Tat transactivation of the human immunodeficiency virus type 1 promoter is influenced by basal promoter activity and the simian virus 40 origin of DNA replication

We examined the activation of transcription from the human immunodeficiency virus type 1 (HIV-1) promoter by the viral Tat protein in a transient expression system. Plasmids contained a HIV-reporter gene cassette and a simian virus 40 origin of DNA replication. Run-on assays of transcription complex distribution and analysis of cytoplasmic RNA accumulation confirmed that Tat is able to activate transcription by two mechanisms: by increasing the rate of transcriptional initiation and the efficiency of transcriptional elongation. The degree to which Tat stimulated initiation is determined by the basal level of HIV-directed transcription, which is influenced by the position of the simian virus 40 replication origin. Tat functions primarily to increase the efficiency of elongation when the origin is located downstream from the HIV-reporter cassette and the basal level of transcription is high. On the other hand, Tat functions primarily to increase the rate of initiation when the origin is upstream from the cassette and the basal level of transcription is 10-fold lower. These studies suggest that the site of integration of the virus into the cellular genome may significantly affect the level of expression from the HIV promoter and consequently the pathobiology of the virus.