Abstract
Bovine leukemia virus (BLV) is a deltaretrovirus infecting bovine B cells and causing enzootic bovine leucosis (EBL). The long terminal repeat (LTR) plays an indispensable role in viral gene expression. The BLV Tax protein acts as the main transactivator of LTR-driven transcription of BLV viral genes. The aim of this study was to analyze mutations in the BLV LTR region and tax gene to determine their association with transcriptional activity. LTRs were obtained from one hundred and six BLV isolates and analyzed for their genetic variability. Fifteen variants were selected and characterized based on mutations in LTR regulatory elements, and further used for in vitro transcription assays. Reporter vectors containing the luciferase gene under the control of each variant BLV promoter sequence, in addition to variant Tax expression vectors, were constructed. Both types of plasmids were used for cotransfection of HeLa cells and the level of luciferase activity was measured as a proxy of transcriptional activity. Marked differences in LTR promoter activity and Tax transactivation activity were observed amongst BLV variants. These results demonstrate that mutations in both the BLV LTR and tax gene can affect the promoter activity, which may have important consequences on proviral load, viral fitness, and transmissibility in BLV-infected cattle.
Highlights
Bovine leukemia virus (BLV), which belongs to the Deltaretrovirus genus of Retroviridae family, is the etiologic agent of enzootic bovine leucosis (EBL)
The long terminal repeat (LTR) sequence analysis revealed the presence of one deletion located in T(−11)del of Catabolite Activator Protein (CAP) site in
Samples of proviral DNA extracted from peripheral blood leukocytes (PBLs) were collected from Polish cattle naturally infected with BLV, and the sequences of the LTR region and the tax gene were determined in order to better understand how genomic variation is tied to BLV
Summary
Bovine leukemia virus (BLV), which belongs to the Deltaretrovirus genus of Retroviridae family, is the etiologic agent of enzootic bovine leucosis (EBL). EBL is a lymphoproliferative disease characterized by B-cell lymphoma, and occurs in cattle throughout the world [1,2,3,4,5]. In the majority of cases, infection is asymptomatic; but, approximately one-third of BLV-infected cattle develop persistent lymphocytosis and less than 5% progress to B-cell lymphoma [6]. Eradication programs have been developed but success has been variable, primarily because of the expense and high prevalence of infection among cattle in the particular countries of the Americas, Eurasia, and Asia. The BLV genome is comprised of two identical copies of single-stranded RNA. This viral RNA is reverse transcribed to form a double-stranded DNA called provirus, flanked by long terminal repeats (LTRs) [7,8]. The LTR contains the Pathogens 2020, 9, 836; doi:10.3390/pathogens9100836 www.mdpi.com/journal/pathogens
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