Highly condensed chromatins are formed adjacent to subtelomeric and decondensed silent chromatin in fission yeast

It is generally believed that silent chromatin is condensed and transcriptionally active chromatin is decondensed. However, little is known about the relationship between condensation levels and gene expression. Here we report condensation levels of interphase chromatin in the fission yeast Schizosaccharomyces pombe examined by super-resolution fluorescence microscopy. Unexpectedly, silent chromatin was observed to be less condensed than the euchromatin. Furthermore, the telomeric silent regions were flanked by highly condensed chromatin bodies, or “knobs”. Knob regions span approximately 50 kb of sequence devoid of methylated histones. Knob condensation was independent of HP1 homolog Swi6 and other gene silencing factors. Disruption of methylation at lysine 36 of histone H3 (H3K36) eliminated knob formation and gene repression at the subtelomeric and adjacent knob regions. Thus, epigenetic marks atH3K36 play crucial roles in the formation of a unique chromatin structure and in gene regulation at those regions in S. pombe. Gene expression profile at exponentially-growing phase in the fission yeast deletion mutant of set2 and histone H3 mutant K36R, K36A.