ISSN 1022-7954, Russian Journal of Genetics, 2017, Vol. 53, No. 2, pp. 157–177. © Pleiades Publishing, Inc., 2017.
Original Russian Text © D.A. Chetverina, P.V. Elizar’ev, D.V. Lomaev, P.G. Georgiev, M.M. Erokhin, 2017, published in Genetika, 2017, Vol. 53, No. 2, pp. 133–154.
Control of the Gene Activity by Polycomb
and Trithorax Group Proteins in Drosophila
D. A. Chetverina*, P. V. Elizar’ev, D. V. Lomaev,
P. G. Georgiev, and M. M. Erokhin**
Institute of Gene Biology, Russian Academy of Sciences, Moscow, 119334 Russia
Received February 12, 2016; in final form, March 30, 2016
Abstract⎯Combinatorial expression of the genes in multicellular organisms leads to the development of dif-
ferent cell types. The important epigenetic regulators of higher eukaryotes are the Polycomb group (PcG) and
Trithorax group (TrxG) proteins. These factors control the transcription of a large number of genes involved
in various cellular processes. Dysregulation of PcG and TrxG systems leads to developmental abnormalities
and cancer. This review focuses on the main characteristics and properties of the Drosophila PRE elements.
Furthermore, we summarize the information on the protein components of the PcG and TrxG groups and
their functional activities and discuss the main aspects of competition between the proteins of these classes as
well as their possible mechanisms of action.
Keywords: Polycomb, Trithorax, epigenetic memory, transcription regulation, chromatin, Drosophila
Proper development of multicellular organisms
requires the individual gene expression patterns to be
established in all cell types and to be stably transmitted
through multiple cell divisions. Epigenetic control of
gene expression in multicellular organisms is carried
out by Polycomb group (PcG) and Trithorax group
(TrxG) proteins [1–5]. Dysregulation of PcG and
TrxG systems leads to developmental abnormalities
and cancer [6–8].
PcG and TrxG proteins were initially identified in
Drosophila as regulators of the HOX gene expression
[9–12]. HOX genes encode transcription factors that
control proper body segmentation. Subsequent studies
showed that PcG and TrxG proteins target many genes
involved in various cellular processes [13–18].
PcG and TrxG proteins act antagonistically: PcG
proteins repress, while TrxG proteins activate gene
transcription [2–5, 19].
It was demonstrated that, in Drosophila, PcG and
TrxG proteins communicate with specialized DNA
elements termed PREs (Polycomb Response Ele-
ments) [20–22]. PREs were shown to be involved in
maintaining proper spatial and temporal gene expres-
sion patterns during development. Moreover, for some
PREs, the ability to switch the activity from silencing
to activation was demonstrated in the transgenic Dro-
sophila model systems. However, at present the
molecular mechanism of the PRE activity switch is
not established .
Detailed analysis of PcG and TrxG proteins
showed that many of them function together in multi-
subunit complexes [1, 5, 23]. It was demonstrated that
the activities of PcG and TrxG proteins are opposite
suggesting the presence of direct competition between
the proteins of these two groups . However, many
aspects of the interactions between PcG and TrxG fac-
tors, as well as their functioning, remains unclear.
This review is devoted to analysis of functional
activities of proteins PcG/TrxG and their role in the
maintenance of the gene expression profile in Dro-
OF PcG AND TrxG FACTORS
PcG factors were identified by mutations that
caused characteristic phenotypes indicating derepres-
sion of HOX genes. In Drosophila, HOX genes are
organized in two complexes: Antennapedia (ANT-C)
(the labial (lab), proboscipedia (pb), Deformed (Dfd),
Sex combs reduced (Scr), Antennapedia (Antp) genes)
and Bithorax (BX-C) (the Ultrabithorax (Ubx), abdom-
inal-A (abd-A), Abdominal-B (Abd-B) genes) (Fig. 1a).
The combination of transcription factors encoded
by maternal, gap, pair-ruled, and segment polarity
groups of genes subdivides the embryo into 14
AND THEORETICAL ARTICLES