RNA interference guides histone modification during the S phase of chromosomal replication.
BACKGROUND
Heterochromatin is chromosomal material that remains condensed throughout the cell division cycle and silences genes nearby. It is found in almost all eukaryotes, and although discovered (in plants) almost 100 years ago, the mechanism by which heterochromatin is inherited has remained obscure. Heterochromatic silencing and histone H3 lysine-9 methylation (H3K9me2) depend, paradoxically, on heterochromatic transcription and RNA interference (RNAi).


RESULTS
Here, we show that heterochromatin protein 1 in fission yeast (Swi6) is lost via phosphorylation of H3 serine 10 (H3S10) during mitosis, allowing heterochromatic transcripts to transiently accumulate in S phase. Rapid processing of these transcripts into small interfering RNA (siRNA) promotes restoration of H3K9me2 and Swi6 after replication when cohesin is recruited. We also show that RNAi in fission yeast is inhibited at high temperatures, providing a plausible mechanism for epigenetic phenomena that depend on replication and temperature, such as vernalization in plants and position effect variegation in animals.


CONCLUSIONS
These results explain how "silent" heterochromatin can be transcribed and lead to a model for epigenetic inheritance during replication.
Kloc
Anna
A
Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724, USA.
Zaratiegui
Mikel
M
Nora
Elphege
E
Martienssen
Rob
R
D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism; D004261:DNA Replication / Q000502:physiology*; D006570:Heterochromatin / Q000378:metabolism*; D006657:Histones / Q000378:metabolism*; D006358:Hot Temperature; D034622:RNA Interference; D016196:S Phase / Q000502:physiology*; D012568:Schizosaccharomyces / Q000378:metabolism*; D029702:Schizosaccharomyces pombe Proteins / Q000378:metabolism
D002868:Chromosomal Proteins, Non-Histone; D006570:Heterochromatin; D006657:Histones; D029702:Schizosaccharomyces pombe Proteins; C465958:Swi6 protein, S pombe