HIV-1 Tat protein is able to efficiently transactivate the HIV-2 LTR through a TAR RNA element lacking both dinucleotide bulge binding sites.
Each of the two stem-loop structures in the HIV-2 TAR (TAR-2) RNA element contains a dinucleotide bulge that specifies a binding site in vitro for the HIV-2 Tat transactivator protein. A TAR-2 RNA with both bulges deleted is very weakly transactivated in vivo by the HIV-2 Tat protein. To gain insight into general features of Tat protein:TAR RNA interactions, we have analyzed the significance of the dinucleotide bulges in TAR-2 RNA for in vitro binding and in vivo transactivation by the related HIV-1 Tat protein. The HIV-1 Tat protein has been shown previously to bind efficiently to wild-type TAR-2 RNA and fully transactivates the HIV-2 LTR. We found that the 5' proximal bulge and the 3' distal bulge appear to specify a high and low affinity binding site in vitro, respectively, for the HIV-1 Tat protein. Wild-type TAR-2 RNA was found to be able to bind HIV-1 Tat proteins simultaneously at each bulge binding site in vitro. A TAR-2 RNA with both bulges deleted was greatly defective for in vitro binding by the HIV-1 Tat protein. Surprisingly, the TAR-2 RNA with both bulges deleted was efficiently transactivated in vivo by the HIV-1 Tat protein, indicating that the HIV-1 Tat protein (but not HIV-2 Tat protein) is able to strongly activate transcription of a TAR RNA with no apparent bulge binding site.
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Division of Molecular Virology, Baylor College of Medicine, Houston, Texas 77030.
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D001483:Base Sequence; D001665:Binding Sites; D015696:Gene Products, tat / Q000502:physiology*; D016325:HIV Long Terminal Repeat; D015498:HIV-2 / Q000235:genetics*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009711:Nucleotides / Q000378:metabolism; D012367:RNA, Viral / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D015533:Transcriptional Activation; D054322:tat Gene Products, Human Immunodeficiency Virus
D015696:Gene Products, tat; D009711:Nucleotides; D012367:RNA, Viral; D054322:tat Gene Products, Human Immunodeficiency Virus