The centromeres of the fission yeast S. pombe consist of a central region onto w

Question

The centromeres of the fission yeast S. pombe consist of a central region onto which kinetochore proteins assemble flanked by pericentromeric regions that are heterochromatic in nature. A histone H3 mutation (H3K4R; Lysine 4 ? Arginine) exhibits pericentromeric silencing defects while a set1 deletion mutant deficient in H3K4 methylation does not. Provide a reasonable hypothesis that would explain how histone modification(s) in S. pombe normally impact pericentromeric chromatin structure. Propose an experiment that would test your hypothesis and describe the experimental results that would support your hypothesis.

Explanation / Answer

The SET1 gene in the yeast is a euchromatin region of the DNA which controls levels of gene expression by regulating the methyltransferase enzymes across the nucleosomes. Thus, over-expression of this gene will lead to gene silencing in the affected area but deletion of this gene would not lead to gene silencing. Further histone methylation is itself a gene-silencing event in which chromatin condensation takes place. Similarly, alteration of the amino acid sequence in the nucleosome can lead to gene silencing. This is why, change in amino acid position in the nucleosome served for gene silencing whereas a deletion mutation in the SET1 gene did not as in the pericentric chromatin of the yeast.

In order to experimentally investigate these findings, one can tag the sequences for SET1 and the target histone moiety by fluorescent labelling in order to highlight the gene silencing complex around the SET1. After cellular division and peri-centric chromatin remodelling, two simultaneous experiments must be performed. The first experiment would target to fluorescently microscope the chromatin and the second experiment would target to identify the genetic manipulation of the associated genes.

Cumulatively, these experiments would result in the observation that whereas amino acid transformation leads to gene silencing whereas a deletion mutation does not.

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