Members of the small Maf family (MafK, MafF, and MafG) are basic region leucine zipper (bZip) proteins that can function as transcriptional activators or repressors. The dimer compositions of their DNA binding forms determine whether the small Maf family proteins activate or repress transcription. Using a yeast two-hybrid screen with a GAL4-MafK fusion protein, we have identified two novel bZip transcription factors, Bach1 and Bach2, as heterodimerization partners of MafK. In addition to a Cap'n'collar-type bZip domain, these Bach proteins possess a BTB domain which is a protein interaction motif; Bach1 and Bach2 show significant similarity to each other in these regions but are otherwise divergent. Whereas expression of Bach1 appears ubiquitous, that of Bach2 is restricted to monocytes and neuronal cells. Bach proteins bind in vitro to NF-E2 binding sites, recognition elements for the hematopoietic transcription factor NF-E2, by forming heterodimers with MafK. Furthermore, a DNA binding complex that contained MafK as well as Bach2 or a protein related closely to Bach2 was found to be present in mouse brain cells. Bach1 and Bach2 function as transcription repressors in transfection assays using fibroblast cells, but they function as a transcriptional activator and repressor, respectively, in cultured erythroid cells. The results suggest that members of the Bach family play important roles in coordinating transcription activation and repression by MafK.

Bach proteins belong to a novel family of BTB-basic leucine zipper transcription factors that interact with MafK and regulate transcription through the NF-E2 site.

Heterochromatin is both necessary for the expression of heterochromatic genes and inhibitory for the expression of euchromatic genes These two properties of heterochromatin have been elucidated from the study of chromosome rearrange­ ments that induce position effect variegation (PEV) in Drosophila melanogaster Novel euchromatin-heteroc hromatin can affect the expression of euchromatic and heterochromatic genes located several megabases away, distin­ guishing higher order chromatin structure from most other regulatory mecha­ nis ms Studies of PEV promis e insights into the basis for heterochromatin formation and the role of higher order chromatin and chromosome structure in gene regulation We eva luate the models and experimental data that address the mechanisms of PEV in different cell types, the potential functions of modifiers of PEV, and the relationship of PEV to other phenomena associated with vari­ egated gene expression in Drosophila

Heterochromatin and gene expression in Drosophila.

Position-effect variegation (PEV) results when a gene normally in euchromatin is juxtaposed with heterochromatin by rearrangement or transposition. When heterochromatin packaging spreads across the heterochromatin/euchromatin border, it causes transcriptional silencing in a stochastic pattern. PEV is intensely studied in Drosophila using the white gene. Screens for dominant mutations that suppress or enhance white variegation have identified many conserved epigenetic factors, including the histone H3 lysine 9 methyltransferase SU(VAR)3-9. Heterochromatin protein HP1a binds H3K9me2/3 and interacts with SU(VAR)3-9, creating a core memory system. Genetic, molecular, and biochemical analysis of PEV in Drosophila has contributed many key findings concerning establishment and maintenance of heterochromatin with concomitant gene silencing.

/pdf/position-effect-variegation-heterochromatin-formation-and-1l0ey1wvj4.pdf

Position-Effect Variegation, Heterochromatin Formation, and Gene Silencing in Drosophila

https://www.cell.com/cell/pdf/S0092-8674(12)00591-0.pdf

DNA Sequence-Dependent Compartmentalization and Silencing of Chromatin at the Nuclear Lamina

The BTB/POZ domain defines a newly characterized protein-protein interaction interface. It is highly conserved throughout metazoan evolution and generally found at the NH2 terminus of either actin-binding or, more commonly, nuclear DNA-binding proteins. By mediating protein binding in large aggregates, the BTB/POZ domain serves to organize higher order macromolecular complexes involved in ring canal formation or chromatin folding.

http://cgd.aacrjournals.org/cgi/reprint/6/9/1193.pdf

The BTB/POZ domain : a new protein-protein interaction motif common to DNA- and actin-binding proteins

LITTLE is known about the way higher-order chromatin structure influences gene expression and chromosome topology in general. Genetic analysis in Drosophila has led to the discovery of two classes of genes, the regulators of homeotic genes and the modifiers of position-effect variegation, which seem to be good candidates for encoding some of the factors regulating chromatin functions1,2. The Trithorax-like gene we describe here is required for the normal expression of the homeotic genes and is a modifier of position-effect variegation. We found that Trithorax-like encodes the GAGA factor which is involved in the formation of an accessible chromatin structure at promoter sequences3. Our genetic analysis suggests that the chromatin modelling function of the GAGA factor is not restricted to promoter regions.

The Trithorax-like gene encodes the Drosophila GAGA factor

The GAGA protein of Drosophila was first identified as a stimulatory factor in in vitro transcription assays using the engrailed and Ultrabithorax promoters. Subsequent studies have suggested that the GAGA factor promotes transcription by blocking the repressive effects of histones; moreover, it has been shown to function in chromatin remodeling, acting together with other factors in the formation of nuclease hypersensitive sites in vitro. The GAGA factor is encoded by the Trithorax-like locus and in the studies reported here we have used the maternal effect allele Trl13C to examine the functions of the protein during embryogenesis. We find that GAGA is required for the proper expression of a variety of developmental loci that contain GAGA binding sites in their upstream regulatory regions. The observed disruptions in gene expression are consistent with those expected for a factor involved in chromatin remodeling. In addition to facilitating gene expression, the GAGA factor appears to have a more global role in chromosome structure and function. This is suggested by the spectrum of nuclear cleavage cycle defects observed in Trl13C embryos. These defects include asynchrony in the cleavage cycles, failure in chromosome condensation, abnormal chromosome segregation and chromosome fragmentation. These defects are likely to be related to the association of the GAGA protein with heterochromatic satellite sequences which is observed throughout the cell cycle.

/pdf/the-gaga-factor-is-required-in-the-early-drosophila-embryo-1omxbn6lq5.pdf

The GAGA factor is required in the early Drosophila embryo not only for transcriptional regulation but also for nuclear division

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The Trithorax-like gene encodes the Drosophila GAGA factor

The GAGA factor is required in the early Drosophila embryo not only for transcriptional regulation but also for nuclear division