Transactivation mechanism of the Jembrana disease virus tat gene

Jembrana disease virus (JDV) is a newly identified bovine lentivirus which causes an acute and sometimes fatal disease in infected animals. In other lentiviruses, the tat gene plays a critical role in viral gene expression and virus replication. To determine whether JDV also encodes a functional Tat, and whether JDV Tat is responsible for high titer JDV expression in infected cattle, we identified and characterized the JDV tat gene, and compared its transactivation function with HIV tat. We first demonstrated that the JDV Tat encoded by the exon 1 possessed strong transactivation activities, and the predicted JDV TAR region in the long terminal repeat (LTR) was important for the transactivation using a co-transfection assay. JDV Tat is a very ubiquitous and potent transactivator of other lentiviral promoters including HIV promoter. JDV Tat, like HIV Tat, transactivated the HIV LTR at least partially in a TAR-dependent manner. The chimeric HIV virus carrying the JDV tat gene replicated in human cells, suggesting that JDV Tat can functionally substitute for HIV Tat. The JDV Tat is entirely encoded by one exon and differs from the HIV Tat in its TAR-recognition mechanism. The function domain swapping experiment demonstrated that JDV Tat binding domain could recognize both HIV and JDV TAR while the HIV Tat binding domain can only specifically recognize its cognate TAR sequences. A gel-shift assay also showed that JDV Tat could directly bind to the HIV TAR RNA in vitro. The cellular factor cyclin T1 may be dispensable for JDV Tat binding to JDV TAR. JDV Tat-mediated transactivation can tolerate the mutations in the loop region of TAR. It is resistant to inhibition of 5,6-Dichloro-1-beta-ribofuranosylbenzimidazole (DRB), a CDK9 inhibitor which specifically blocks the cyclin T pathway. The bovine cyclin T may be the species-determinant that prevents HIV Tat from transactivating the HIV LTR in bovine cells. Overexpression of the bovine cyclin T competitively inhibits HIV Tat function in human cells. To our surprise, the fetal bovine lung (FBL) cells transfected with proviral HIV DNA produced high levels of p24. However, FBL cells cannot be infected by HIV productively