The Scientist - 03.2020

BINDING

SITES FOR

CTCF

(CCCTC-

binding factor)

are enriched in

escapees as well as at

the boundaries between

silenced and active regions.

The mechanism by which this

transcription factor and manager

of chromosome looping supports

escape is unclear.

BINDING SITES FOR YY1

are common at escapee

promoters. YY1 is a

transcription factor that

may work in part by

infl uencing histone

acetylation, which

infl uences gene

expression.

PLOTTING AN ESCAPE

The silencing of the one X chromosome in XX cells is mediated by XIST, a long noncoding RNA that is randomly
transcribed from only one X early in development. It coats the DNA and shuts down gene expression on that X. For genes
to escape, researchers hypothesize, certain sequences on the X, so-called escape elements, attract proteins that help
nearby genes evade silencing. In addition, sequences known as boundary elements and their associated proteins seem to
act as divisions between active and quiet regions. The identities of these regulators haven’t been conclusively pinned down,
but there are several suspects.

XIST RNA

XIST

gene

Escaped

gene’s

transcripts

YY1

CTCF

Escaped

gene

Xist RNA

Histones

Escapee

TA D

Silenced

TA D

Gene

promoter

TOPOLOGICALLY ASSOCIATED DOMAINS (TADS)distinguish

segments of the genome and typically contain genes that physically

interact. Researchers haven’t yet identifi ed the nucleic acid

sequences and proteins that set TA D boundaries, but the domains

may help delineate silenced and escaping regions on the X.

© N.R.FULLER, SAYO-ART, LLC