An adenosine at position 27 in the human immunodeficiency virus type 1 trans-activation response element is not critical for transcriptional or translational activation by Tat.

Tat protein binds to the trans-activation response (TAR) element of human immunodeficiency virus type 1 RNAs and activates gene expression at the level of transcription in mammalian cell lines and translation in Xenopus oocytes. Certain residues within TAR are important for Tat binding in vitro, including residue A-27, which appears to be able to be modified in a Tat-dependent manner in Xenopus oocytes (L. Sharmeen, B. Bass, N. Sonenberg, H. Weintraub, and M. Groudine, Proc. Natl. Acad. Sci. USA 88:8096-8100, 1991). Activation by Tat in oocytes occurs via a covalent modification of TAR-containing RNA. We have found that in both mammalian cells and Xenopus oocytes, conversion of A-27.U-38 or C-27.G-38 or C-27.G-38 reduces activation. However, conversion to G-27.U-38 or G-27.C-38 had little or no effect on activation, and in oocytes, these mutant RNAs were still covalently modified. These data exclude a specific role for the adenosine at residue 27 for Tat activation but suggest a requirement for a purine at this position.