RNA oligonucleotide fingerprint analyses indicate tht the genome RNA obtained from Trinidad donkey (TRD) Venezuelan equine encephalomyelitis (VEE) virus serotype I A, its vaccine strain derivative TC-83, and the VEE I B virus isolate PTF-39, have almost identical patterns of characteristic ribonuclease T1 resistant oligonucleotides. The TC-83 strain and the I B isolate can, on the basis of these analyses, be considered as variants of the TRD virus and categorized as I AB serotypes. Comparisons made by single and co-electrophoreses of the ribonuclease T1 digests of the RNA species of TC-83 and a VEE I C isolate P6-76 indicate that 16 of 37 large oligonucleotides of the TC-83 virus co-migrate with the oligonucleotides obtained from the I C isolate. Similar single and co-electrophoreses of ribonuclease T1 digests of the RNA species of TC-83 and a VEE I D isolate 3880 indicate that 18 of 41 TC-83 large oligonucleotides co-migrate with the oligonucleotides obtained from the I D virus isolate. At least nine of the TC-83 large oligonucleotides appear on the basis of these analyses, to be present in the digests of the genome RNA obtained from these selected I B, I C and I D virus isolates. The ribonuclease T1 digests of three I E virus isolates (Mena II, 63U2 and 71U388) give oligonucleotide fingerprints which, although comparable to each other, are more distinct from the I A and I B RNA fingerprints than are those of the I C and I D RNA species. The ribonuclease T1 resistant oligonucleotide fingerprints of VEE virus isolates belonging to serotypes (VEE subtypes) II, III and IV show little similarity to each other or to those of the serotype I virus isolates we have studied. The results obtained here agree with the reported close antigenic relationships of VEE, I A, I B, I C and I D virus isolates, and our studies suggest that these viruses have conserved nucleotide sequences. The I E virus isolates appear to have more distinct nucleotide sequences than do the other serotype I viruses. The results also agree with the serological differentiation of VEE, I, II, III and IV subtypes in that the oligonucleotide fingerprints of subtypes II to IV are different from each other and from those of the different serotype I virus isolates. On the basis of antigenic and genome relationships, VEE isolates can be classified as serotypes I to IV with serotype I viruses differentiated into the categories I AB, I C, I D and I E.
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