Centromeres and pericentromeric heterochromatin are accessible in living Schizosaccharomyces pombe cells

Chromatin is intrinsically repressive, limiting access to DNA, implying a major regulatory role. Studies with nuclei support this model. However, we have shown previously that genomic DNA is almost completely accessible in living budding yeast and human cells, except for centromeric chromatin. The fission yeast, Schizosaccharomyces pombe, possesses heterochromatin similar to mammalian heterochromatin at the pericentromeric repeats, telomeres and the silenced mating type loci. S. pombe heterochromatin is marked by histone H3K9 di- and tri-methylation (H3K9me2/3) and heterochromatin protein 1 (HP1/Swi6), potentially repressing genes by preventing access to the DNA. Here, we developed a copper-inducible DNA methyltransferase system to measure accessibility in living S. pombe cells. We find that euchromatin and heterochromatin are generally accessible, indicating that heterochromatin does not represent a significant block to DNA methyltransferases in vivo. S. pombe centromeres are much more accessible than budding yeast and human centromeres. In contrast, S. pombe chromatin is mostly inaccessible in isolated nuclei, primarily due to tight nucleosome spacing on gene bodies, with very little linker DNA. We conclude that S. pombe euchromatin and heterochromatin are both highly dynamic in vivo, suggesting that the H3K9me/HP1 system does not repress transcription by preventing access to DNA.