HIV-1 (human immunodeficiency virus) transcription is primarily controlled by the virally encoded Tat (transactivator of transcription) protein and its interaction with the viral TAR (transcription response element) RNA element. Specifically, binding of a Tat-containing complex to TAR recruits cellular factors that promote elongation of the host RNA polymerase engaging the viral DNA template. Disruption of this interaction halts viral RNA transcription. In the present study, we investigated the effect of pokeweed antiviral protein (PAP), an RNA glycosidase (EC#: synthesized by the pokeweed plant (Phytolacca americana), on transcription of HIV-1 mRNA. We show that co-expression of PAP with a proviral clone in culture cells resulted in a Tat-dependent decrease in viral mRNA levels. PAP reduced HIV-1 transcriptional activity by inhibiting Tat protein synthesis. The effects of PAP expression on host factors AP-1 (activator protein 1), NF-κB (nuclear factor kappa-light-chain-enhancer of activated B-cells) and specificity protein 1, which modulate HIV-1 transcription by binding to the viral LTR (5′-long terminal repeat), were also investigated. Only AP-1 showed a modest JNK pathway-dependent increase in activity in the presence of PAP; however, this activation was not sufficient to significantly enhance transcription from a partial viral LTR containing AP-1 binding sites. Therefore, the primary effect of PAP on HIV-1 transcription is to reduce viral RNA synthesis by decreasing the abundance of Tat. These findings provide a mechanistic explanation for the observed decrease in viral RNAs in cells expressing PAP and contribute to our understanding of the antiviral effects of this plant protein.

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