Tetracycline-reversible silencing of eukaryotic promoters

How knockout mouse lines will be used to study the role of drug-metabolizing enzymes and their receptors during reproduction and development, and in environmental toxicity, cancer, and oxidative stress

The dioxin-inducible mouse [Ah] battery contains at least six genes that "cross-talk" with one another and are believed to play important roles in reproduction and development, and in environmental toxicity, cancer, and oxidative stress. In addition to two P450 genes, Cyp1a1 and Cyp1a2, this laboratory has shown that the four Phase II [Ah] genes include: NAD(P)H:menadione oxidoreductase (Nmo1); a cytosolic "class 3" aldehyde dehydrogenase (Ahd4); a UDP glucuronosyltransferase having 4-methylumbelliferone as substrate (Ugt1a6); and a glutathione transferase having 2,4-dinitro-1-chlorobenzene as substrate (Gsta1, Ya). The Ah receptor-mediated coordinate induction is controlled positively in all six [Ah] battery genes. Oxidative stress up-regulates the four Phase II [Ah] genes. This laboratory is generating conventional, plus inducible, knockout mouse lines having homozygous disruptions in the above-mentioned genes; this novel methodology is described herein. If the conventional knockout is healthy and viable, the mouse line would be useful for studies involving environmental agents. If the conventional knockout is lethal during development, this model would be important for developmental biology, but the inducible (also called conditional) knockout can still be used--at selected ages and even in selected tissue or cell types--for studies designed to understand the mechanisms involved in reproduction and development, and in environmental toxicity, cancer, and oxidative stress.

Construction and characterization of a one-plasmid system for the controlled expression of genes in mammalian cells by tetracycline

The two-plasmid system of Gossen and Bujard [Gossen and Bujard (1992) Proc. Natl. Acad. Sci. USA 89, 5547-5551] to express mammalian genes in a tetracycline-repressed fashion was combined into a single-plasmid system. Two variants of this single-plasmid system that differ in the multiple cloning site (MCS) region are described. These vectors were used to stably transfect raf kinase domain into the normal rat kidney epithelial cell line (NRKE) to obtain a conditionally transformed cell line. These vectors were also used to stably transfect wild-type and mutant human p53 into the human osteosarcoma cell line, SAOS-2. Tetracycline repressed gene expression in both cell lines; about 12-fold in NRKE and about 80-fold in SAOS-2 cell line.

A novel member of the RING finger family, KRIP-1, associates with the KRAB-A transcriptional repressor domain of zinc finger proteins

The Krüppel-associated box A (KRAB-A) domain is an evolutionarily conserved transcriptional repressor domain present in approximately one-third of zinc finger proteins of the Cys2-His2 type. Using the yeast two-hybrid system, we report the isolation of a cDNA encoding a novel murine protein, KRAB-A interacting protein 1 (KRIP-1) that physically interacts with the KRAB-A region. KRIP-1 is a member of the RBCC subfamily of the RING finger, or Cys3HisCys4, family of zinc binding proteins whose other members are known to play important roles in differentiation, oncogenesis, and signal transduction. The KRIP-1 protein has high homology to TIF1, a putative modulator of ligand-dependent activation function of nuclear receptors. A 3.5-kb mRNA for KRIP-1 is ubiquitously expressed among all adult mouse tissues studied. When a GAL4-KRIP-1 fusion protein is expressed in COS cells with a chloramphenicol acetyltransferase reporter construct with five GAL4 binding sites, there is dose-dependent repression of transcription. Thus, KRIP-1 interacts with the KRAB-A region of C2H2 zinc finger proteins and may mediate or modulate KRAB-A transcriptional repressor activity.

Transcriptional repression by RING finger protein TIF1 beta that interacts with the KRAB repressor domain of KOX1

Many of the vertebrate zinc finger factors of the Kruppel type (C2H2 zinc fingers) contain in their N-terminus a conserved sequence referred to as the KRAB (Kruppel-associated box) domain that, when tethered to DNA, efficiently represses transcription. Using the yeast two-hybrid system, we have isolated an 835 amino acid RING finger (C3HC4 zinc finger) protein, TIF1 beta (also named KAP-1), that specifically interacts with the KRAB domain of the human zinc finger factor KOX1/ZNF10. TIF1 beta, TIF1 alpha, PML and efp belong to a characteristic subgroup of RING finger proteins that contain one or two other Cys/His-rich clusters (B boxes) and a putative coiled-coil in addition to the classical C3HC4 RING finger motif (RBCC configuration). Like TIF1 alpha, TIF1 beta also contains an additional Cys/His cluster (PHD finger) and a bromo-related domain. When tethered to DNA, TIF1 beta can repress transcription in transiently transfected mammalian cells both from promoter-proximal and remote (enhancer) positions, similarly to the KRAB domain itself. We propose that TIF1 beta is a mediator of the transcriptional repression exerted by the KRAB domain.

A possible involvement of TIF1 alpha and TIF1 beta in the epigenetic control of transcription by nuclear receptors

Nuclear receptors (NRs) are ligand-inducible transcription factors that mediate complex effects on development, differentiation and homeostasis. They regulate the transcription of their target genes through binding to cognate DNA sequences as homodimers or heterodimers. The molecular mechanisms underlying transcriptional activation by NRs are still poorly understood, although intermediary factors (mediators) appear to be involved in mediating the transactivation functions of NRs. TIF1 has been identified previously as a protein that interacts specifically with the ligand binding domain of several nuclear receptors, both in yeast and in vitro. The characteristics of these interactions have led us to suggest that TIF1 might be a mediator of the NR ligand-inducible activation function AF-2. Using a two-hybrid screening in yeast, we have now identified two TIF1-binding proteins, mHP1 alpha and mMOD1, that are mouse homologues of the Drosophila heterochromatinic protein 1. Using mHP1 alpha as a bait in a second two-hybrid screening, we have isolated cDNAs encoding proteins that are also very likely to be involved in chromatin structure and function, as well as a protein structurally and functionally related to TIF1 (renamed TIF1 alpha), which was named TIF1 beta. Here we discuss how the function of members of the TIF1 family in the control of transcription could be exerted at the level of the structure of the chromatin template.

Fast large-scale purification of tetracycline repressor variants from overproducing Escherichia coli strains

We constructed a plasmid for overexpression of Tn10 Tet repressor (TetR) by placing a synthetic tetR gene under control of the Pc promoter. Active TetR is expressed up to 30% of the total soluble cell protein. A protocol containing anion-exchange, cation-exchange, and size-exclusion chromatography steps is described for the large-scale purification of milligram amounts of TetR in three days. Cation-exchange chromatography already yields almost homogenous TetR. Purification of about fifty TetR mutants demonstrates that this protocol is generally applicable. No correlation between net charge of TetR variants and elution behaviour was detected for the anion-exchange column. On the other hand, TetR mutants with increased negative charge in their DNA binding domain eluted at lower NaCl concentration from the cation-exchange column. The applicability of this purification protocol to the wide variety of TetR variants suggests that it can be used for the rapid purification of other DNA binding proteins as well.

Bi-directional gene switching with the tetracycline repressor and a novel tetracycline antagonist

We have screened a panel of tetracycline (tc)-like compounds for their potential use with tc-repressor (tetR) based gene switches. The interaction between tc and tetR appears quite specific, as only tc itself and its close homologues anhydro-tc and doxycycline strongly inhibited DNA binding. However, a single tc-like compound, GR33076X, increased DNA binding of the tetR-VP16 fusion protein, both in eukaryotic cells and in bacteria. We provide evidence that this antagonist of tetracycline is potentially useful for accelerated gene switching, especially in whole animals.

Zfp60, a mouse zinc finger gene expressed transiently during in vitro muscle differentiation

The complete cDNA coding sequence of the zinc finger gene Zfp60 is reported. The predicted amino acid sequence of the Zfp60 protein has been found to contain 19 zinc finger motives clustered at the C-terminus. At its N-terminus, Zfp60 shares with other members of the zinc finger gene family two additional conserved amino acid modules named Kruppel Associated Boxes (KRAB). The expression patterns of Zfp60, MyoD and MHC mRNAs have been followed during in vitro myogenic differentiation of C2 cells. We show that the bacterial produced Zfp60 protein binds DNA only in presence of zinc ions. Zfp60 locus has been mapped in chromosome 7, where other Zfp loci are localised.

Chromosomal location of fifteen unique mouse KRAB-containing zinc finger loci

The mammalian genome contains hundreds if not thousands of zinc finger protein (Zfp) genes. While the function of most of these genes remains to be determined, it is clear that a few of them play important roles in gene regulation and development. In studies described here, we have used an interspecific mouse backcross mapping panel to determine the chromosomal location of 15 KRAB-containing zinc finger loci. These loci map to nine different mouse autosomes and the X Chromosome (Chr). Two Chrs, 7 and 9, contain cosegregating pairs of KRAB-containing Zfp genes, indicating that the KRAB-containing Zfp genes have evolved through processes involving regional as well as genome-wide duplication events.

Efficient control of gene expression by single step integration of the tetracycline system in transgenic mice

Tetracycline-regulated gene expression in eukaryotic cell lines, plants, and transgenic mice has become a powerful tool for the analysis of eukaryotic gene expression and function. The system consists of two plasmids, one encoding the transactivator protein under control of a viral cytomegalovirus promoter, and the second being the tet-operator minimal promoter driving the gene of interest. Here we show that these control elements, when integrated in cis on a single plasmid, allow efficient and tight control of reporter gene expression in vitro and in vivo. Dependent on the route of administration of tetracycline, gene expression can be partially or fully repressed in transgenic mice, whereas removal of the antibiotic induces the reporter gene in various tissues to levels up to 800-fold more than the two-plasmid system. In addition, crossing and analysis of animals transgenic for the individual components of the system are unnecessary, and genetic segregation of the control elements during breeding is prevented.

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.