Biology Reference
In-Depth Information
heterochromatin formation (
Luff, Pawlowski, & Bender, 1999
). These
repeats serve as template for the production of noncoding RNA (ncRNA)
from both DNA strands, which is often associated with the generation of
dsRNA. Although heterochromatin is a silent chromatin structure that
blocks transcription, RNA Pol II transcribes these repeats in an exquisitely
regulated temporal fashion at particular stages of the cell cycle (
Chen et al.,
2008; Kloc et al., 2008
). Therefore, there seems to be an important and
conserved role for both ncRNA and RNAi in this context (
Matzke &
Birchler, 2005
).
The RNAi machinery was originally defined as a regulator of posttran-
scriptional silencing (
Muller et al., 2002
), but it is now clear that the RNAi
machinery also alters chromatin structure and effects silencing at the tran-
scriptional level. Most importantly, RNAi is central for initiating hetero-
chromatin assembly not only at repetitive DNA in the centromeres and
mating-type loci of fission yeast but also at retrotransposons in the
Arabidopsis
germ line (
Grewal & Elgin, 2007
). In fission yeast, siRNAs derived from
heterochromatic repeats are present within the cell and are loaded into
the RNA-induced transcriptional silencing (RITS) complex, which is
composed of Ago1, Tas3, and the chromodomain-containing protein
Chp1(
Verdel et al., 2004
). RITS is thought to bind to heterochromatic
ncRNA using siRNA as a guiding molecule. It further participates in
“amplifying” the response since it recruits the RNA-dependent RNA poly-
merase complex (RDRC), which consists of Rdp1, a poly(A) polymerase
(Cid12), and a putative helicase (Hrr1), most likely via physical interactions
(
Verdel et al., 2004
). The RDRC enhances the generation of siRNA by
synthesizing dsRNAs from centromeric transcripts as substrates for Dcr1
(
Colmenares, Buker, Buhler, Dlakic, & Moazed, 2007; Sugiyama, Cam,
Verdel, Moazed, & Grewal, 2005
). RITS also recruits Clr4 through the
LIM domain protein, Stc1, such that the heterochromatin spreads onto the
dg
and
dh
repeats (
Zhang, Mosch, Fischle, & Grewal, 2008
). Stc1 associates
with RITS on centromeric transcripts and recruits the Clr4-containing
complex (CLRC), thereby coupling RNAi to chromatin modification
(
Bayne et al., 2010
). Methylation of H3K9me3 is also recognized by
Swi6/HP1, which further reinforces the silencing environment by mediat-
ing targeting of HDACs and is also responsible of recruiting the JmjC
domain-containing antisilencing factor Epe1 that facilitates the transcription
of heterochromatic repeats (see below) (
Zofall & Grewal, 2006
).
In the RNAi-mediated heterochromatin assembly system, siRNA
generation and heterochromatin formation are interdependent, forming a
