cDNA clones of the St. Louis encephalitis (SLE) virus genome have been obtained and the nucleotide sequence of 4.7 kb corresponding to the 5′ terminal half of the genome determined. The genome contains a 5′ noncoding region of 98 nucleotides followed by a single continuous open reading frame that encodes three structural proteins in the order capsid (C), membrane precursor (prM)-membrane (M), and envelope (E). Immediately following the C-terminus of E are located nonstructural proteins NS1 through NS3. The SLE amino acid sequence homology with yellow fever (YF), Murray Valley encephalitis (MVE), West Nile (WN), and dengue-2 (DEN) viruses over the sequenced region is 39, 66, 64, and 43%, respectively. The start of each SLE protein has been assigned on the basis of N-terminal sequence data and potential proteolytic cleavage sites homologous with YF and MVE viruses. Flaviviruses have conserved glycosylation sites in prM and NS1 proteins, although only one of the two glycosylation sites in the SLE E protein is conserved in MVE and DEN viruses. An evolutionary tree showing relationships of SLE, MVE, WN, YF, and DEN-2 flaviviruses is proposed on the basis of the amino acid sequences of the C proteins.
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