Anti-termination of transcription within the long terminal repeat of HIV-1 by tat gene product

Abstract

Human immunodeficiency virus-1 (HIV-1) gene expression is controlled by cellular transcription factors and by virally encoded trans-activation proteins of the HIV-1 tat and art/trs genes, which are essential for viral replication. Tat trans-activates HIV-1 gene expression by interacting with the trans-acting response element (TAR) located within the HIV-1 long terminal repeat (LTR) (ref. 2). In transient expression assays, tat mediates its effects largely by increasing the steady-state levels of messenger RNA species that contain the TAR sequence at or near their 5' ends, suggesting a function for tat either in transcription or in subsequent RNA processing. The tat gene could also facilitate translation of mRNA containing the TAR sequence. To determine the mechanism of trans-activation by tat, we analysed the structure and rate of synthesis of RNA species directed by the HIV-1 LTR in transient expression assays both in the presence and absence of tat. Although the rate of HIV-1 transcription initiation was not affected by tat, transcriptional elongation beyond position +59 was seen only in the presence of tat. Thus, tat trans-activates HIV-1 transcription by relieving a specific block to transcriptional elongation within the TAR sequence.

Binding of basal transcription factor TFIIH to the acidic activation domains of VP16 and p53.

“…One could therefore explain part of the synergism in transcriptional activation if interaction of acidic activators with TFIIH stimulates melting of the DNA at the promoter. This phenomenon would have been difficult to detect in vitro because acidic activators also stimulate formation of the closed preinitiation complex (65,112,114 18) and increasing the processivity of chain elongation by RNA polymerase 11 (52,54,61,71). Similarly, other typical activators, including VP16, may also generally stimulate chain elongation by RNA polymerase 11 (119).…”

Regulation of eukaryotic gene expression by transcriptional attenuation.

“…A U-rich bulge located just below the loop is important for TAT recognition. It has been suggested that TAT may regulate both transcriptional initiation and elongation with Kao et al (1987) being the first to demonstrate that it activates HIV transcription by behaving as an antiterminator like protein. By analyzing the expression of an HIV LTR-driven CAT gene in transient expression assays, it was shown that transcripts did not elongate past the viral TAR region in the absence of a cotransfected TAT gene; this was indicated both by runoff transcription analysis and by the detection of truncated RNAs in the cytoplasm.…”

The Human Immunodeficiency Virus Transactivator Tat Interacts with the RNA Polymerase II Holoenzyme

“…In the absence of Tat, RNA polymerase II (pol II) terminates transcription of the HIV genome prematurely, resulting in primarily short transcripts. Interactions between Tat and TAR convert pol II into its processive form and lead to the efficient production of full-length viral transcripts (15,24,27,32,37).…”

“…Following phenol-chloroform extraction, the transcripts were precipitated with ethanol and separated on a 5% denaturing polyacrylamide gel. Wild-type (LTR) and mutated (U23A) HIV-1 promoter constructions yielded specific HIV-1 runoff transcripts of 750 and 620 nt, respectively (24,48). Recombinant Tat (100 ng) was expressed in E. coli and bound to streptavidin-agarose beads as described above.…”