No abstract

The heterochromatin domain at the mat locus of Schizosaccharomyces pombe is bounded by the IR-L and IR-R barriers. A genetic screen for mutations that promote silencing beyond IR-L revealed a novel gene named epe1, encoding a conserved nuclear protein with a jmjC domain. Disruption of epe1 promotes continuous spreading of heterochromatin-associated histone modifications and Swi6 binding to chromatin across heterochromatic barriers. It also enhances position effect variegation at heterochromatic domains, suppresses mutations in silencing genes, and stabilizes the repressed epigenetic state at the mat locus. However, it does not enhance silencing establishment. Our analysis suggests that the jmjC domain is essential for Epe1 activity and that Epe1 counteracts transcriptional silencing by negatively affecting heterochromatin stability. Consistent with this proposition, the meiotic stability of established heterochromatin beyond IR-L is diminished by Epe1 activity, and overexpression of Epe1 disrupts heterochromatin through acetylation of H3-K9 and H3-K14 and methylation of H3-K4. Furthermore, overexpression of Epe1 elevates the rate of chromosome loss. We propose that Epe1 helps control chromatin organization by down-regulating the stability of epigenetic marks that govern heterochromatization.Epigenetic control of gene expression by clonally inherited higher-order chromatin structures accounts for diverse biological phenomena, such as dosage compensation, imprinting, and position effect variegation (PEV) (26,44,45). In addition to transcription regulation, epigenetic effects have an important impact on cellular differentiation, accurate chromosome segregation, recombination, neocentromere maintenance, and mating-type switching in yeast (reviewed in references 1, 18, 24, 25, 28, 37, and 47). Molecular models that explain chromosomal inheritance of epigenetic states assume a long-range memory mechanism that marks imprinted chromosomal domains by self-templating higher-order chromatin structures and covalent modification of DNA (20,46). Evidence for chromatin modifications in imprinted domains has been obtained through experiments that demonstrate the association of silent chromosomal regions with modified histones and chromatinassociated nonhistone proteins like Saccharomyces cerevisiae SIR3, Drosophila HP1, and Schizosaccharomyces pombe Swi6 (15,22,33,36).The mat locus of S. pombe shares an extended sequence homology with the centromeric outer repeats (cenH) and provides a useful paradigm for the highly conserved process of heterochromatin assembly and inheritance (17,18,25). Constitutive heterochromatin extends along the 15-kb mat2-K-mat3 interval that is bound on its centromere-proximal end by the REII protosilencer (Fig. 1A) (7,21,33,41). Silencing diminishes gradually at the L region as the distance from REII toward the IR-L heterochromatin barrier increases (6). IR-L, like its IR-R homologous counterpart at the centromere-distal end of the heterochromatic domain, is a distinct transition point for Swi6 associa...

SUMMARY DNA binding by numerous transcription factors including the p53 tumor suppressor protein constitutes a vital early step in transcriptional activation. While the role of the central core DNA binding domain (DBD) of p53 in site-specific DNA binding has been established, the contribution of the sequence-independent C-terminal domain (CTD) is still not well understood. We investigated the DNA-binding properties of a series of p53 CTD variants using a combination of in vitro biochemical analyses and in vivo binding experiments. Our results provide several unanticipated and interconnected findings. First, the CTD enables DNA binding in a sequence-dependent manner that is drastically altered by either its modification or deletion. Second, dependence on the CTD correlates with the extent to which the p53 binding site deviates from the canonical consensus sequence. Finally, the CTD enables stable formation of p53-DNA complexes to divergent binding sites via DNA-induced conformational changes within the DBD itself.

The mechanisms by which pathogens sense the host and respond by remodeling gene expression are poorly understood. Enteropathogenic (EPEC), the cause of severe intestinal infection, employs a type III secretion system (T3SS) to inject effector proteins into intestinal epithelial cells. These effectors subvert host cell processes to promote bacterial colonization. We show that the T3SS also functions to sense the host cell and to trigger in response posttranscriptional remodeling of gene expression in the bacteria. We further show that upon effector injection, the effector-bound chaperone (CesT), which remains in the EPEC cytoplasm, antagonizes the posttranscriptional regulator CsrA. The CesT-CsrA interaction provokes reprogramming of expression of virulence and metabolic genes. This regulation is likely required for the pathogen's adaptation to life on the epithelium surface.

No abstract