Interaction between acidic transcriptional activation domains of herpes simplex virus activator protein VP16 and transcriptional initiation factor IID

TFIIH operates through an expanded proximal promoter to fine-tune c-myc expression

A continuous stream of activating and repressing signals is processed by the transcription complex paused at the promoter of the c-myc proto-oncogene. The general transcription factor IIH (TFIIH) is held at promoters prior to promoter escape and so is well situated to channel the input of activators and repressors to modulate c-myc expression. We have compared cells expressing only a mutated p89 (xeroderma pigmentosum complementation group B [XPB]), the largest TFIIH subunit, with the same cells functionally complemented with the wild-type protein (XPB/wt-p89). Here, we show structural, compositional, and functional differences in transcription complexes between XPB and XPB/wt-89 cells at the native c-myc promoter. Remarkably, although the mean levels of c-Myc are only modestly elevated in XPB compared to those in XPB/wt-p89 cells, the range of expression and the cell-to-cell variation of c-Myc are markedly increased. Our modeling indicates that the data can be explained if TFIIH integrates inputs from multiple signals, regulating transcription at multiple kinetically equivalent steps between initiation and promoter escape. This helps to suppress the intrinsic noise of transcription and to ensure the steady transcriptional output of c-myc necessary for cellular homeostasis.

Interaction between BRCA2 and replication protein A is compromised by a cancer-predisposing mutation in BRCA2

Mutations in the BRCA1 and BRCA2 genes predispose women to familial, early-onset breast cancer. Both the BRCA1 and BRCA2 proteins appear to function in the homologous recombination pathway of DNA double-strand break repair. Both BRCA1 and BRCA2 have also been implicated in transcription by RNA polymerase II, for both proteins have domains which, when tethered adjacent to a promoter, can activate transcription. In experiments reported here, we have used protein affinity chromatography and coimmunoprecipitation techniques to show that the putative N-terminal acidic transcriptional activation domain of BRCA2 interacts with replication protein A (RPA), a protein essential for DNA repair, replication and recombination. This interaction was not mediated by DNA and was specific for human RPA but not yeast RPA. Since the cancer-predisposing mutation Y42C in BRCA2 significantly compromised the interaction between RPA and BRCA2, this interaction may be biologically important. That BRCA2 protein in HeLa cell extract also coimmunoprecipitated with RPA suggested that this interaction occurs in vivo. Therefore, the transcriptional activation domains within BRCA2, and perhaps BRCA1, may provide links to RPA and DNA repair processes rather than transcription.

LEDGF binds to heat shock and stress-related element to activate the expression of stress-related genes

We have investigated the mechanism by which LEDGF protects cells against environmental stress. Our earlier report showed that a low level of LEDGF was present in the nucleus of most cell types and significant elevation of LEDGF level was induced by heat and oxidative stress. The cells overexpressing LEDGF-activated expression of heat shock proteins and enhanced survival of many cell types. Here we show that LEDGF binds to heat shock element (HSE) and stress-related regulatory element (STRE) to activate the expression of stress-related genes (Hsp27 and alphaB-crystallin). Apparently, HSE and STRE are present in promoters of many stress-related genes. Elevation of many stress-related proteins (STRPs) induced by LEDGF may protect cells against environmental stress. In yeast, it has been demonstrated that a single stress can activate the expression of multiple STRPs. This is known as "cross-protection," and now similar mechanism has been found in mammalian cells and LEDGF plays a vital role in it.

Characterization of the transcriptional activator CBF1 from Arabidopsis thaliana. Evidence for cold denaturation in regions outside of the DNA binding domain

A transcriptional activator, CBF1, from Arabidopsis thaliana, which has the AP2 domain for DNA binding and regulates the cold acclimation response, was overexpressed in Escherichia coli, purified, and characterized. Analyses of the interaction between CBF1 and the C-repeat/dehydration-responsive element by fluorescence measurement showed that CBF1 binds to C-repeat/dehydration-responsive element as a monomer irrespective of the temperature. CD spectra of the intact and truncated CBF1 proteins (1-213, 41-213, 41-157, and 41-146) were measured to examine the temperature-dependent changes of the secondary structure of CBF1. The results suggested that the CBF1 protein has regions exhibiting reversible cold denaturation in the range between 30 and -5 degrees C and also has a region exhibiting thermal denaturation between 40 and 60 degrees C. This cold denaturation occurred in both the N-terminal and acidic regions. The thermal denaturation occurred in the region encompassing the AP2 domain. The difference between the retention time of CBF1 at 4 degrees C and that at 25 degrees C in gel filtration, and the decrease of the sedimentation coefficient, s20,w, caused by the temperature change from 25 to 3 degrees C, strongly suggested that the cold denaturation was accompanied by the extension of the molecule. The possible cold denaturation observed here might be a physiologically important structural response of CBF1 to cold stress.

Isolation of cDNAs encoding novel transcription coactivators p52 and p75 reveals an alternate regulatory mechanism of transcriptional activation

Transcriptional activation in human cell-free systems containing RNA polymerase II and general initiation factors requires the action of one or more additional coactivators. Here, we report the isolation of cDNAs encoding two novel human transcriptional coactivators (p52 and p75) that are derived from alternatively spliced products of a single gene and share a region of 325 residues, but show distinct coactivator properties. p52 and p75 both show strong interactions with the VP16 activation domain and several components of the general transcriptional machinery. p52, like the previously described PC4, is a potent broad-specificity coactivator, whereas p75 is less active for most activation domains. These results suggest that p52 is a general transcriptional coactivator that mediates functional interactions between upstream sequence-specific activators and the general transcription apparatus, possibly through a novel mechanism.