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Chemically induced differentiation of cultured murine erythroleukaemia (MEL) cells results in a several hundred-fold increase in transcription of the adult mouse globin genes and thus serves as a model for gene activation during erythropoiesis. One approach to study gene regulation in this system has been to analyse the expression of foreign globin genes introduced into MEL cells. By introducing cosmid DNA containing the human adult(beta), fetal(gamma) and embryonic(epsilon)-globin genes, we have shown here that expression of the beta, but not the gamma or epsilon genes, is regulated during MEL differentiation. Regulated expression of the human beta-globin gene was observed when it was introduced either as part of the intact globin gene cluster or as an individual gene with 1.5 kilobases (kb) of 5' flanking DNA. Transcription from a herpes simplex virus (HSV) promoter adjacent to the thymidine kinase (tk) gene is also inducible in MEL cells.

Protein ubiquitination in eukaryotic cells is mediated by diverse E3 ligase enzymes that each target specific substrates. The cullin E3 ligase complexes are the most abundant class of E3 ligases; they contain various cullin components that serve as scaffolds for interaction with substrate-recruiting adaptor proteins. SPOP is a BTB-domain adaptor of the cullin-3 E3 ligase complexes; it selectively recruits substrates via its N-terminal MATH domain, whereas its BTB domain mediates dimerization and interactions with cullin-3. It has recently been recognized that the high-order oligomerization of SPOP enhances the ubiquitination of substrates. Here, a dimerization interface in the SPOP C-terminus is identified and it is shown that the dimerization interfaces of the BTB domain and of the C-terminus act independently and in tandem to generate high-order SPOP oligomers. The crystal structure of the dimeric SPOP C-terminal domain is reported at 1.5 Å resolution and it is shown that Tyr353 plays a critical role in high-order oligomerization. A model of the high-order SPOP oligomer is presented that depicts a helical organization that could enhance the efficiency of substrate ubiquitination.

Expression of the protooncogene myc is regulated by multiple mechanisms and is probably involved in the control of cellular proliferation. Modulation of transcriptional elongation by attenuation within exon 1 of the myc gene is thought to play an important role in determining levels of myc RNA in both normal and tumor cells. We show that the first exon of mQuse myc contains specific DNA sequences that may mediate transcriptional attenuation. A 180-nucleotide DNA fragment derived from the 3' end of exon 1 reduced transcriptional elongation by polymerase II when placed within an intron of the human a1-globin gene and assayed by transfection into HeLa cells. A 36-nucleotide sequence that resembles a variety of transcriptional termination signals was located within this myc fragment but was by itself insufficient to cause attenuation when placed within the a-globin gene. Modulation of transcriptional elongation through specific DNA sequences within a gene may thus provide a mechanism for regulating its expression.The product of myc is a nuclear protein (1, 2) that has been implicated in the control of cellular proliferation, although its precise function remains unknown. Levels of myc RNA increase during the response of a variety of cells to growth factors (3) and decrease upon growth arrest or differentiation (4,5). Moreover, alteration of the pattern of myc expression by diverse mechanisms, including chromosomal translocation (6,7), retroviral insertion (8, 9), and gene amplification (10), may be an important step in the development of a variety of neoplasms. Changes in steady-state levels of myc RNA in the cell have been attributed to regulation of both the rate of gene transcription (11) and the stability of myc RNA (12, 13). In addition, measurement of the rate of transcription across different regions of the myc gene has suggested that levels of myc RNA may be determined by modulation of transcription elongation through a "block" near the 3' end of the first exon of the myc gene (14,15). This mechanism by which transcriptional termination or pausing is used to modulate expression of downstream sequences is commonly known as attenuation.As an initial step in elucidating the role of transcriptional attenuation in the regulation of myc and other eukaryotic genes, we sought to define DNA sequences within the myc gene that mediate this effect. DNA fragments from the murine myc gene were inserted into an intron of the human a1-globin gene, and their effect on globin expression was tested after transfection into HeLa cells. We show that a 180-nucleotide (nt) fragment derived from the 3' end of exon 1 acts in an orientation-dependent manner to partially block transcription elongation by polymerase II through the human a-globin gene. Our data complement those of Bentley and Groudine who have shown that a 95-nt sequence at the 3' end of exon 1 of human MYC mediates premature termination of transcription (16). MATERIALS AND METHODSCell Culture and Transfection. HeLa cells were grown in a-MEM medium supplemented w...

SUMMARYThe POZ/BTB domain is an evolutionarily conserved motif found in approximately forty zinc-finger transcription factors (POZ-ZF factors). Several POZ-ZF factors are implicated in human cancer, and POZdomain interaction interfaces represent an attractive target for therapeutic intervention. Miz-1 is a POZ-ZF factor that regulates DNA-damage-induced cell cycle arrest, and plays an important role in human cancer by virtue of its interaction with the c-Myc and Bcl6 oncogene products. The Miz-1 POZ domain mediates both self-association and the recruitment of non-POZ partners. POZ-ZF factors generally function as homo-dimers, although higher-order associations and heteromeric interactions are known to be physiologically important; crucially, the interaction interfaces in such large complexes have not been characterised. We report here the crystal structure of the Miz-1 POZ domain to 2.1Å resolution. The tetrameric organisation of Miz-1 POZ reveals two types of interaction interface between subunits; an interface of alpha-helices resembles the dimerisation interface of reported POZ domain structures, whereas a novel beta-sheet interface directs the association of two POZ domain dimers. We show that the beta-sheet interface directs tetramerisation of the Miz-1 POZ domain in solution, and therefore represents a newly described candidate interface for the higherorder homo-and hetero-oligomerisation of POZ-ZF proteins in vivo. Keywords:The POZ/BTB (poxvirus and zinc finger/bric-à-brac, tramtrack and broad complex) domain mediates Higher-order oligomers 12 and heteromeric interactions 13 between different POZ-ZF factors are thought to be physiologically important, although the interaction interfaces in these complexes have not been characterised.We report here the crystal structure of the tetrameric Miz-1 POZ domain. The oligomeric organisation of Miz-1 POZ reveals a novel beta-sheet interaction interface that directs the association of two POZ domain dimers. Weshow that this region mediates the association of Miz-1 POZ dimers in solution, and therefore represents a newly described candidate interface for the higher-order homo-and hetero-oligomerisation of POZ-ZF proteins in vivo. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 4General Organisation of the Miz-1 POZ DomainThe entire Miz-1 POZ domain (Miz-1 residues 2 -115) was expressed in E.coli, purified and crystallized. The crystal structure was solved by molecular replacement and refined to R = 18.1%, R free = 22.9% at 2.1 Å resolution (Table 1 and Figure 1A), with a single tetramer per asymmetric unit. All residues were built in the model.The Miz-1 POZ tetramer may be described as an association of two dimers, each of which resembles the reported PLZF, BCL6 and LRF POZ structures ( Figure 1). The assembly has two distinct types of interface between subunits: the A:B and C:D interfaces resemble those described in PLZF, BCL6 and LRF, whereas A:D The Miz-1 POZ Domain Core and Dimerisation Interface POZ domains of the POZ-ZF factors are highly conserved, an...