Aromatic polyamidines inhibiting the Tat-induced HIV-1 transcription recognize structured TAR-RNA

We have investigated the effects of aromatic polyamidines on HIV-1 transcri ption. We found a block to Tat-induced HIV-1 transcription assessed by inhi bition of CAT activity in RL3T1 cells at a concentration lower than the IC5 0 value, suggesting that molecules with three (TAPB) and four (TAPP) benzam idine rings could be useful against HIV-1. In contrast, aromatic polyamidin es with only two benzamidine rings (DAPP) did not block Tat-induced transcr iption. We reasoned that this effect could be due to binding of TAPB and TA PP to HIV-1 TAR RNA. By EMSA and filter binding assays, we studied possible interactions of aromatic polyamidines with HIV-1 TAR RNA. Wild-type TAR RN A or TAR RNA with mutations in the stem or bulge sequences, but retaining t he stem-loop structure, was used to define the RNA-binding activities of th ese compounds. Our data suggest that aromatic polyamidines with two (DAPP) and four (TAPP) benzamidine rings, respectively, do not bind to TAR RNA or bind without sequence selectivity. Interestingly, an aromatic polyamidine w ith three benzamidine rings (TAPB) recognizes the wild-type TAR RNA in a sp ecific manner. Furthermore, we found that introduction of one halogen atom into the benzamidine rings strongly increases the RNA-binding activity of t hese compounds.