Chromosome number changes in yeast

ScienceDaily (July 21, 2011) — Researchers from Trinity College Dublin have uncovered the evolutionary mechanisms that have caused increases or decreases in the numbers of chromosomes in a group of yeast species during the last 100-150 million years. The study, to be published on July 21st in the open-access journal PLoS Genetics, offers an unprecedented view of chromosome complement (chromosome number) changes in a large group of related species.See Also:Plants & AnimalsGeneticsMolecular BiologyEpigenetics ResearchEvolutionary BiologyCell BiologyNew SpeciesReferenceChromosomal crossoverSomatic cellVector (biology)Sex linkage

A few specific cases of chromosome number changes have been studied in plants and animals, for example the fusion of two great ape chromosomes that gave rise to chromosome 2 in humans, giving humans a chromosome count of 23 pairs compared to 24 pairs in great apes. The family of yeasts studied in this new research spans a similar evolutionary distance to that of vertebrates. The availability of completely sequenced genomes facilitated the reconstruction of ancestral genome structures at different evolutionary time points. Tracing the positions of essential parts of chromosomes (centromeres and telomeres) through time allowed for the identification of specific genome rearrangement events that resulted in chromosome complement changes.

The addition of large numbers of genes is not often tolerated by cells, and neither are deletions of large numbers of genes. This restricts the types of possible changes in chromosome complement to rearrangements of genes on chromosomes that maintain the same number of genes.

The researchers show that, in yeasts, chromosome complement has decreased in all except one notable event, a whole genome duplication -- an event that doubled the complement of an ancestor of several of the species from 8 chromosomes to 16. The decreases in chromosome number were mostly by the fusion of whole chromosomes, similar to the one that gave rise to chromosome 2 in humans. One exception to this mechanism was the breakage of a chromosome and the subsequent fusion of the two broken edges to two different chromosome ends.

Although some aspects of the research are specific to yeast, many of the mechanisms of chromosome number change in yeast are similar to those found in other organisms and therefore shed light on how chromosome complements evolve.

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Cellular stress can induce yeast to promote prion formation

ScienceDaily (July 24, 2011) — It's a chicken and egg question. Where do the infectious protein particles called prions come from? Essentially clumps of misfolded proteins, prions cause neurodegenerative disorders, such as mad cow/Creutzfeld-Jakob disease, in humans and animals. Prions trigger the misfolding and aggregation of their properly folded protein counterparts, but they usually need some kind of "seed" to get started.See Also:Health & MedicineHuman BiologyDiseases and ConditionsMind & BrainMad Cow DiseaseHuntington's DiseasePlants & AnimalsPrionsCell BiologyReferenceAmyloidPrionTransmissible spongiform encephalopathyProtein folding

Biochemists at Emory University School of Medicine have identified a yeast protein called Lsb2 that can promote spontaneous prion formation. This unstable, short-lived protein is strongly induced by cellular stresses such as heat. Lsb2's properties also illustrate how cells have developed ways to control and regulate prion formation. Research in yeast has shown that sometimes, prions can actually help cells adapt to different conditions.

The results are published in the July 22 issue of the journal Molecular Cell. The senior author is Keith Wilkinson, PhD, professor of biochemistry at Emory University School of Medicine The first author is senior associate Tatiana Chernova, PhD.

The aggregated form of proteins connected with several other neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's can, in some circumstances, act like prions. So the Emory team's finding provides insight into how the ways that cells deal with stress might lead to poisonous protein aggregation in human diseases.

"A direct human homolog of Lsb2 doesn't exist, but there may be a protein that performs the same function," Wilkinson says. "The mechanism may say more about other types of protein aggregates than about classical prions in humans, This mechanism of seeding and growth may be more important for aggregate formation in diseases such as Huntington's."

Lsb2 does not appear to form stable prions by itself. Rather, it seems to bind to and encourage the aggregation of another protein, Sup35, which does form prions.

"Our model is that stress induces high levels of Lsb2, which allows the accumulation of misfolded prion proteins," Wilkinson says. "Lsb2 protects enough of these newborn prion particles from the quality control machinery for a few of them to get out."

The research was supported by the National Institutes of Health.

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