The Drosophila Polycomb Group proteins ESC and E(Z) are present in a complex containing the histone-binding protein p55 and the histone deacetylase RPD3

The Drosophila Polycomb Group (PcG) proteins are required for stable long term transcriptional silencing of the homeotic genes. Among PcG genes, esc is unique in being critically required for establishment of PcG-mediated silencing during early embryogenesis, but not for its subsequent maintenance throughout development. We previously showed that ESC is physically associated in vivo with the PcG protein E(Z). We report here that ESC, together with E(Z), is present in a 600 kDa complex that is distinct from complexes containing other PcG proteins. We have purified this ESC complex and show that it also contains the histone deacetylase RPD3 and the histone-binding protein p55, which is also a component of the chromatin remodeling complex NURF and the chromatin assembly complex CAF-1. The association of ESC and E(Z) with p55 and RPD3 is conserved in mammals. We show that RPD3 is required for silencing mediated by a Polycomb response element (PRE) in vivo and that E(Z) and RPD3 are bound to the Ubx PRE in vivo, suggesting that they act directly at the PRE. We propose that histone deacetylation by this complex is a prerequisite for establishment of stable long-term silencing by other continuously required PcG complexes.

Mouse homolog of the Drosophila Pc-G gene esc exerts a dominant negative effect in Drosophila

The Polycomb group genes are involved in maintaining long term transcriptional repression of the homeotic genes in both Drosophila and mammals. The mouse eed locus encodes the highly conserved ortholog of the Drosophila ESC protein. To test the functional conservation between the two genes, eed was introduced into the fly to determine whether it could rescue the esc mutant phenotype. eed exerted a dominant negative effect on the leg transformation phenotype associated with the esc mutation. This result is interpreted in light of in vitro protein-protein binding data and in vivo polytene chromosome staining indicating the lack of significant interaction between Eed and fly E(Z), a molecular partner of ESC. genesis 26:67-76, 2000

An extended alpha-helix and specific amino acid residues opposite the DNA-binding surface of the cAMP response element binding protein basic domain are important for human T cell lymphotropic retrovirus type I Tax binding

The human T cell lymphotropic retrovirus type I (HTLV-I) trans-activator, Tax, interacts specifically with the basic-domain/leucine-zipper (bZip) protein, cAMP response element binding protein (CREB), bound to the viral Tax-responsive element consisting of three imperfect 21-base pair repeats, each with a cAMP response element core flanked by G/C-rich sequences. Here, the minimal CREB-bZip necessary for Tax binding is shown to be composed of amino acid residues 280-341. The Tax-CREB interaction involves an uninterrupted and extended alpha-helix in CREB that spans most of its basic domain to include amino acid residues localized to the NH2 terminus of the DNA binding region. Mutational analyses indicate that three residues, Arg284, Met291, and Glu299 unique to this region of the CREB/activating transcription factor-1 subfamily of bZip proteins, constitute the contact surface for Tax. Amino acid substitutions in these positions had little impact on CREB-bZip binding to DNA but abrogated its binding to Tax. Each of the contact residues for Tax are spaced approximately two helical turns apart on the side of the bZip helix directly opposite to that of the invariant DNA-binding residues. Molecular modeling reveals the Tax-contact residues to be near the minor groove of the G/C-rich DNA in the 21-base pair repeat. They most likely position Tax for minor groove contact with the G/C-rich sequences.

The Drosophila Polycomb Group proteins ESC and E(Z) bind directly to each other and co-localize at multiple chromosomal sites

The Polycomb Group gene esc encodes an evolutionarily conserved protein required for transcriptional silencing of the homeotic genes. Unlike other Polycomb Group genes, esc is expressed and apparently required only during early embryogenesis, suggesting it is required for the initial establishment of silencing but not for its subsequent maintenance. We present evidence that the ESC protein interacts directly with E(Z), another Polycomb Group protein required for silencing of the homeotic genes. We show that the most highly conserved region of ESC, containing seven WD motifs that are predicted to fold into a beta-propeller structure, mediate its binding to a conserved N-terminal region of E(Z). Mutations in the WD region that perturb ESC silencing function in vivo also perturb binding to E(Z) in vitro. The entire WD region forms a trypsin-resistant structure, like known beta -propeller domains, and mutations that would affect the predicted ESC beta-propeller perturb its trypsin-resistance, while a putative structure-conserving mutation does not. We show by co-immunoprecipitation that ESC and E(Z) are directly associated in vivo and that they also co-localize at many chromosomal binding sites. Since E(Z) is required for binding of other Polycomb Group proteins to chromosomes, these results suggest that formation of an E(Z):ESC complex at Polycomb Response Elements may be an essential prerequisite for the establishment of silencing.

Cytoplasmic forms of human T-cell leukemia virus type 1 Tax induce NF-kappaB activation

Human T-cell leukemia virus type 1 (HTLV-1) Tax targets I-kappaB alpha and I-kappaB beta for phosphorylation, ubiquitination, and proteasome-mediated degradation, causing the nuclear translocation of NF-kappaB/Rel proteins and transcription induction of many cellular genes. The mechanism by which a nuclear protein such as Tax stimulates I-kappaB phosphorylation and degradation remains unclear. Here, we describe two cytoplasmic mutants of Tax, designated TaxDeltaN81 and TaxDeltaN109, from which the domains important for cyclic AMP response element binding factor (CREB) and serum response factor (SRF) binding and nuclear transport have been removed. These mutants were unable to trans activate from the HTLV-1 21-bp repeats or the serum response element in the c-fos promoter. In contrast, they activated NF-kappaB reporters, suggesting that activation of NF-kappaB by Tax occurs in the cytoplasm. Incorporation of the nuclear localization signal (NLS) of the simian virus 40 large T antigen into TaxDeltaN81 and TaxDeltaN109 redirected both proteins predominantly to the nucleus yet did not restore trans activation via CREB or SRF. The NLS fusion had little effect on TaxDeltaN81 but reduced NF-kappaB trans activation by TaxDeltaN109, possibly because of its proximity to the NF-kappaB-activating domain of Tax. In contrast to wild-type Tax, the cytoplasmic TaxDeltaN mutants are not cytotoxic. Stable expression of TaxDeltaN109 in HeLa cells resulted in a significant reduction in the intracellular level of I-kappaB alpha, with the constitutive presence of NF-kappaB in the nucleus and concomitant activation of the NF-kappaB enhancer. These results are suggestive of a potential application of the TaxDeltaN109-like mutants in targeting I-kappaB degradation and NF-kappaB activation. Interestingly, a Tax species with a molecular mass similar to that of TaxDeltaN109 was identified in many HTLV-1-transformed T cells, suggesting that TaxDeltaN109-like species might play a role in HTLV-1-induced leukemogenesis.

Interaction of the human T-lymphotropic virus type 1 Tax dimer with CREB and the viral 21-base-pair repeat

Human T-lymphotropic virus type 1 Tax interacts specifically with the cellular transcription factor CREB and the viral 21-bp repeat element to form a Tax-CREB-DNA ternary complex which mediates activation of viral mRNA transcription. Analyses of Tax and Tax mutants indicate that, like CREB, Tax incorporates into the ternary complex as a dimer. The ability of Tax to form a dimer is necessary for its interaction with CREB and the 21-bp element. Analyses of several Tax mutants with amino acid substitutions spanning residues 123 to 204 indicate that intersubunit Tax dimerization correlates with its ability to assemble into the ternary complex and activate transcription. Tax also enhances the DNA binding activities of specific bZip domains in vitro. The ability of Tax to enhance DNA binding of bZip proteins can be explained in part by Tax dimerization. This activity alone is not sufficient for transactivation. A dual amino acid substitution mutant of Tax, M47 (L319R, L320S), completely abrogated for activation of the human T-lymphotropic virus type 1 long terminal repeat as a result of a defect in the transactivation domain, continues to stimulate binding of bZip proteins to DNA.

Interaction of bovine leukemia virus transactivator Tax with bZip proteins

The bovine leukemia virus transactivator, BLV Tax, augments transcription from three imperfect 21-bp repeats in the viral transcriptional regulatory region. Each BLV 21-bp repeat contains a cAMP response element (CRE) in the center and unique 5' and 3' neighboring sequences which are crucial for the transcriptional activation by BLV Tax. Here we describe the interactions of recombinant BLV Tax with cellular bZip proteins. The recombinant BLV Tax, tagged at the carboxy terminus with a hexahistidine extension, was prepared by solubilization in 6 M guanidine hydrochloride and renaturation on the Ni(2+)-chelating Sepharose gel matrix. The renatured BLV TaxH6 activates the BLV LTR when introduced into HeLa cells by scrape loading. Furthermore, the purified BLV TaxH6 enhances binding of members of the CREB/ATF family of bZip proteins to CRE motifs by interacting with their bZip domains in vitro. Chemical cross-linking indicates that dimerization of bZip proteins such as CREB becomes greatly facilitated in the presence of BLV Tax. These results suggest that BLV Tax interacts directly with CREB/ATF-like factors to activate viral mRNA transcription.

Alpha-domain of human metallothionein IA can bind to metals in transgenic tobacco plants

With a view to exploring its use as a metal-binding factor in transgenic plants we prepared the alpha-domain of metallothionein by reconstitution of rabbit apometallothionein and proteolysis of MT-1 and MT-2 with subtilisin. The isolated alpha-domains were characterised by UV and CD spectroscopy Double-Stranded. DNA encoding the alpha-domain (106 bp) of the human MT-IA was constructed from chemically synthesized oligomers by repair synthesis and enzymatic ligation, cloned into pUC19 and sequenced. A expression construct containing the cloned alpha-domain was introduced into tobacco cells on a disarmed Agrobacterium tumefaciens Ti-plasmid. Transformed tobacco cells were selected and regenerated on medium containing cadmium and kanamycin. The growth of roots and shoots of transformants was unaffected by up to 100 microM cadmium, whereas control plants showed severe inhibition of root and shoot growth, and chlorosis of leaves on medium containing only 10 microM cadmium. Southern hybridization confirmed the presence of the transgene in the transformed plant tissues. The concentration of human alpha-domain peptides in transgenic tobacco leaves was determined by the Cd/hemoglobin saturation assay and polarography using the rabbit alpha-domain as standard. The results indicate that the alpha-domain, one of two domains in MT molecules, is not only stable in vitro, but is also expressed efficiently and functions independently in transgenic plant cells.

Expression of mouse metallothionein-I gene confers cadmium resistance in transgenic tobacco plants

Transgenic tobacco plants containing a mouse metallothionein-I (MT-I) gene fused to the cauliflower mosaic virus 35S (CaMV 35S) promoter and nopaline synthase (nos) polyadenylation site were obtained by transforming tobacco leaf discs with an Agrobacterium tumefaciens strain carrying the chimaeric gene. Transformants were directly selected and rooted on medium containing cadmium and kanamycin. A total of 49 individual transgenic tobacco plants were regenerated. Among them 20% showed a very high expression level and their growth was unaffected by up to 200 microM cadmium, whereas the growth of control plants was severely affected leaf chlorosis occurred on medium containing only 10 microM cadmium. The concentration of microM cadmium. The concentration of MT-I in leaves of control and transgenic tobacco was determined with Cd/haemoglobin saturation assay, a polarographic method and western blotting. In addition, seeds from self-fertilized transgenic plants were germinated on medium containing toxic levels of cadmium and scored for tolerance/susceptibility to this heavy metal. The ratio of tolerant to susceptible plants was 3:1 indicating that the metallothionein gene is inherited as a single locus.

Construction of multiple copy of alpha-domain gene fragment of human liver metallothionein IA in tandem arrays and its expression in transgenic tobacco plants

Metallothioneins (MT) are low molecular weight, cysteine-rich, metal-binding proteins. An MT molecule contains two domains which appear to act independently--an alpha-domain, which is characterized by cadmium-binding, and a beta-domain, which binds preferentially to copper. Based on this conception, DNA duplex encoding the alpha-domain (106 bp) of human MT-IA was constructed from a chemically-synthesized oligomer by repair synthesis and enzymatic ligation and cloned into pUC19. The genes cloned were sequenced and found to be in the correct order as designed. Synthetic directional adapters were attached to the terminals of the alpha-domain gene fragment of human MT-IA to establish complete control over fragment orientation during ligation. The use of these directional adapters thereby ensured the production of multiple copies of the alpha-domain in tandem arrays. The successive alpha-domains were linked by a peptide linker consisting of 10 residues. A chimeric gene containing 12 cloned tandemly repeated copies of the 106 bp alpha-domain DNA was introduced into tobacco cells on a disarmed Ti-plasmid of Agrobacterium tumefaciens. A total of 10 different transgenic tobacco plants were generated, of which two showed root and shoot growth unaffected by up to 200 mg/l kanamycin and 100 microM cadmium, whereas root growth of control plants was severely inhibited and leaf chlorosis developed on media containing only 10 microM cadmium.

Purification and characterization of hedgehog liver metallothioneins

Two forms of liver metallothioneins (MTs) were purified from hedgehog exposed to zinc, using gel filtration on Sephacryl S-100 and DEAE Sepharose Fast Flow chromatography. The peptide chain weight of both MT-1 and MT-2 was found to be about 10,000, as determined by high performance liquid chromatography. This value was higher than that calculated from amino acid analysis. The amino acid composition of hedgehog liver MT-1 and MT-2 resembles that of liver to MTs from rabbit and other species. Their distinctive features include an extremely high cysteine content, about 33% of all the amino acid residues, and an absence of aromatic amino acids and histidine. In addition, a rapid method for the determination of MTs during animal tissue purification has been established. The samples were directly added in an ammoniacal solution of a Co(II) salt for recording linear sweep polarograms. By comparison with the commonly used metal determination method, our method is direct, rapid, credible and suitable for all the MTs or MT-like samples.

An improved method for isolation and identification of Zn-metallothionein from cadmium-induced rat liver

Zn-metallothioneins (MT-1 and MT-2) were isolated and purified from Wistar rat liver induced by subcutaneous injection with cadmium chloride over a short time. Instead of Sephadex G-50 and DEAE Sephadex A-50, new chromatographic media produced by Pharmacia, Sephacryl S-200, S-100 and DEAE Sepharose Fast Flow were used in the purification of metallothioneins. The time required for purification with the new method was only 1/3 that required with the usual method and had the same purification effect and rate of recovery. The number of mercapto groups measured with modified Ellman's reagent and cysteine as standard is 20 in MT molecules. Zn and Cd concentrations in each fraction were measured by single sweep polarography rather than atomic absorption spectrophotometry. MT-1 and MT-2 contained 6 gram atoms of zinc, but no cadmium. Purified MT-1 and MT-2 were shown by high performance liquid chromatographic analysis to be highly homogeneous and had an amino acid composition similar to that of Cd-MT.

An improved ELISA with linear sweep voltammetry detection

An improved ELISA combined with linear sweep voltammetry detection of p-nitrophenol generated by an enzyme has been investigated in this study. p-nitrophenol, produced from alkaline phosphatase catalysing p-nitrophenyl phosphate, yielded an oxidative peak at 1.06 V (vs. Ag/AgCl) with a wax-impregnated tubular graphite anode. Without separation, the small three-electrode system was directly inserted in the well of an ELISA plate for detection. The detection limit for p-nitrophenol was 1 x 10(-6) M, lower than that obtained by measuring the absorbance of p-nitrophenol. The feasibility of utilizing linear sweep voltammetry as a detection scheme was demonstrated by determining metallothionein, granulocyte-colony stimulating factor and Xenopus laevis keratin using the above new system. The method was simple, reproducible and much more sensitive than traditional spectrophotometry.