Biology Reference
In-Depth Information
S phase
SH
RE
C
Acetylation
K36me
Set2
K9me
Ch
p
1
Ago1
Transcript
Ri
k
1
R
dp
1
Dcr1
RDRC
siRNA
Clr4
G2 phase
Chromatin
antisilencing
SHRE
C
TGS
Heterochromatin
spreading
Epe1
HP
HP
HP
HP
HP
Ri
k
1
Ch
p
1
Rik1
Ch
p
1
Ri
k
1
Rdp1
Clr4
Clr4
cis-PTGS
Ch
p
1
Clr4
siRNA
Figure 1.4 Heterochromatin assembly in fission yeast requires the coordinated action
of histone-modifying enzymes and RNAi. During S-phase (top), RNA Pol II activity at cen-
tromeric repeats occurs. This, in turn, stimulates the recruitment of heterochromatin
assembly factors such as the ClRC subunit Rik1 and the RITS subunit Argonaute 1
(Ago1), as well as histone H3 lysine 36 methylation by the Set2 methyltransferase. Inter-
action between ClrC and RITS stabilizes their binding to chromatin and facilitates the
processing of centromeric repeat RNAs to siRNAs. Recruitment of ClrCmay also be medi-
ated by downstream siRNA products such as double-stranded RNAs. Methylation of
lysine 9 on histone H3 (H3K9me) by the Clr4 subunit of ClrC not only recruits HP1 pro-
teins but also establishes a positive feedback loop by stabilizing the chromatin associ-
ation of ClrC (via Clr4 chromodomain) and RNAi components such as RITS (via Chp1
chromodomain). In G2 phase (bottom), HP1 proteins bound to H3K9me recruit not only
silencing factors such as the HDAC complex SHREC but also an antisilencing factor,
Epe1, that promotes Pol II transcription. Spreading of HP1 proteins and H3K9me
from the original nucleation sites allows heterochromatin to serve as a recruiting
platform to reinforce its nature,
including RITS components. Adapted from
Cam,
Chen, & Grewal (2009)
.
