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Genetic peculiarities of bovine leukemia virus isolates (BLV) spread throughout Russia and Ukraine (BLV-1, BLV-2, and BLV-4) have been characterized based on pol gene polymorphism. Seven viral forms have been detected. The variability of BLV isolates did not exceed 1% within one form. Despite the recent inhabitation of BLV in Russia in the middle of 20th century, Russian BLV variants are characterized by several specific nucleotide substitutions. Point mutations that result in the changes in the aminoacid sequence of reverse transcriptase of BLV specific to distinct viral forms were observed. C --> G transition at the 2752 position (relatively to the reference genome AF033818), which results in the substitution of glutamic to asparaginic acid (GAG --> GAC), is specific to form BLV-2. This mutation was demonstrated in BLV isolates from Ukraine. The T --> G substitution at the 2758 position, which results in the substitution of isoleucine for methionine (ATT --> ATG), is specific to BLV-4 and BLV-7 forms. The BLV-4 form was only detected in Russia and Ukraine. The present study also includes a review of the published data concerning BLV variability. The existing classifications of BLV forms have been critically conceived and the new optimal classification of BLV forms with the maximal resolution has been suggested.

Bovine leukemia virus (BLV) is a widespread specific pathogen of cattle. Analysis of the pol viral gene polymorphism has been used to characterize the polymorphism of BLV isolates at stock-breeding farms in Russia and Ukraine. The fragments of the pol gene corresponding to the reverse transcriptase and integrase 494 and 233 bp in size, respectively, have been used for analysis. Phylogenetic analysis has revealed several variants of BLV clustered with a high bootstrap support in Russia and Ukraine. A new classification of BLV variants is suggested. Comparison of phylograms based on the polymorphism of the nucleotide sequences of the integrase and reverse transcriptase domains did not show topological conflicts. Therefore, recombination between BLV variants has not been found.

Bovine leucosis virus (BLV), circulating in the Ukrainian territory, was characterized through the definition of its subspecies affiliation. The pro-viral BLV DNA was isolated from peripheral-blood lymphocytes of naturally-HIV-infected black-variegate animals taken from leucosis-affected farms in the Kharkov Region. The env-gene fragment of pro-viral DNA was amplified, sequenced and analyzed after the amplicon had been treated by three restriction enzymes, i.e. BamH I, Bcl I and Pvu II. According to the analysis of restriction-fragments' length polymorphism, the Ukrainian BLV isolate can be classified as belonging to the Australian subspecies, i.e. to one of the 3 known subspecies. Multiple alignment and phylogenetic analysis of the env-gene fragment of BLV isolates from the EMBL database showed that evolutionally the Ukrainian isolate is distantly located from the isolates' clusters of the Belgian, Japanese and Australian subspecie and has the biggest quantity (4) of non-coinciding nucleotides for the analyzed highly conservative locus of the BLV env-gene with a length of 444 pair of nucleotides.