The transactivator of transcription protein (Tat) of human immunodeficiency virus (HIV) plays an important role in both viral replication and AIDS pathogenesis. Tat is responsible for enhancing HIV-specific transcription, without which production of infectious virus is severely limited. Ongoing research, however, has identified a plethora of additional functions attributable to Tat including regulating additional HIV processes, altering the homeostasis of both infected and uninfected cells, and being directly involved in AIDS-related neuropathology and cancer. Tat thus plays a diverse role in both viral replication and pathogenesis. What is the relative importance of Tat’s additional functions compared to its principle function of transactivation? This question is explored in Chapter 2 in which tat clones from an epidemiologicallylinked transmission cohort were functionally tested. tat sequences were isolated and cloned from the cohort members at various time points and assessed for transactivation potential. The data revealed that transmission of HIV resulted in the selection of mutations in tat that were host specific and that impacted on the Tat clones’ abilities to transactivate. No correlation, however, was observed between the transactivation potentials of the Tat clones and the AIDS progression statuses of the hosts. The host-specific tat mutations may instead reflect the selection of variants fitter in one or more of Tat’s other functions. This therefore leads to the hypothesis that Tat functions in addition to transactivation are influential in AIDS pathogenesis and are subject to selective pressures during HIV transmission. Tat’s flexibility and diversity of functions are mediated by defined functional domains. One of the most important of these is the basic domain, a highly conserved region encompassing a unique nuclear localisation signal (NLS). The NLS allows Tat to localise to the cell nucleus to participate in transactivation. However, many of Tat’s additional functions require Tat to be outside of the nucleus or even outside of the cell. It is currently unknown how the NLS is modulated to enable Tat nuclear egress, but it may involve post-translational modifications of NLS residues. In Chapter 3, protein arginine methyltransferase 6 (PRMT6) is demonstrated to alter the localisation of Tat within the nucleus and increase the half-life of Tat within the cell. Both of these effects require the methylation activity of PRMT6 and two NLS residues, arginines 52 and 53, previously described to be substrates for PRMT6 methylation. The results suggest that PRMT6 primes Tat for functions outside of the nucleus by altering its subcellular localisation and increasing its stability. The critical role of the basic domain in Tat function is highlighted in Chapter 4 in which mutation of the basic domain led to dysregulation of HIV replication. The mutant, referred to as Nullbasic, has characteristics similar to transdominant Tat mutants, such as suppressing transactivation and localising to the cell cytoplasm. Surprisingly, Nullbasic potently inhibits HIV infectivity by strongly downregulating the expression of envelope, the key molecule involved in cell attachment and entry. Part of this downregulation is attributable to Nullbasic interfering with the nuclear export of envelope-encoding mRNA as a result of altering the subcellular localisation of Rev. However, an additional mechanism of interference is required to fully explain the strong suppression of envelope protein observed in cells. The data allude to a role for Tat in promoting envelope mRNA translation, a role subverted by mutating the basic domain. Taken together, these studies highlight the importance of a diversity of Tat functions to HIV replication and AIDS pathogenesis. Further, they emphasise the critical role of the basic domain to Tat function. Modifications of the basic domain not only influence function, but also regulate Tat spatially and temporally. Dysregulation of the basic domain by mutation not only subverts Tat function, but also elicits antiviral activity at multiple steps of the HIV lifecycle. The study of Tat and its basic domain is crucial in order to understand its role in HIV/AIDS and to recognise Tat as a pathogenic agent in its own right.