Repression of the human immunodeficiency virus type 1 promoter by the human KRAB domain results in inhibition of virus production.

The Krüppel-associated box (KRAB) domain has been described as a eukaryotic repressor of transcription. We show that fusion of KRAB to DNA-binding-domains provides a novel approach to inhibit expression of a replication-competent human immunodeficiency virus (HIV) genome. The KRAB domain from the human zinc finger protein KOX1 was combined with the DNA binding domain of the Escherichia coli tetracycline repressor (TetR). Constitutive expression of the TetR-KRAB protein in HeLa cells inhibited virus production from an HIV genome encoding TetR target sequences by 80%. The same inhibition was observed with HIV-promoter-driven reporter plasmids. The specificity of inhibition was shown with informative KRAB mutants, plasmids lacking the respective target sequences, and by reversal of the TetR-KRAB-mediated inhibition with tetracycline. Virus production was suppressed by binding of TetR-KRAB at a distance of 6 kbp to the promoter. We therefore conclude that any site of the genuine HIV genome could serve as target of a chimeric KRAB repressor protein. Specific targeting of the KRAB domain by artificially selected binding domains may be generally applicable to control transcription in mammalian cells.

Interaction of STAT5 dimers on two low affinity binding sites mediates interleukin 2 (IL-2) stimulation of IL-2 receptor alpha gene transcription.

Stimulation of the interleukin 2 receptor alpha (IL-2Ralpha) gene by IL-2 is important for the proliferation of antigen-activated T lymphocytes. IL-2 regulates IL-2Ralpha transcription via a conserved 51-nucleotide IL-2 responsive enhancer. Mouse enhancer function depends on cooperative activity of three distinct sites. Two of these are weak binding sites for IL-2-activated STAT5 (signal transducer and activator of transcription) proteins, and mutational analysis indicates that binding of STAT5 to both sites is required for IL-2 responsiveness of the enhancer. The STAT5 dimers interact to form a STAT5 tetramer. The efficiency of tetramerization depends on the relative rotational orientation of the two STAT motifs on the DNA helix. STAT5 tetramerization on enhancer mutants correlates well with the IL-2 responsiveness of these mutants. This provides strong evidence that interactions between STAT dimers binding to a pair of weak binding sites play a biological role by controlling the activity of a well characterized, complex cytokine-responsive enhancer.

Repression of transcriptional activity by heterologous KRAB domains present in zinc finger proteins.

We report the characterization of three novel members of the KRAB-domain containing C2-H2 zinc finger family (ZNF133, 136 and 140). KRAB (Krüppel-associated box) is an evolutionarily conserved protein domain found N-terminally with respect to the zinc finger repeats that encodes the DNA binding domain. ZNF133 and ZNF140 have both the KRAB A- and KRAB B-boxes present at their N-terminus, whereas ZNF136 contains only the KRAB A-box. We have previously demonstrated that the KRAB domains derived from ZNF133 and ZNF140 are potent transcriptional repression domains [Margolin et al. (1994) Proc. Natl. Acad. Sci. USA 91, 4509-4513]. The KRAB domain from ZNF136, containing only subdomain A, is a considerable weaker suppression domain; however, when fused to the heterologous KRAB B subdomain of ZNF10 (KOX1) the two subdomains from a KRAB domain which induces repression as potently as previously reported KRAB domains.

Tetracycline-reversible silencing of eukaryotic promoters.

A tetracycline-controlled transrepressor protein has been engineered to silence transcriptional activities of eukaryotic promoters that are stably integrated into the chromatin of human cells. By fusing the KRAB domain of human Kox1 to the Tet repressor derived from Tn10 of Escherichia coli, a tetracycline-controlled hybrid protein (TetR-KRAB) was generated and constitutively expressed in HeLa cells. The TetR-KRAB protein binds to tet operator (tetO) sequences in the absence but not in the presence of tetracycline. When TetR-KRAB bound to tetO sequences upstream of the immediate-early promoter-enhancer of human cytomegalovirus (CMV), the expression of a CMV-driven luciferase reporter construct (ptetO7-CMV-L) was repressed in transient transfection experiments. This silencing was found to operate on different promoters and from tetO sequences placed more than 3 kb from the transcriptional start site. We constructed a stable, doubly transfected cell line (TIS-10) carrying a chromosomally integrated ptetO7-CMV-L reporter construct and expressing the TetR-KRAB protein. Upon addition of tetracycline, luciferase expression was induced more than 50-fold above the baseline level, with half-maximal induction by 2 days. Furthermore, a protein of around 110 kDa was found to coimmunoprecipitate with the TetR-KRAB fusion protein. This protein might play a role as an adaptor protein mediating the silencing exerted by the TetR-KRAB protein. The TetR-KRAB silencing system should be useful as a genetic switch for regulating the expression of chromosomally integrated heterologous and endogenous genes present in mammalian genomes.

Krüppel-associated boxes are potent transcriptional repression domains.

The Krüppel-associated box (KRAB) is a highly conserved, 75-aa region containing two predicted amphipathic alpha-helices. The KRAB domain is present in the amino-terminal regions of more than one-third of all Krüppel-class Cys2His2 zinc finger proteins and is conserved from yeast to man; however, its function is unknown. Here it is shown that the KRAB domain functions as a DNA binding-dependent transcriptional repressor when fused to a heterologous DNA-binding domain from the yeast GAL4 protein. A 45-aa segment containing one of the predicted KRAB amphipathic helices was necessary and sufficient for repression. Amino acid substitutions in the predicted helix abolished the repression function. These results assign a function, transcriptional repression, to the highly conserved KRAB box and define a minimal repression domain which may aid in identifying mechanisms of repression.