Premature termination and processing of human immunodeficiency virus type 1-promoted transcripts.

We have used transient expression assays to study transcription directed by the human immunodeficiency virus (HIV) type 1 promoter. A plasmid containing an HIV-reporter gene fusion and a simian virus 40 origin of DNA replication was transfected into COS-1 cells in the presence or absence of a Tat expression vector. HIV-promoted RNA was analyzed by in vivo labeling, by RNase protection mapping, and in run-on transcription assays. As observed previously, two populations of HIV RNA accumulate in vivo: short, attenuated transcripts and long, polyadenylated mRNA. The short transcripts labeled in vivo were longer and more heterogeneous than expected from RNase protection assays. Moreover, comparison of transcripts labeled in vivo with run-on transcription products revealed that similar, if not identical, short RNAs accumulate in vitro. Utilizing the run-on assay, we show that following transcriptional termination, the attenuated transcripts undergo processing to generate one species of RNA. We also provide evidence that Tat does not act as an antiterminator to relieve a discrete elongation block but instead modifies transcriptional complexes, enabling them to overcome putative pause sites and continue transcription of the template.