Regulation of gene expression directed by the long terminal repeat of the feline immunodeficiency virus

The long terminal repeat (LTR) of a retrovirus contains sequence elements that constitute a promoter for controlling viral gene expression in infected cells. We have examined regulation of LTR-directed gene expression in feline immunodeficiency virus (FIV), a T-lymphocytopathic lentivirus associated with a fatal AIDS-like disease in domestic cats. Two independent virus isolates, designated FIV-petaluma and FIV-PPR, have been molecularly cloned and show greater than 85% sequence homology. Both clones (termed pF34 and pPPR) produce infectious virus after transfection of permissive feline cells. Basal promoter activity of the LTRs was measured in various cell lines in transient expression assays using plasmids containing the viral LTR linked to the bacterial chloramphenicol acetyltransferase gene. Both LTRs were strong promoters in several cell lines, although in some cell lines the pF34 LTR had four- to fivefold higher basal activity than the pPPR LTR. FIV LTR mutations affecting the first AP4 site, AP1 site, ATF site, or NF-KB site resulted in decreased basal activity of the FIV promoter. Mutational analysis also revealed a negative regulatory element. In cotransfection experiments, both pF34 proviral DNA and pPPR proviral DNA appeared to transactivate either the pF34 LTR or the pPPR LTR; however, levels of transactivation were very low. Cotransfection of both LTRs with FIV subgenomic clones containing various viral open reading frames resulted in low level or no transactivation. The LTRs of both FIV clones responded to cell activation signals in human T-lymphoid cells (Jurkat) treated with phytohemagglutinin and phorbol-12-myristate-13-acetate. Promoter function of both FIV LTRs was also enhanced in cells treated with either forskolin, an inducer of intracellular cyclic-AMP (c-AMP), or dibutyryl c-AMP. Analysis of site-specific mutants showed that a potential AP1 site in the U3 domain of the LTR was required for T-cell activation responses mediated by protein kinase C, whereas a putative ATF site was the target for c-AMP-induced responses mediated by protein kinase A. These studies revealed that cellular transcription factors play a significant role in regulation of FIV gene expression.