Conventional generation of such cDNA clones requires the production of an initial virus stock, viral RNA isolation, reverse transcription, PCR amplification of subfragments and engineering into the final transcription units. These approaches are sometimes hampered by low fidelity

of reverse transcriptase BLU9931 nmr or sequence variations in the starting isolate, which may lead to undesired alterations of the genomic sequence. As a consequence, in most reports in which the viral cDNA clones or generated viruses were analyzed by sequence analysis, nucleotide variations were detected compared to the published sequence of the parent virus [6], [7], [9], [13], [14], [16] and [19]. In 2002, a landmark publication proved the feasibility of de novo synthesis of a poliovirus by biochemical synthesis precluding any preformed components. The viral cDNA encoding the 7.5 kb genome was assembled from overlapping oligonucleotides and yielded infectious virus after transcription SNS-032 molecular weight of genomic RNA and inoculation into cell lysates [23]. Taking advantage of the rapid progression of gene synthesis technology (for review [24]), we intended to adopt such a synthetic approach to produce a flavivirus cDNA system

for the generation of a synthetic WNV seed virus for use in vaccine development. In this study we report the generation of a fully functional WNV virus from a completely synthetic source. The whole 11,029-nucleotide WNV genomic sequence was generated by gene synthesis without using

natural viral templates. The production and characterization of the resulting West Nile Virus, which fully matched the sequence of the in silico designed viral genome, confirms the feasibility and accuracy of the synthetic flavivirus reverse genetic system. WNV wild-type virus strain NY99-flamingo 382-99 was obtained from Centers for Disease Control (CDC, Atlanta) corresponding to GenBank accession #AF196835. This sequence information was also used as template for in silico design for de novo synthesis of the genomic cDNAs. The cell lines Vero (ATCC CCL-81), BHK (ATCC CCL-10) and C6-36 (ECEACC 123.P. #03D016) were obtained from the American Type Culture Collection Phosphatidylinositol diacylglycerol-lyase or European Collection of Cell Cultures and grown in Dubecco’s modified Eagle’s medium (DMEM) or TC-Vero Media (Baxter). TC-Vero is an animal protein-free medium based on DMEM/Ham’s F12 medium. Six DNA fragments corresponding to WNV strain NY99-flamingo 382-99 (GenBank accession #AF196835) were generated by chemical synthesis (GENEART, Regensburg, Germany). Plasmid p5′TL-AB carried DNA corresponding to WNV genomic sequence nt 1–1792, plasmid p5′TL-CD to nt 1789–3632, plasmid p3′TL-AB to nt 3622–5801, plasmid p3′TL-CD to nt 5792–8028, plasmid p3′TL-EF to nt 8022–10,025 and plasmid p3′TL-GH to nt 10,022–11,029.