 in non-responsive compared with responsive advanced squamous cervical cancer, both in the initial set of 21 cervical cancer samples and the second set of 24 samples. CONCLUSIONS: Our findings suggest that FA/BRCA pathway plays an important role in treatment failure in advanced cervical cancer. The assessment of FANCD2, RAD51, BRCA1 and BRIP1 nuclear proteins could provide important information about the patients at risk for treatment failure.

The Forkhead Box M1 protein regulates BRIP1 expression and DNA damage repair in epirubicin treatment.

FOXM1 is implicated in genotoxic drug resistance but its role and mechanism of action remain unclear. Here, we establish that γH2AX foci, indicative of DNA double-strand breaks (DSBs), accumulate in a time-dependent manner in the drug-sensitive MCF-7 cells but not in the resistant counterparts in response to epirubicin. We find that FOXM1 expression is associated with epirubicin sensitivity and DSB repair. Ectopic expression of FOXM1 can increase cell viability and abrogate DSBs sustained by MCF-7 cells following epirubicin, owing to an enhancement in repair efficiency. Conversely, alkaline comet and γH2AX foci formation assays show that Foxm1-null cells are hypersensitive to DNA damage, epirubicin and γ-irradiation. Furthermore, we find that FOXM1 is required for DNA repair by homologous recombination (HR) but not non-homologous end joining (NHEJ), using HeLa cell lines harbouring an integrated direct repeat green fluorescent protein reporter for DSB repair. We also identify BRIP1 as a direct transcription target of FOXM1 by promoter analysis and chromatin-immunoprecipitation assay. In agreement, depletion of FOXM1 expression by small interfering RNA downregulates BRIP1 expression at the protein and mRNA levels in MCF-7 and the epirubicin-resistant MCF-7 Epi(R) cells. Remarkably, the requirement for FOXM1 for DSB repair can be circumvented by reintroduction of BRIP1, suggesting that BRIP1 is an important target of FOXM1 in DSB repair. Indeed, like FOXM1, BRIP1 is needed for HR. These data suggest that FOXM1 regulates BRIP1 expression to modulate epirubicin-induced DNA damage repair and drug resistance.

Pathological changes in the pancreas of fulminant type 1 diabetes and slowly progressive insulin-dependent diabetes mellitus (SPIDDM): innate immunity in fulminant type 1 diabetes and SPIDDM.

OBJECTIVE: The contribution of innate immunity responsible for beta-cell destruction in fulminant type 1 diabetes (FT1D) and slowly progressive insulin-dependent diabetes mellitus (SPIDDM) is unclear. RESEARCH DESIGN AND METHODS: Islet-cell expression of Toll-like receptors (TLRs) including TLR3 and TLR4, the cytoplasmic retinoic acid-inducible protein I (RIG-I)-like helicases, RIG-I, melanoma differentiation-associated gene-5 and laboratory of genetics and physiology 2 in the affected islets were studied immuno-histochemically on three pancreases obtained 2-5 days after the onset of FT1D and a pancreas from a patient with SPIDDM. RESULTS: Laboratory of genetics and physiology 2 and RIG-I strongly expressed in beta cells in all three FT1D pancreases infected with enterovirus (VP1 antigen). Melanoma differentiation-associated gene-5 was hyper-expressed in all subsets of islet cells including beta cells and alpha cells. TLR3 and TLR4 were expressed in mononuclear cells that infiltrated to islets. IFN-alpha/beta was strongly expressed in islet cells. In contrast, pancreas of a patient with SPIDDM, enterovirus and expression of innate immune receptors including RIG-I, melanoma differentiation-associated gene-5, hyperexpression of laboratory of genetics and physiology 2 and mononuclear cells, which were positive for TLR3 and TLR4, and infiltration to the islets were not detected. CONCLUSIONS: These findings demonstrate that retinoic acid-inducible protein I (RIG-I)-like helicases and TLRs play a crucial role on beta-cell destruction in enterovirus-induced FT1D. The presence of distinct mechanism(s) of slowly progressive beta-cell failure in SPIDDM was suggested.

Expression and functional characterization of the RIG-I-like receptors MDA5 and LGP2 in Rainbow trout (Oncorhynchus mykiss).

The retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) comprise three homologues: RIG-I, melanoma differentiation-associated gene 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2). They activate the host interferon (IFN) system upon recognition of viral RNA pathogen-associated molecular patterns (PAMPs) in the cytoplasm. Bioinformatic analysis of the sequenced vertebrate genomes suggests that the cytosolic surveillance system is conserved in lower vertebrates, and recent functional studies have confirmed that RIG-I is important to fish antiviral immunity. In this study, we have identified MDA5 and LGP2 homologues from rainbow trout Oncorhynchus mykiss and an additional LGP2 variant with an incomplete C-terminal domain of RIG-I. Trout MDA5 and LGP2 were constitutively produced in fibroblast and macrophage cell lines and upregulated by poly(I:C), recombinant IFN, or infection by RNA viruses (viral hemorrhagic septicemia virus and salmon alphavirus) with a single-stranded positive or negative genome. Overexpression of MDA5 and LGP2 but not of the LGP2 variant resulted in significant accumulation of Mx transcripts in cultured cells, which correlated with a marked enhancement of protection against viral infection. These results demonstrate that both MDA5 and LGP2 are important RLRs in host surveillance against infection of both negative and positive viruses and that the LGP2 variant with a deletion of 54 amino acids at the C terminus acts as a negative regulator for LGP2-elicited antiviral signaling by competing for the viral RNA PAMPs. Interestingly, MDA5 expression was not affected by overexpressed LGP2 in transfected cells and vice versa, suggesting that they likely act in parallel as positive regulators for IFN production.

A key gene of the RNA interference pathway in the black tiger shrimp, Penaeus monodon: identification and functional characterisation of Dicer-1.

RNA interference (RNAi) is an evolutionarily conserved mechanism by which double-stranded RNA (dsRNA) initiates post-transcriptional silencing of homologous genes. Here we report the amplification and characterisation of a full length cDNA from black tiger shrimp (Penaeus monodon) that encodes the bidentate RNAase III Dicer, a key component of the RNAi pathway. The full length of the shrimp Dicer (Pm Dcr1) cDNA is 7629bp in length, including a 5' untranslated region (UTR) of 130bp, a 3' UTR of 77bp, and an open reading frame of 7422bp encoding a polypeptide of 2473 amino acids with an estimated molecular mass of 277.895kDa and a predicted isoelectric point of 4.86. Analysis of the deduced amino acid sequence indicated that the mature peptide contains all the seven recognised functional domains and is most similar to the mosquito (Aedes aegypti) Dicer-1 sequence with a similarity of 34.6%. Quantitative RT-PCR analysis showed that Pm Dcr1 mRNA is most highly expressed in haemolymph and lymphoid organ tissues (P<0.05). However, there was no correlation between Pm Dcr1 mRNA levels in lymphoid organ and the viral genetic loads in shrimp naturally infected with gill-associated virus (GAV) and Mourilyan virus (P>0.05). Treatment with synthetic dsRNA corresponding to Pm Dcr1 sequence resulted in knock-down of Pm Dcr1 mRNA expression in both uninfected shrimp and shrimp infected experimentally with GAV. Knock-down of Pm Dcr1 expression resulted in more rapid mortalities and higher viral loads. These data demonstrated that Dicer is involved in antiviral defence in shrimp.

Oncoprotein EWS-FLI1 activity is enhanced by RNA helicase A.

RNA helicase A (RHA), a member of the DEXH box helicase family of proteins, is an integral component of protein complexes that regulate transcription and splicing. The EWS-FLI1 oncoprotein is expressed as a result of the chromosomal translocation t(11;22) that occurs in patients with the Ewing's sarcoma family of tumors (ESFT). Using phage display library screening, we identified an EWS-FLI1 binding peptide containing homology to RHA. ESFT cell lines and patient tumors highly expressed RHA. GST pull-down and ELISA assays showed that EWS-FLI1 specifically bound RHA fragment amino acids 630 to 1020, which contains the peptide region discovered by phage display. Endogenous RHA was identified in a protein complex with EWS-FLI1 in ESFT cell lines. Chromatin immunoprecipitation experiments showed both EWS-FLI1 and RHA bound to EWS-FLI1 target gene promoters. RHA stimulated the transcriptional activity of EWS-FLI1 regulated promoters, including Id2, in ESFT cells. In addition, RHA expression in mouse embryonic fibroblast cells stably transfected with EWS-FLI1 enhanced the anchorage-independent phenotype above that with EWS-FLI1 alone. These results suggest that RHA interacts with EWS-FLI1 as a transcriptional cofactor to enhance its function.

Gene expression profiling of the bone marrow mononuclear cells from patients with myelodysplastic syndrome.

The gene expression pattern of bone marrow mononuclear cells (BMNCs) from 10 patients with myelodysplastic syndrome (MDS) was studied by two-color cDNA microarray techniques. To confirm the microarray results, a semiquantitative RT-PCR was performed to analyze gene expression in fifty additional MDS patients. Ninety-five genes were shown to be abnormally expressed in at least five MDS patients compared to normal controls, involving cell growth and differentiation regulation, cell cycle control, signaling and redox; such as thrombospondin 1, phosphatase and tensin homolog, MAD, DNA-damage-inducible transcript 3 (DDIT3), ets variant gene 1 (ETV1), and G1 to S phase transition 1. CD36 was also revealed up-regulated in 4 cases. MDS patients in early and advanced stages could be clustered into two distinct groups by hierarchical clustering, wherein a case with isolated thrombocytopenia and other RA patients were clustered into two subgroups. Consistent expression patterns of 3/5 (60%) genes were confirmed by semiquantitative RT-PCR. Further analysis showed the different transcript levels of RNAHP, DDIT3 in patients with MDS in different stages, AML, and normal controls. Meanwhile, the different significance of RNAHP and ETV1 expression was revealed between RA and untypical anaplastic anemia, iron deficiency anemia, and megaloblastic anemia patients. We propose that the technology of microarray may reveal the intrinsic molecular features and the expression levels of RNAHP, DDIT3, and ETV1 may provide useful markers for the diagnosis of MDS.

DHX32 expression suggests a role in lymphocyte differentiation.

RNA helicases constitute a large group of enzymes involved in all aspects of RNA metabolism. Several RNA helicases are dysregulated in cancer, whereas several others are involved in differentiation. DHX32 has previously been identified as a novel RNA helicase with a unique structure and expression pattern. DHX32 message was down-regulated in acute lymphoblastic leukemia cell lines and patient samples. In this report, anti-DHX32 was used to study its expression in thymus. Immunohistochemistry and flow cytometry showed positive correlation of DHX32 expression with thymocyte maturation. These results suggest that DHX32 might play a role in normal lymphocyte differentiation.

p68 DEAD box RNA helicase expression in keratinocytes. Regulation, nucleolar localization, and functional connection to proliferation and vascular endothelial growth factor gene expression.

Nitric oxide (NO) represents a short lived mediator that pivotally drives keratinocyte movements during cutaneous wound healing. In this study, we have identified p68 DEAD box RNA helicase (p68) from an NO-induced differential keratinocyte cDNA library. Subsequently, we have analyzed regulation of p68 by wound-associated mediators in human and murine keratinocytes. NO, serum, growth factors, and pro-inflammatory cytokines were potent inducers of p68 expression in the cells. p68 was constitutively expressed in the epithelial compartment of murine skin. Upon injury, we found a transient down-regulation of overall p68 protein in wound tissue. However, p68 did not completely disappear during early wound repair, as we found an expression of p68 protein in isolated wound margin tissue 24 h after wounding. Moreover, immunohistochemistry and cell fractionation analysis revealed a restricted localization of p68 in keratinocyte nuclei of the developing epithelium. Accordingly, cultured keratinocytes also showed a nuclear localization of the helicase. Moreover, confocal microscopy revealed a strong localization of p68 protein within the nucleoli of the cells. Functional analyses demonstrated that p68 strongly participated in keratinocyte proliferation and gene expression. Keratinocytes that constitutively overexpressed p68 protein were characterized by a marked increase in serum-induced proliferation and vascular endothelial growth factor expression, whereas down-regulation of endogenous p68 using small interfering RNA markedly attenuated serum-induced proliferation and vascular endothelial growth factor expression. Altogether, our results suggest a tightly controlled expression and nucleolar localization of p68 in keratinocytes in vitro and during skin repair in vivo that functionally contributes to keratinocyte proliferation and gene expression.

Ectopic expression of DICE1 suppresses tumor cell growth.

The tumor suppressor gene DICE1 is located within a previously reported critical region of loss of heterozygosity on chromosome 13q14.3. Expression of the remaining DICE1 allele is down-regulated in non-small cell lung carcinomas. Ectopic expression of DICE1 cDNA by DICE1-green fluorescent protein fusion constructs resulted in inhibition of colony formation of human non-small cell lung carcinoma cell line SK-MES-1 and NCI-H520 and prostate carcinoma cell line DU145. In IGF-IR transformed Balb/c 3T3, DICE1 substantially sup-pressed growth in soft agar. These results demonstrate that DICE1 has a growth-suppressing activity and interferes with anchorage-independent growth of IGF-IR transformed tumor cells dependent upon IGF-I signaling.

Retinoic acid-inducible gene-I is induced by interferon-gamma and regulates the expression of interferon-gamma stimulated gene 15 in MCF-7 cells.

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH box family proteins, which have diverse roles in regulation of gene expression and cellular functions. We found RIG-I mRNA and protein were expressed in MCF-7 human breast cancer cells stimulated with interferon-gamma (IFN-gamma). This effect of IFN-gamma was observed in concentration- and time-dependent manners, and IFN-gamma also induced promoter activity of RIG-I. Transfection of GFP-RIG-I cDNA into MCF-7 cells resulted in the expression of RIG-I protein in cytoplasm. Overexpression of RIG-I induced the upregulation of IFN-gamma stimulated gene 15, which has the potential to amplify the immunomodulatory effects. We conclude that IFN-gamma induces the expression of RIG-I, which may play a role in the immunological effects of IFN-gamma.

Over-expression of RNA helicases in cancer.

RNA helicases constitute a large group of essential enzymes involved in all aspects of RNA metabolism. With few exceptions, human RNA helicases of DDX and DHX gene families have not been well characterized. However, several of them have been shown to be dysregulated in cancer, including over-expression in various types of tumors. Although the exact contribution of RNA helicases to carcinogenesis has not been determined, their over-expression in cancer render them potential targets for novel anti-cancer agents. In this review, RNA helicases whose expression is up-regulated in cancer are highlighted.

A novel cellular RNA helicase, RH116, differentially regulates cell growth, programmed cell death and human immunodeficiency virus type 1 replication.

In an effort to define novel cellular factors regulating human immunodeficiency virus type 1 (HIV-1) replication, a differential display analysis has been performed on endogenously infected cells stimulated with the HIV-suppressive immunomodulator Murabutide. In this study, the cloning and identification of a Murabutide-downregulated gene, named RH116, bearing classical motifs that are characteristic of the DExH family of RNA helicases, are reported. The 116 kDa encoded protein shares 99.9 % similarity with MDA-5, an inducible RNA helicase described recently. Ectopic expression of RH116 in HeLa-CD4 cells inhibited cell growth and cell proliferation but had no measurable effect on programmed cell death. RH116 presented steady state cytoplasmic localization and could translocate to the nucleus following HIV-1 infection. Moreover, the endogenous expression of RH116, at both the transcript and protein levels, was found to be considerably upregulated after infection. Overexpression of RH116 in HIV-1-infected HeLa-CD4 cells also resulted in a dramatic increase in the level of secreted viral p24 protein. This enhancement in virus replication did not stem from upregulated proviral DNA levels but correlated with increased unspliced and singly spliced viral mRNA transcripts. These findings implicate RH116 in the regulation of HIV-1 replication and point to an apoptosis-independent role for this novel helicase in inducing cell growth arrest.

Mechanisms and enzymes involved in SARS coronavirus genome expression.

A novel coronavirus is the causative agent of the current epidemic of severe acute respiratory syndrome (SARS). Coronaviruses are exceptionally large RNA viruses and employ complex regulatory mechanisms to express their genomes. Here, we determined the sequence of SARS coronavirus (SARS-CoV), isolate Frankfurt 1, and characterized key RNA elements and protein functions involved in viral genome expression. Important regulatory mechanisms, such as the (discontinuous) synthesis of eight subgenomic mRNAs, ribosomal frameshifting and post-translational proteolytic processing, were addressed. Activities of three SARS coronavirus enzymes, the helicase and two cysteine proteinases, which are known to be critically involved in replication, transcription and/or post-translational polyprotein processing, were characterized. The availability of recombinant forms of key replicative enzymes of SARS coronavirus should pave the way for high-throughput screening approaches to identify candidate inhibitors in compound libraries.