PURIFICATION AND CHARACTERIZATION OF FBI-1, A CELLULAR FACTOR THAT BINDS TO THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 INDUCER OF SHORT TRANSCRIPTS

The human immunodeficiency virus (HIV-1) promoter directs the synthesi s of two classes of RNA molecules, short transcripts and full-length t ranscripts. The synthesis of short transcripts depends on a bipartite DNA element, the inducer of short transcripts (IST), located in large part downstream of the HIV-1 start site of transcription. IST does not require any viral product for function and is thought to direct the a ssembly of transcription complexes that are incapable of efficient elo ngation. Nothing is known, however, about the biochemical mechanisms t hat mediate IST function. Here, we report the identification and purif ication of a factor that binds specifically to the IST. This factor, F BI-1, recognizes a large bipartite binding site that coincides with th e bipartite IST element. It is constituted as least in part by an 86-k Da polypeptide that can be specifically cross-linked to IST. FBI-1 als o binds to promoter and attenuation regions of a number of cellular an d viral transcription units that are regulated by a transcription elon gation block. This observation, together with the observation that the binding of FBI-1 to IST mutants correlates with the ability of these mutants to direct IST function, suggests that FBI-1 may be involved in the establishment of abortive transcription complexes.