Trans-activation of human immunodeficiency virus gene expression is mediated by nuclear events

Human immunodeficiency virus encodes a gene product termed tat that is able to activate viral gene expression when present in trans. The mechanism of action of the tat gene product appears to be bimodal, resulting in both an increase in the steady-state level of viral mRNA and the enhanced translation of that RNA. In this report we have examined the mechanism by which tat elevates viral mRNA levels. Data are presented demonstrating that tat acts by increasing the rate of viral transcription, rather than by modulating the stability of viral mRNA. Indirect immunofluorescence was used to show that tat is predominantly localized in the nucleus of expressing cells, a location consistent with a role in the regulation of viral transcription. These results suggest that tat could play a role in human immunodeficiency virus replication essentially similar to that proposed for the trans-acting nuclear gene products described for several other virus species.