Jennifer L. Gerton

Psychological impact of biculturalism: Evidence and theory.

A vital step in the development of an equal partnership for minorities in the academic, social, and economic life of the United States involves moving away from assumptions of the linear model of cultural acquisition. In this article we review the literature on the psychological impact of being bicultural. Assimilation, acculturation, alternation, multicultural, and fusion models that have been used to describe the psychological processes, social experiences, and individual challenges and obstacles of being bicultural are reviewed and summarized for their contributions and implications for investigations of the psychological impact of biculturalism. Emphasis is given to the alternation model, which posits that an individual is able to gain competence within 2 cultures without losing his or her cultural identity or having to choose one culture over the other. Finally, a hypothetical model outlining the dimensions of bicultural competence is presented.

Genome-Wide Mapping of the Cohesin Complex in the Yeast Saccharomyces cerevisiae

In eukaryotic cells, cohesin holds sister chromatids together until they separate into daughter cells during mitosis. We have used chromatin immunoprecipitation coupled with microarray analysis (ChIP chip) to produce a genome-wide description of cohesin binding to meiotic and mitotic chromosomes of Saccharomyces cerevisiae. A computer program, PeakFinder, enables flexible, automated identification and annotation of cohesin binding peaks in ChIP chip data. Cohesin sites are highly conserved in meiosis and mitosis, suggesting that chromosomes share a common underlying structure during different developmental programs. These sites occur with a semiperiodic spacing of 11 kb that correlates with AT content. The number of sites correlates with chromosome size; however, binding to neighboring sites does not appear to be cooperative. We observed a very strong correlation between cohesin sites and regions between convergent transcription units. The apparent incompatibility between transcription and cohesin binding exists in both meiosis and mitosis. Further experiments reveal that transcript elongation into a cohesin-binding site removes cohesin. A negative correlation between cohesin sites and meiotic recombination sites suggests meiotic exchange is sensitive to the chromosome structure provided by cohesin. The genome-wide view of mitotic and meiotic cohesin binding provides an important framework for the exploration of cohesins and cohesion in other genomes.

Homologous chromosome interactions in meiosis: diversity amidst conservation

Proper chromosome segregation is crucial for preventing fertility problems, birth defects and cancer. During mitotic cell divisions, sister chromatids separate from each other to opposite poles, resulting in two daughter cells that each have a complete copy of the genome. Meiosis poses a special problem in which homologous chromosomes must first pair and then separate at the first meiotic division before sister chromatids separate at the second meiotic division. So, chromosome interactions between homologues are a unique feature of meiosis and are essential for proper chromosome segregation. Pairing and locking together of homologous chromosomes involves recombination interactions in some cases, but not in others. Although all organisms must match and lock homologous chromosomes to maintain genome integrity throughout meiosis, recent results indicate that the underlying mechanisms vary in different organisms.

Cse4 Is Part of an Octameric Nucleosome in Budding Yeast

The budding yeast CenH3 histone variant Cse4 localizes to centromeric nucleosomes and is required for kinetochore assembly and chromosome segregation. The exact composition of centromeric Cse4-containing nucleosomes is a subject of debate. Using unbiased biochemical, cell-biological, and genetic approaches, we have tested the composition of Cse4-containing nucleosomes. Using micrococcal nuclease-treated chromatin, we find that Cse4 is associated with the histones H2A, H2B, and H4, but not H3 or the nonhistone protein Scm3. Overexpression of Cse4 rescues the lethality of a scm3 deletion, indicating that Scm3 is not essential for the formation of functional centromeric chromatin. We also find that octameric Cse4 nucleosomes can be reconstituted in vitro. Furthermore, Cse4-Cse4 dimerization occurs in vivo at the centromeric nucleosome, and this requires the predicted Cse4-Cse4 dimerization interface. Taken together, our experimental evidence supports the model that the Cse4 nucleosome is an octamer, containing two copies each of Cse4, H2A, H2B, and H4.

The Kinetochore Is an Enhancer of Pericentric Cohesin Binding

The recruitment of cohesins to pericentric chromatin in some organisms appears to require heterochromatin associated with repetitive DNA. However, neocentromeres and budding yeast centromeres lack flanking repetitive DNA, indicating that cohesin recruitment occurs through an alternative pathway. Here, we demonstrate that all budding yeast chromosomes assemble cohesin domains that extend over 20–50 kb of unique pericentric sequences flanking the conserved 120-bp centromeric DNA. The assembly of these cohesin domains requires the presence of a functional kinetochore in every cell cycle. A similar enhancement of cohesin binding was also observed in regions flanking an ectopic centromere. At both endogenous and ectopic locations, the centromeric enhancer amplified the inherent levels of cohesin binding that are unique to each region. Thus, kinetochores are enhancers of cohesin association that act over tens of kilobases to assemble pericentric cohesin domains. These domains are larger than the pericentric regions stretched by microtubule attachments, and thus are likely to counter microtubule-dependent forces. Kinetochores mediate two essential segregation functions: chromosome movement through microtubule attachment and biorientation of sister chromatids through the recruitment of high levels of cohesin to pericentric regions. We suggest that the coordination of chromosome movement and biorientation makes the kinetochore an autonomous segregation unit.

Psh1 Is an E3 Ubiquitin Ligase that Targets the Centromeric Histone Variant Cse4

Cse4 is a variant of histone H3 that is incorporated into a single nucleosome at each centromere in budding yeast. We have discovered an E3 ubiquitin ligase, called Psh1, which controls the cellular level of Cse4 via ubiquitylation and proteolysis. The activity of Psh1 is dependent on both its RING and zinc finger domains. We demonstrate the specificity of the ubiquitylation activity of Psh1 toward Cse4 in vitro and map the sites of ubiquitylation. Mutation of key lysines prevents ubiquitylation of Cse4 by Psh1 in vitro and stabilizes Cse4 in vivo. While deletion of Psh1 stabilizes Cse4, elimination of the Cse4-specific chaperone Scm3 destabilizes Cse4, and the addition of Scm3 to the Psh1-Cse4 ubiquitylation reaction prevents Cse4 ubiquitylation, together suggesting Scm3 may protect Cse4 from ubiquitylation. Without Psh1, Cse4 overexpression is toxic and Cse4 is found at ectopic locations. Our results suggest Psh1 functions to prevent the mislocalization of Cse4.

Cell-Cycle-Coupled Structural Oscillation of Centromeric Nucleosomes in Yeast

The centromere is a specialized chromosomal structure that regulates chromosome segregation. Centromeres are marked by a histone H3 variant. In budding yeast, the histone H3 variant Cse4 is present in a single centromeric nucleosome. Experimental evidence supports several different models for the structure of centromeric nucleosomes. To investigate Cse4 copy number in live yeast, we developed a method coupling fluorescence correlation spectroscopy and calibrated imaging. We find that centromeric nucleosomes have one copy of Cse4 during most of the cell cycle, whereas two copies are detected at anaphase. The proposal of an anaphase-coupled structural change is supported by Cse4-Cse4 interactions, incorporation of Cse4, and the absence of Scm3 in anaphase. Nucleosome reconstitution and ChIP suggests both Cse4 structures contain H2A/H2B. The increase in Cse4 intensity and deposition at anaphase are also observed in Candida albicans. Our experimental evidence supports a cell-cycle-coupled oscillation of centromeric nucleosome structure in yeast.

Cohesinopathies, gene expression, and chromatin organization

The cohesin protein complex is best known for its role in sister chromatid cohesion, which is crucial for accurate chromosome segregation. Mutations in cohesin proteins or their regulators have been associated with human diseases (termed cohesinopathies). The developmental defects observed in these diseases indicate a role for cohesin in gene regulation distinct from its role in chromosome segregation. In mammalian cells, cohesin stably interacts with specific chromosomal sites and colocalizes with CTCF, a protein that promotes long-range DNA interactions, implying a role for cohesin in genome organization. Moreover, cohesin defects compromise the subnuclear position of chromatin. Therefore, defects in the cohesin network that alter gene expression and genome organization may underlie cohesinopathies.

Cohesinopathy mutations disrupt the subnuclear organization of chromatin

Nuclear morphology, chromosomal condensation, and transcriptional-mediated localization of genes to the nuclear periphery are disturbed by mutations in cohesin pathway genes.

Mnd1p: An evolutionarily conserved protein required for meiotic recombination

We used a functional genomics approach to identify a gene required for meiotic recombination, YGL183c or MND1. MND1 was spliced in meiotic cells, extending the annotated YGL183c ORF N terminus by 45 aa. Saccharomyces cerevisiae mnd1–1 mutants, in which the majority of the MND1 coding sequence was removed, arrested before the first meiotic division with a phenotype reminiscent of dmc1 mutants. Physical and genetic analysis showed that these cells initiated recombination, but did not form heteroduplex DNA or double Holliday junctions, suggesting that Mnd1p is involved in strand invasion. Orthologs of MND1 were identified in protists, several yeasts, plants, and mammals, suggesting that its function has been conserved throughout evolution.