Transcription-Driven Translocation of Cohesive and Non-Cohesive Cohesin In Vivo.

Cohesin is a central architectural element of chromosomes that regulates numerous DNA-based events. The complex holds sister chromatids together until anaphase onset and organizes individual chromosomal DNAs into loops and self-associating domains. Purified cohesin diffuses along DNA in an ATP-independent manner but can be propelled by transcribing RNA polymerase.

Nucleoporin TPR promotes tRNA nuclear export and protein synthesis in lung cancer cells.

The robust proliferation of cancer cells requires vastly elevated levels of protein synthesis, which relies on a steady supply of aminoacylated tRNAs. Delivery of tRNAs to the cytoplasm is a highly regulated process, but the machinery for tRNA nuclear export is not fully elucidated. In this study, using a live cell imaging strategy that visualizes nascent transcripts from a specific tRNA gene in yeast, we identified the nuclear basket proteins Mlp1 and Mlp2, two homologs of the human TPR protein, as regulators of tRNA export.

Binding, sliding, and function of cohesin during transcriptional activation.

The ring-shaped cohesin complex orchestrates long-range DNA interactions to mediate sister chromatid cohesion and other aspects of chromosome structure and function. In the yeast , the complex binds discrete sites along chromosomes, including positions within and around genes. Transcriptional activity redistributes the complex to the 3' ends of convergently oriented gene pairs.

Determinants of Sir2-Mediated, Silent Chromatin Cohesion.

Cohesin associates with distinct sites on chromosomes to mediate sister chromatid cohesion. Single cohesin complexes are thought to bind by encircling both sister chromatids in a topological embrace. Transcriptionally repressed chromosomal domains in the yeast Saccharomyces cerevisiae represent specialized sites of cohesion where cohesin binds silent chromatin in a Sir2-dependent fashion.

A series of conditional shuttle vectors for targeted genomic integration in budding yeast.

The capacity of Saccharomyces cerevisiae to repair exposed DNA ends by homologous recombination has long been used by experimentalists to assemble plasmids from DNA fragments in vivo. While this approach works well for engineering extrachromosomal vectors, it is not well suited to the generation, recovery and reuse of integrative vectors. Here, we describe the creation of a series of conditional centromeric shuttle vectors, termed pXR vectors, that can be used for both plasmid assembly in vivo and targeted genomic integration.

Silencing sounds off.

Silent chromatin in budding yeast is propagated from one generation to the next, even though 'silenced' genes are occasionally expressed.

Coordination of tRNA transcription with export at nuclear pore complexes in budding yeast.

tRNAs are encoded by RNA polymerase III-transcribed genes that reside at seemingly random intervals along the chromosomes of budding yeast. Existing evidence suggests that the genes congregate together at the nucleolus and/or centromeres. In this study, we re-examined spatial and temporal aspects of tRNA gene (tDNA) expression.

Palmitoylation controls the dynamics of budding-yeast heterochromatin via the telomere-binding protein Rif1.

The posttranslational addition of palmitate to cysteines occurs ubiquitously in eukaryotic cells, where it functions in anchoring target proteins to membranes and in vesicular trafficking. Here we show that the Saccharomyces cerevisiae palmitoyltransferase Pfa4 enhanced heterochromatin formation at the cryptic mating-type loci HMR and HML via Rif1, a telomere regulatory protein. Acylated Rif1 was detected in extracts from wild-type but not pfa4Δ mutant cells.

Nucleoporin mediated nuclear positioning and silencing of HMR.

The organization of chromatin domains in the nucleus is an important factor in gene regulation. In eukaryotic nuclei, transcriptionally silenced chromatin clusters at the nuclear periphery while transcriptionally poised chromatin resides in the nuclear interior. Recent studies suggest that nuclear pore proteins (NUPs) recruit loci to nuclear pores to aid in insulation of genes from silencing and during gene activation.

Heterochromatin and the cohesion of sister chromatids.

Heterochromatin, once thought to be the useless junk of chromosomes, is now known to play significant roles in biology. Underlying much of this newfound fame are links between the repressive chromatin structure and cohesin, the protein complex that mediates sister chromatid cohesion. Heterochromatin-mediated recruitment and retention of cohesin to domains flanking centromeres promotes proper attachment of chromosomes to the mitotic and meiotic spindles.

Condensin goes with the family but not with the flow.

Condensin and cohesin are loaded onto yeast chromosomes by a common mechanism at RNA polymerase III transcribed genes. Whereas cohesin translocates from these loading sites to mediate cohesion at secondary locations, condensin remains, bringing distant sites together into clusters.

Bypassing Sir2 and O-acetyl-ADP-ribose in transcriptional silencing.

The yeast Sir2/3/4 complex forms a heterochromatin-like structure that represses transcription. The proteins nucleate at silencers and spread distally, utilizing the Sir2 NAD(+)-dependent histone deacetylase activity and the affinity of Sir3/4 for deacetylated histone tails. A by-product of the Sir2 reaction, O-acetyl-ADP-ribose (OAADPr), is thought to aid spreading by binding one of the Sir proteins.

Long-range communication between the silencers of HMR.

Gene regulation involves long-range communication between silencers, enhancers, and promoters. In Saccharomyces cerevisiae, silencers flank transcriptionally repressed genes to mediate regional silencing. Silencers recruit the Sir proteins, which then spread along chromatin to encompass the entire silenced domain.

Controlled exchange of chromosomal arms reveals principles driving telomere interactions in yeast.

The 32 telomeres in the budding yeast genome cluster in three to seven perinuclear foci. Although individual telomeres and telomeric foci are in constant motion, preferential juxtaposition of some telomeres has been scored. To examine the principles that guide such long-range interactions, we differentially tagged pairs of chromosome ends and developed an automated three-dimensional measuring tool that determines distances between two telomeres.

A tDNA establishes cohesion of a neighboring silent chromatin domain.

DNA replication generates sister chromatid pairs that are bound to one another until anaphase onset. The process, termed sister chromatid cohesion, requires the multisubunit cohesin complex that resides at centromeres and sites where genes converge. At the HMR mating-type locus of budding yeast, cohesin associates with a heterochromatin-like structure known as silent chromatin.

Swapping the gene-specific and regional silencing specificities of the Hst1 and Sir2 histone deacetylases.

Sir2 and Hst1 are NAD(+)-dependent histone deacetylases of budding yeast that are related by strong sequence similarity. Nevertheless, the two proteins promote two mechanistically distinct forms of gene repression. Hst1 interacts with Rfm1 and Sum1 to repress the transcription of specific middle-sporulation genes.

Geropsychology:integrated training at the interim level.

The profession of psychology is responding to this culture's rapidly changing problems related to an aging population. A central theme regarding the training of clinical geropsychologists has been that education should be sequentially organized across levels, from undergraduate, to graduate, internship, postdoctoral, and continuing education for current practitioners. This paper addresses issues related to geropsychology training at the internship level.

Multiple pathways tether telomeres and silent chromatin at the nuclear periphery: functional implications for sir-mediated repression.

The positioning of chromosomal domains within interphase nuclei is thought to contribute to establishment and maintenance of epigenetic control. Using GFP-tagged chromosomal domains, LexA-fusion targeting and live microscopy, we investigated mechanisms through which chromatin can be anchored to the nuclear envelope (NE). We find that a subdomain of the silencing information regulator Sir4 (Sir4(PAD)) and yKu80 are sufficient to tether a chromatin region to the nuclear periphery, independently of their silencing function.