Ras-responsiveness of the HIV-1 LTR requires RBF-1 and RBF-2 binding sites

Lentiviruses characteristically form latent integrated proviruses whose transcription can be induced by cell regulatory signals. The HIV-1 LTR responds to multiple signals, including the Ras pathway. We report here that Ras-responsive HIV-1 transcription requires two previously undescribed factors, RBF-1 and RBF-2 in Jurkat T cells. RBF-1 binds to Ets-like motifs located between nucleotides -151 and -142, and within the NF-kappaB binding sites, but is distinct from Ets-1 or Elf-1. RBF-2 binds the HIV-1 LTR at nucleotides -131 and -121 and immediately 3' of the TATA box. Both RBF-1 and RBF-2 contain DNA binding subunits of relative molecular weight 100 kilodaltons. Mutation of the RBF-1 and RBF-2 binding elements (RBEs) prevents Ras stimulation of HIV-1 LTR-directed transcription. These data define a mechanism for Ras responsiveness of HIV-1 transcription that involves the previously uncharacterized factors RBF-1 and RBF-2.