Initiation of HIV-1 reverse transcription is regulated by a primer activation signal

Reverse transcription of the human immunodeficiency virus type 1 (HIV-1) RN A genome appears to be strictly regulated at the level of initiation. The p rimer binding site (PBS), at which the tRNA(3)(LYS) molecule anneals and re verse transcription is initiated, is present in a highly structured region of the untranslated leader RNA. Detailed mutational analysis of the U5 lead er stem identified a sequence motif in the U5 region that is critical for a ctivation of the PBS-bound tRNA(3)(Lys) primer. This U5 motif, termed the p rimer activation signal Lys (PAS), may interact with the T psiC arm of the tRNA(3)(Lys) primer, similar to the additional interaction proposed for the genome of Rous sarcoma virus and its tRNA(Trp) primer. This suggests that reverse transcription is regulated by a common mechanism in all retroviruse s. In HIV-1, the PAS is masked through base pairing in the U5 leader stem. This provides a mechanism for positive and negative regulation of reverse t ranscription. Based on structure probing of the mutant and wild-type RNAs, an RNA secondary structure model is proposed that juxtaposes the critical P AS and PBS motifs.