The gene expression of human foamy virus does not require a post-transcriptional transactivator

Human foamy virus (HFV) comprises a complex genomic organization of gag, pol, env, and several nonstructural genes such as bel1, bel2, bel3, bet, beo, and bes located between env and 3' LTR. Among these viral nonstructural genes, bel1 appears to encode an essential transactivator of LTR-directed gene expression. To investigate the roles of the other nonstructural proteins for the viral replication, a series of proviral mutants were generated and tested for their in vitro replication. The mutations in the other than bel1 open reading frame did not show any significant effect on viral replication. The bel1 protein is the only essential transactivator for the LTR-directed transcription. To determine whether HFV has an essential post-transcriptional transactivator like the rev protein of human immunodeficiency virus type 1, or the rex protein of human T cell leukemia virus type 1, we investigated the bel1-independent expression of the HFV gag structural gene under the control of the heterologous simian virus 40 promoter. We demonstrated that the expression of the HFV gag structural gene does not require an essential post-transcriptional transactivator. Thus, it appears that the regulation of HFV gene expression is distinct from that of other human retroviruses.