FBI-1, a factor that binds to the HIV-1 inducer of short transcripts (IST), is a POZ domain protein

The HIV-1 promoter directs the synthesis of two classes of transcripts, sho rt, non-polyadenylated transcripts and full-length, polyadenylated transcri pts. The synthesis of short transcripts is activated by a bipartite DNA ele ment, the inducer of short transcripts or IST, located downstream of the HI V-1 transcriptional start site, while the synthesis of full-length transcri pts is activated by the viral activator Tat, Tat binds to the RNA element T AR, which is encoded largely between the two IST half-elements. Upon activa tion by Tat, the synthesis of short RNAs is repressed, We have previously p urified a factor called FBI-1 (for factor that binds to IST) whose binding to wild-type and mutated ISTs correlated well with the abilities of these I ST to direct the synthesis of short transcripts, Here, we report the clonin g of cDNAs encoding FBI-1. FBI-1 contains a POZ domain at its N-terminus an d four Kruppel-type zinc fingers at its C-terminus, The C-terminus is suffi cient for specific binding, and FBI-1 can form homomers through its POZ dom ain and, in vivo, through its zinc finger domain as well. In addition, FBI- l associates with Tat, suggesting that repression of the short transcripts by Tat may be mediated through interactions between the two factors.