The Drosophila kismet gene is related to chromatin-remodeling factors and is required for both segmentation and segment identity.
The Drosophila kismet gene was identified in a screen for dominant suppressors of Polycomb, a repressor of homeotic genes. Here we show that kismet mutations suppress the Polycomb mutant phenotype by blocking the ectopic transcription of homeotic genes. Loss of zygotic kismet function causes homeotic transformations similar to those associated with loss-of-function mutations in the homeotic genes Sex combs reduced and Abdominal-B. kismet is also required for proper larval body segmentation. Loss of maternal kismet function causes segmentation defects similar to those caused by mutations in the pair-rule gene even-skipped. The kismet gene encodes several large nuclear proteins that are ubiquitously expressed along the anterior-posterior axis. The Kismet proteins contain a domain conserved in the trithorax group protein Brahma and related chromatin-remodeling factors, providing further evidence that alterations in chromatin structure are required to maintain the spatially restricted patterns of homeotic gene transcription. (+info
Transcriptional repression by the Drosophila giant protein: cis element positioning provides an alternative means of interpreting an effector gradient.
Early developmental patterning of the Drosophila embryo is driven by the activities of a diverse set of maternally and zygotically derived transcription factors, including repressors encoded by gap genes such as Kruppel, knirps, giant and the mesoderm-specific snail. The mechanism of repression by gap transcription factors is not well understood at a molecular level. Initial characterization of these transcription factors suggests that they act as short-range repressors, interfering with the activity of enhancer or promoter elements 50 to 100 bp away. To better understand the molecular mechanism of short-range repression, we have investigated the properties of the Giant gap protein. We tested the ability of endogenous Giant to repress when bound close to the transcriptional initiation site and found that Giant effectively represses a heterologous promoter when binding sites are located at -55 bp with respect to the start of transcription. Consistent with its role as a short-range repressor, as the binding sites are moved to more distal locations, repression is diminished. Rather than exhibiting a sharp 'step-function' drop-off in activity, however, repression is progressively restricted to areas of highest Giant concentration. Less than a two-fold difference in Giant protein concentration is sufficient to determine a change in transcriptional status of a target gene. This effect demonstrates that Giant protein gradients can be differentially interpreted by target promoters, depending on the exact location of the Giant binding sites within the gene. Thus, in addition to binding site affinity and number, cis element positioning within a promoter can affect the response of a gene to a repressor gradient. We also demonstrate that a chimeric Gal4-Giant protein lacking the basic/zipper domain can specifically repress reporter genes, suggesting that the Giant effector domain is an autonomous repression domain. (+info
Activation of systemic acquired silencing by localised introduction of DNA.
BACKGROUND: In plants, post-transcriptional gene silencing results in RNA degradation after transcription. Among tobacco transformants carrying a nitrate reductase (Nia) construct under the control of the cauliflower mosaic virus 35S promoter (35S-Nia2), one class of transformants spontaneously triggers Nia post-transcriptional gene silencing (class II) whereas another class does not (class I). Non-silenced plants of both classes become silenced when grafted onto silenced stocks, indicating the existence of a systemic silencing signal. Graft-transmitted silencing is maintained in class II but not in class I plants when removed from silenced stocks, indicating similar requirements for spontaneous triggering and maintenance. RESULTS: Introduction of 35S-Nia2 DNA by the gene transfer method called biolistics led to localised acquired silencing (LAS) in bombarded leaves of wild-type, class I and class II plants, and to systemic acquired silencing (SAS) in class II plants. SAS occurred even if the targeted leaf was removed 2 days after bombardment, indicating that the systemic signal is produced, transmitted and amplified rapidly. SAS was activated by sense, antisense and promoterless Nia2 DNA constructs, indicating that transcription is not required although it does stimulate SAS. CONCLUSIONS: SAS was activated by biolistic introduction of promoterless constructs, indicating that the DNA itself is a potent activator of post-transcriptional gene silencing. The systemic silencing signal invaded the whole plant by cell-to-cell and long-distance propagation, and reamplification of the signal. (+info
Association of snRNA genes with coiled bodies is mediated by nascent snRNA transcripts.
BACKGROUND: Coiled bodies are nuclear organelles that are highly enriched in small nuclear ribonucleoproteins (snRNPs) and certain basal transcription factors. Surprisingly, coiled bodies not only contain mature U snRNPs but also associate with specific chromosomal loci, including gene clusters that encode U snRNAs and histone messenger RNAs. The mechanism(s) by which coiled bodies associate with these genes is completely unknown. RESULTS: Using stable cell lines, we show that artificial tandem arrays of human U1 and U2 snRNA genes colocalize with coiled bodies and that the frequency of the colocalization depends directly on the transcriptional activity of the array. Association of the genes with coiled bodies was abolished when the artificial U2 arrays contained promoter mutations that prevent transcription or when RNA polymerase II transcription was globally inhibited by alpha-amanitin. Remarkably, the association was also abolished when the U2 snRNA coding regions were replaced by heterologous sequences. CONCLUSIONS: The requirement for the U2 snRNA coding region indicates that association of snRNA genes with coiled bodies is mediated by the nascent U2 RNA itself, not by DNA or DNA-bound proteins. Our data provide the first evidence that association of genes with a nuclear organelle can be directed by an RNA and suggest an autogenous feedback regulation model. (+info
TIF1gamma, a novel member of the transcriptional intermediary factor 1 family.
We report the cloning and characterization of a novel member of the Transcriptional Intermediary Factor 1 (TIF1) gene family, human TIF1gamma. Similar to TIF1alpha and TIF1beta, the structure of TIF1beta is characterized by multiple domains: RING finger, B boxes, Coiled coil, PHD/TTC, and bromodomain. Although structurally related to TIF1alpha and TIF1beta, TIF1gamma presents several functional differences. In contrast to TIF1alpha, but like TIF1beta, TIF1 does not interact with nuclear receptors in yeast two-hybrid or GST pull-down assays and does not interfere with retinoic acid response in transfected mammalian cells. Whereas TIF1alpha and TIF1beta were previously found to interact with the KRAB silencing domain of KOX1 and with the HP1alpha, MODI (HP1beta) and MOD2 (HP1gamma) heterochromatinic proteins, suggesting that they may participate in a complex involved in heterochromatin-induced gene repression, TIF1gamma does not interact with either the KRAB domain of KOX1 or the HP1 proteins. Nevertheless, TIF1gamma, like TIF1alpha and TIF1beta, exhibits a strong silencing activity when tethered to a promoter. Since deletion of a novel motif unique to the three TIF1 proteins, called TIF1 signature sequence (TSS), abrogates transcriptional repression by TIF1gamma, this motif likely participates in TIF1 dependent repression. (+info
Telomerase reverse transcriptase gene is a direct target of c-Myc but is not functionally equivalent in cellular transformation.
The telomerase reverse transcriptase component (TERT) is not expressed in most primary somatic human cells and tissues, but is upregulated in the majority of immortalized cell lines and tumors. Here, we identify the c-Myc transcription factor as a direct mediator of telomerase activation in primary human fibroblasts through its ability to specifically induce TERT gene expression. Through the use of a hormone inducible form of c-Myc (c-Myc-ER), we demonstrate that Myc-induced activation of the hTERT promoter requires an evolutionarily conserved E-box and that c-Myc-ER-induced accumulation of hTERT mRNA takes place in the absence of de novo protein synthesis. These findings demonstrate that the TERT gene is a direct transcriptional target of c-Myc. Since telomerase activation frequently correlates with immortalization and telomerase functions to stabilize telomers in cycling cells, we tested whether Myc-induced activation of TERT gene expression represents an important mechanism through which c-Myc acts to immortalize cells. Employing the rat embryo fibroblast cooperation assay, we show that TERT is unable to substitute for c-Myc in the transformation of primary rodent fibroblasts, suggesting that the transforming activities of Myc extend beyond its ability to activate TERT gene expression and hence telomerase activity. (+info
B-MYB transactivates its own promoter through SP1-binding sites.
B-MYB is an ubiquitous protein required for mammalian cell growth. In this report we show that B-MYB transactivates its own promoter through a 120 bp segment proximal to the transcription start site. The B-MYB-responsive element does not contain myb-binding sites and gel-shift analysis shows that SP1, but not B-MYB, protein contained in SAOS2 cell extracts binds to the 120 bp B-myb promoter fragment. B-MYB-dependent transactivation is cooperatively increased in the presence of SP1, but not SP3 overexpression. When the SP1 elements of the B-myb promoter are transferred in front of a heterologous promoter, an increased response to B-MYB results. In contrast, c-MYB, the prototype member of the Myb family, is not able to activate the luciferase construct containing the SP1 elements. With the use of an SP1-GAL4 fusion protein, we have determined that the cooperative activation occurs through the domain A of SP1. These observations suggest that B-MYB functions as a coactivator of SP1, and that diverse combinations of myb and SP1 sites may dictate the responsiveness of myb-target genes to the various members of the myb family. (+info
Differential stability of the DNA-activated protein kinase catalytic subunit mRNA in human glioma cells.
DNA-dependent protein kinase (DNA-PK) functions in double-strand break repair and immunoglobulin [V(D)J] recombination. We previously established a radiation-sensitive human cell line, M059J, derived from a malignant glioma, which lacks the catalytic subunit (DNA-PKcs) of the DNA-PK multiprotein complex. Although previous Northern blot analysis failed to detect the DNA-PKcs transcript in these cells, we show here through quantitative studies that the transcript is present, albeit at greatly reduced (approximately 20x) levels. Sequencing revealed no genetic alteration in either the promoter region, the kinase domain, or the 3' untranslated region of the DNA-PKcs gene to account for the reduced transcript levels. Nuclear run-on transcription assays indicated that the rate of DNA-PKcs transcription in M059J and DNA-PKcs proficient cell lines was similar, but the stability of the DNA-PKcs message in the M059J cell line was drastically (approximately 20x) reduced. Furthermore, M059J cells lack an alternately spliced DNA-PKcs transcript that accounts for a minor (5-20%) proportion of the DNA-PKcs message in all other cell lines tested. Thus, alterations in DNA-PKcs mRNA stability and/or the lack of the alternate mRNA may result in the loss of DNA-PKcs activity. This finding has important implications as DNA-PKcs activity is essential to cells repairing damage induced by radiation or radiomimetric agents. (+info