Epigenetics in the Extreme: Prions and the Inheritance of
Environmentally Acquired Traits
Randal Halfmann, et al.
Science 330, 629 (2010);
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cytosine methylation within the b1 tandem repeats in B′ relative to B-I (7), whereas histones
associated with the b1 repeats in bothalleles did
not carry modifications characteristic of silent
chromatin. Future studies on the paramutation
properties of mutants impaired in DNA methylation and various histone modifications should
shed light on the potential role for these marks
in paramutation. The observations that RdDM in
Arabidopsis is associated with cytosine methylation and heterochromatin histone modifications
(4),yet paramutation does not occur between
RdDM silenced alleles (see below), leads to the
speculation that paramutation involves additional mechanisms, such as RNA or proteins that
remain associated with the b1 repeats during
mitosis and meiosis.
It is puzzling that RNAi-mediated heterochromatin in S. pombe and RdDM-silenced genes in
Arabidopsis do not undergo paramutation (4, 5).
For example,specific alleles of b1 and FWA in
Arabidopsis are both silent when cytosine residues of the respective tandem repeats are methylated and active when hypomethylated. In both
systems, the tandem repeats required for silencing are transcribed and produce small RNAs regardless of whether the alleles are active or silent.
The methylated, silenced FWA allele can initiate
trans methylation of anunmethylated transgene,
yet, unlike the maize paramutation system, the
unmethylated allele segregates normally and is
active and unchanged (12). It is unclear whether
the “natural” active FWA allele is protected from
silencing, or the transgene is hypersensitive to
silencing, or both (12). Additionally, the mechanism that makes B-I in maize highly sensitized
to silencing is also unknown,although several
hypotheses have been proposed (13).
The relationship with other RNA silencing
pathways suggests that paramutation, despite
being rare, may underlie fundamental mechanisms
for gene regulation (2). Speculations on potential
roles and consequences include that paramutation provides an adaptive mechanism through the
transfer of favorable expression states to progeny,
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