Chemistry Reference
In-Depth Information
The HRR process (Figure 6.5) requires the assembly of multienzymatic com-
plexes.
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These complexes include the Rad51 family members such as Rad51, Rad54,
Rad51B, Rad51C, Rad51D, Xrcc2 and Xrcc3.
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To initiate the repair of double-
strand breaks in DNA, RAD51 needs to bind to the single-stranded DNA that is
produced by nucleolytic resection at the break site. The protein shows a weak spe-
cifi city for binding single-stranded DNA compared with double-stranded DNA, but
this specifi city is enhanced by interactions with another key recombination protein,
which is known as RAD52. Invasion of a resected end of the double-strand break
into duplex DNA takes place in the RAD51 fi lament and requires the binding of a
high energy nucleotide cofactor such as ATP.
6.2.4 DNA Damage Bypass
Even when DNA repair and cell cycle checkpoint control are fully functional, some
DNA lesions often persist through replication of the genome. Factors that contrib-
ute to the persistence of DNA damage include: (i) high levels of damage; (ii) poorly
repaired lesions; (iii) ineffi ciently repaired genomic regions and (iv) DNA damage
incurred during the S phase of the cell cycle. Since many lesions that persist despite
DNA repair and cell cycle checkpoints hamper or counteract the replication appa-
ratus, cells have evolved a damage tolerance system to allow complete replication
in the presence of DNA damage. There are two reasons why it is important for the
cell to be able to move replication forks past unrepaired damage. First, long-term
blockage of replication forks leads to cell death. Second, replication of damaged
DNA provides a sister chromatid that can be used as template for subsequent repair
by homologous recombination.
DNA damage bypass or translesion synthesis (TLS) is a DNA damage toler-
ance process that allows the DNA replication machinery to replicate past DNA
lesions (Figure 6.6). It involves exchanging regular DNA polymerases by specialized
translesion polymerases (Table 6.2), often with larger active sites that can facilitate
the insertion of bases opposite damaged nucleotides. The polymerase switching is
thought to be mediated by, among other factors, the post-translational modifi cation
of the replication processivity factor - PCNA. Many of the TLS enzymes (Table 6.2)
belong to the recently described 'Y-family' of DNA polymerases. By possessing a
spacious preformed active site, these enzymes can physically accommodate a variety
of DNA lesions and facilitate their bypass. Flexible DNA-binding domains and a
variable binding pocket for the replicating base pair further allow these TLS
polymerases to select specifi c lesions to bypass and favour distinct nonWatson-Crick
base pairs. Consequently, TLS polymerases tend to exhibit much lower fi delity
(higher propensity to insert wrong bases) than the cell's regular polymerases when
copying normal DNA, which results in a dramatic increase in mutagenesis. Occa-
sionally this can be benefi cial, but it often speeds the onset of cancer in humans.
However, many TLS polymerases (Table 6.2) are extremely effi cient at inserting
correct bases opposite specifi c types of damage. For example, polymerase h medi-
ates error-free bypass of lesions induced by UV irradiation, whereas polymerase z
introduces mutations at these sites. From a cellular perspective, risking the introduc-
tion of point mutations during TLS may be preferable to resorting to more drastic
