In human immunodeficiency virus type 1 (HIV-1)-infected patients treated with combination antiretroviral therapy (cART), the persistence of latent viral reservoirs remains the major barrier to virus eradication1. The molecular mechanisms that govern HIV-1 latency at the transcriptional level are not well characterized, but a key player is the HIV-1 Transactivator of transcription (Tat) protein, which promotes viral gene transcription. The sequence of Tat can vary within and between viral subtypes2,3, and this has been shown to translate into differential Tat activity4,5. However, emerging data show that the transmitted/founder (T/F) viruses have a consensus-like sequence for other key genes, including gag, pol and env6. Consistent with this, our group has shown that Gag and Protease sequences obtained from infants in mother-to-child HIV-1 subtype C (HIV-1C) transmission pairs are similar to the HIV-1C consensus sequences7. Taken together, these data suggest that consensus-like sequences for gag, pol and env genes are selected for at transmission. Interestingly, data from our group also demonstrated that HIV-1C tat genetic variation that translates into differential LTR activity exists at transmission (Mkhize et al. unpublished), thus HIV-1C T/F virus Tat variation may influence disease outcome. However, the molecular mechanisms by which Tat variation results in diminished transactivation activity remain unknown. Therefore, here I will investigate the effect of an HIV-1C Tat variant, P21A, alone and/or in combination with other Tat mutations on Tat protein stability, binding properties, and recruitment of P-TEFb for efficient viral gene transcription (Aim 1). In Aim 2, I will investigate the effect of the Tat P21A variant alone or in combination with other Tat mutations on virus latency development or reversal. This project will build on a collaboration with Melanie Ott’s group, who made the key observation that monomethylation of Tat enhances Tat-dependent HIV-1 transcription.