Rotational symmetry in ribonucleotide strand requirements for binding of HIV-1 Tat protein to TAR RNA

MYB elongation is regulated by the nucleic acid binding of NFκB p50 to the intronic stem-loop region

MYB transcriptional elongation is regulated by an attenuator sequence within intron 1 that has been proposed to encode a RNA stem loop (SLR) followed by a polyU tract. We report that NFκBp50 can bind the SLR polyU RNA and promote MYB transcriptional elongation together with NFκBp65. We identified a conserved lysine-rich motif within the Rel homology domain (RHD) of NFκBp50, mutation of which abrogated the interaction of NFκBp50 with the SLR polyU and impaired NFκBp50 mediated MYB elongation. We observed that the TAR RNA-binding region of Tat is homologous to the NFκBp50 RHD lysine-rich motif, a finding consistent with HIV Tat acting as an effector of MYB transcriptional elongation in an SLR dependent manner. Furthermore, we identify the DNA binding activity of NFκBp50 as a key component required for the SLR polyU mediated regulation of MYB. Collectively these results suggest that the MYB SLR polyU provides a platform for proteins to regulate MYB and reveals novel nucleic acid binding properties of NFκBp50 required for MYB regulation.

HIV-1 Tat induces the expression of the interleukin-6 (IL6) gene by binding to the IL6 leader RNA and by interacting with CAAT enhancer-binding protein beta (NF-IL6) transcription factors

Human immunodeficiency virus type 1 (HIV-1) infection is associated with severe psoriasis, B cell lymphoma, and Kaposi's sarcoma. A deregulated production of interleukin-6 (IL6) has been implicated in the pathogenesis of these diseases. The molecular mechanisms underlying the abnormal IL6 secretion of HIV-1-infected cells may include transactivation of the IL6 gene by HIV-1. Here we report the molecular mechanisms of Tat activity on the expression of the IL6 gene. By using 5' deletion mutants of pIL6Pr-CAT and using IL6:HIV-1-LTR hybrid constructs where discrete regions of the IL6 promoter replaced the TAR sequence in HIV-1 LTR, we identified a short sequence of the 5'-untranslated region of the IL6 mRNA that is required for Tat to trans-activate the IL6 promoter. This sequence acquires a stem-loop structure and includes a UCU sequence that binds to Tat and is necessary for full trans-activation. In addition, we provide the evidence that Tat can function by enhancing the CAAT enhancer-binding protein (C/EBP) DNA binding activity and is able to complex with in vitro translated C/EBPbeta, which is a major mediator of IL6 promoter function. By using the yeast two-hybrid system and immunoprecipitation, we observed that the interaction of Tat with C/EBP proteins also occurred in vivo. The data are consistent with the possibility that Tat may function on heterologous genes by interacting with RNA structures possibly present in a large number of cellular and viral genes. In addition, Tat may function by protein-protein interactions, leading to the generation of heterodimers with specific transcription factors.

A small circular TAR RNA decoy specifically inhibits Tat-activated HIV-1 transcription

Linear TAR RNA has previously been used as a decoy to inhibit HIV-1 transcription in vitro and HIV-1 replication in vivo. A 48 nucleotide circular RNA containing the stem, bulge and loop of the HIV-1 TAR element was synthesized using the self-splicing activity of a group I permuted intron-exon and was tested for its ability to function as a TAR decoy in vitro. This small circular TAR molecule was exceptionally stable in HeLa nuclear extracts, whereas a similar linear TAR molecule was rapidly degraded. The TAR circle bound specifically to Tfr38, a peptide containing the TAR-binding region of Tat. The ability of Tat to trans-activate transcription from the HIV-1 promoter in vitro was efficiently inhibited by circular TAR RNA but not by TAR circles that contained either bulge or loop mutations. TAR circles did not inhibit transactivation exclusively by binding to Tat since this inhibition was not reversed by adding excess Tat to the transcription reaction. Together, these data suggest that TAR circles act as decoys that inhibit transactivation by binding to Tat and at least one cellular factor. These data also demonstrate the utility of small circular RNA molecules as tools for biochemical studies.