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The Plant Cell, Vol. 21: 365, February 2009, www.plantcell.org ª 2009 American Society of Plant Biologists
IN BRIEF
Functions of DNA Polymerase e
Eukaryotes contain multiple DNA polymerases
which ensure the accurate and efficient replication of the genome as well as protection and
repair from endogenous and environmental
DNA damaging agents. In most eukaryotes,
the main replicative enzymes are DNA polymerases a, d, and e (reviewed in Garcia-Diaz
and Bebenek, 2007). In yeast, DNA polymerase
a (DNA pol a) initiates synthesis on the lagging
strand. The bulk of chain elongation proceeds
via the activities of DNA pol d and e, which
appear to function principally on the lagging
and leading strands, respectively (Pursell et al.,
2007). Some functions of the DNA polymerases
likely are conserved across eukaryotes, but the
functions of the plant enzymes have not been
well characterized.
DNA polymerase e (Pol e) in yeast consists
of a single catalytic and three regulatory
The Arabidopsis Pol2a mutant allele abo4-1
increases frequency of HR. Seedlings were
transformed with a marker for HR (1415) that
produces visible spots on leaves. (Adapted from
Figure 4 of Yin et al. [2009].)
www.plantcell.org/cgi/doi/10.1105/tpc.109.210212
subunits, and this structure appears to be
conserved in other organisms. There are two
genes encoding the Pol e catalytic subunit in
Arabidopsis, Pol2a and Pol2b, and single
genes encoding each of the regulatory subunits. The catalytic subunit Pol2A and the
regulatory subunit DPB2 in Arabidopsis are
known to be essential for early embryogenesis
(Ronceret et al., 2005). Pol2a appears to encode the main catalytic subunit active during
embryogenesis and in meristems, whereas
Pol2b is expressed mainly under stress conditions and null mutations produce no noticeable phenotype. Arabidopsis mutants with
partial loss of function of Pol2a (homozygous
weak alleles) have longer cell cycles, larger
cells, and delayed development of embryos,
emphasizing the critical role of this subunit in
proper cell cycle progression and patterning
in the embryo (Jenik et al., 2005).
Now, Yin et al. (pages 386–402) have identified a nonlethal mutation of Pol2a in Arabidopsis
that causes an ABA overly sensitive phenotype,
suggesting a link between Pol e activity and ABA
signaling. In addition, the mutant showed a dramatic 60-fold increase in the frequency of
somatic homologous recombination (HR; see
figure), enhanced sensitivity to DNA-damaging
agents, activation of silenced loci, and early
flowering, indicating a potential role for Pol e in
HR and the maintenance of epigenetic states.
The authors show that the new allele of
Pol2a, called abo4-1, releases transcriptional
gene silencing at tested loci without affecting
DNA methylation. They further show that an
early flowering phenotype is associated with
reduced expression of FLC and increased
expression of FT concurrent with altered
histone H3 modifications at these loci. The
results suggest a role for Pol e in ABAdependent effects on genome stability,
epigenetic states of chromatin, and chromatin-mediated gene expression in plants.
The link to HR is intriguing, although further
studies are needed to show if Pol2a plays
a direct or indirect role. It was shown that
mutations in Pol2a lead to lengthening of the
S-phase of DNA replication (Jenik et al., 2005;
Yin et al., 2009), and Yin et al. suggest that this
leads to increased formation of double-strand
breaks, activation of cell cycle checkpoints,
and the induction of HR. This work significantly extends our knowledge of DNA Pol e
activity in plants and suggests exciting new
avenues for future research.
Nancy A. Eckardt
Senior Features Editor
[email protected]
REFERENCES
Garcia-Diaz, M., and Bebenek, K. (2007).
Multiple functions of DNA polymerases. Crit.
Rev. Plant Sci. 26: 105–122.
Jenik, P.D., Jurkuta, R.E., and Barton, M.K.
(2005). Interactions between the cell cycle and
embryonic patterning in Arabidopsis uncovered by a mutation in DNA polymerase
epsilon. Plant Cell 17: 3362–3377.
Pursell, Z.F., Isoz, I., Lundstrom, E.B.,
Johansson, E., and Kunkel, T.A. (2007).
Yeast DNA polymerase epsilon participates in
leading-strand DNA replication. Science 317:
127–130.
Ronceret, A., Guilleminot, J., Lincker, F.,
Gadea-Vacas, J., Delorme, V., Bechtold,
N., Pelletier, G., Delseny, M., Chaboute,
M.E., and Devic, M. (2005). Genetic analysis
of two Arabidopsis DNA polymerase epsilon
subunits during early embryogenesis. Plant J.
44: 223–236.
Yin, H., Zhang, X., Liu, J., Wang, Y., He, J.,
Yang, T., Hong, X., Yang, Q., and Gong, Z.
(2009). Epigenetic regulation, somatic homologous recombination, and abscisic acid signaling are influenced by DNA polymerase e
mutation in Arabidopsis. Plant Cell 21: 386–402.
Functions of DNA Polymerase ε
Nancy A. Eckardt
Plant Cell 2009;21;365; originally published online February 24, 2009;
DOI 10.1105/tpc.109.210212
This information is current as of June 18, 2017
References
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