Biology Reference
In-Depth Information
rely almost exclusively on canonical Watson
Crick base
pairing rules to achieve efficient and faithful DNA poly-
merization. Many of these classical polymerases can
achieve synthetic rates as high as 1000 nucleotide incor-
poration events per second while displaying error rates
as low as one error per 1,000,000 incorporations.
33,34
In terms of biological function, pol
a
is first on the list
as this polymerase functions as a primase to synthesize
a short RNA primer
35
that is necessary for the initiation
of leading and lagging strand DNA synthesis catalyzed
by the highly processive DNA polymerases, pol
d
and
pol
3
.
36
e
38
Telomerase, the sole eukaryotic polymerase
that possesses reverse transcriptase activity, is respon-
sible for replicating telomeric regions of the chromo-
some.
39
This polymerase is also noteworthy for its role
in cancer initiation and progression.
40,41
Finally, Pol
g
is involved in the replication and repair of the mitochon-
drial genome.
42
Pol
g
is similar to pol
d
and pol
3
in that
all three possess a rigorous 3'
participate in certain DNA repair pathways, especially
when large stretches of DNA need to be re-synthesized
as a result of excision or recombination.
50
The final group of DNA polymerases are viewed as
“specialized” polymerases due to their ability to effi-
ciently replicate various DNA lesions. These polymer-
ases are also referred to as “non-classical” as they
generally do not obey Watson
e
Crick base pairing rules
since they must replicate DNA lesions that either lack or
have altered hydrogen bonding coding information.
These polymerases are arguably the most diverse family
due to their activity and efficiency for replicating
various DNA lesions. Members of this family include
pol
h
, pol
i
, pol
k
, pol
q
, pol
4
, pol
s
, pol
z
, and Rev1.
As expected by their role in replicating DNA lesions,
these specialized polymerases also lack 5'
e
3' exonu-
/
clease
activity
that
could provide proofreading
capabilities.
Of these non-classical DNA polymerases, the biolog-
ical function of pol
h
is the most clear as it is primarily
responsible for the proper replication of naturally occur-
ring crosslinked lesions such as thymine dimers.
51
e
52
In
this case, the activity of pol
h
is essential toward main-
taining genomic fidelity as this polymerase efficiently
incorporates dATP opposite both the 5'- and 3'-thymine
of the crosslinked lesion.
52
In fact, a lack of pol
h
in
humans causes a disease known as the variant form of
xeroderma pigmentosum, a cancer-prone genetic
disorder
53,54
that arises from a hypersensitivity of indi-
viduals to sunlight. In addition to replicating thymine
dimers, pol
h
also incorporates nucleotides opposite
DNA lesions caused by chemotherapeutic agents, the
most notable of which is cisplatin that creates inter-
and intra-strand crosslinks in DNA.
55
As expected, cells
deficient in pol
h
are hypersensitive to cisplatin and
other DNA crosslinking agents,
56
and this feature is dis-
cussed again below.
Compared to pol
h
, the cellular roles of other special-
ized DNA polymerases are less well understood. Much
of this deficiency in knowledge is caused by the diver-
sity in the number and types of lesions that act as effec-
tive substrates for these polymerases. Pol
i
is an
interesting DNA polymerase that can replicate a wide
variety of DNA lesions including minor groove
adducted purines
57,58
and bulky major groove-adducted
purines.
59
Rev1 is yet another unique polymerase that
displays dual cellular functions. As a polymerase,
Rev1 catalyzes the incorporation of C opposite non-
instructional DNA lesions such as abasic sites and
a variety of DNA lesions that occur at G residues.
60,61
In addition, Rev1 reportedly acts as a scaffold to recruit
other non-classical polymerases
5' exonuclease proof-
reading activity that plays an important role in main-
taining fidelity during replication. Pol
a
and
telomerase do not possess exonuclease activity and are
thus more prone to lapses in replicative fidelity. Defects
in the fidelity of pol
a
do not pose significant complica-
tions for the cell since the RNA primer that is synthe-
sized is degraded after completion of DNA synthesis.
The cellular effect of unfaithful DNA synthesis by telo-
merase is not completely understood at this time.
In addition to replicative polymerases, there are
a large number of DNA polymerases that are essential
for efficient DNA repair in humans. These polymerases
are also considered to be “classical” as they rely on
canonical Watson
/
Crick base pairing information for
efficient synthesis. However, in contrast to polymerases
involved in nuclear or mitochondrial replication, these
polymerases do not possess a 5'
e
3' exonuclease
activity that could provide proofreading capabilities.
Pol
b
is arguably the most important DNA polymerase
that functions in DNA repair as it is the primary enzyme
involved in base excision repair and gap-filling
synthesis during nucleotide excision repair.
43
Pol
l
and pol
m
are two DNA polymerases involved in non-
homologous end joining, a mechanism for rejoining
double-strand DNA breaks.
44
Finally, lymphoid tissue
express a unique DNA polymerase, denoted as terminal
deoxynucleotidyl transferase (TdT), that adds random
deoxynucleotides to double-strand DNA breaks formed
during V(D)J recombination.
45
The function of this poly-
merase is to promote immunological diversity during an
immune response.
46
This polymerase is also noteworthy
as it is commonly overexpressed in acute lymphoblastic
leukemia,
47,48
and this increase often correlates with
poor response to many chemotherapeutic agents that
induce DNA damage.
49
Although pol
d
and pol
3
play
essential roles in nuclear DNA synthesis, they can also
/
to sites of DNA
damage.
62,63
Pol
k
is another DNA polymerase that has dual cellu-
lar functions. This polymerase incorporates nucleotides
