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FIGURE 4.1 (A) NAD þ substrate of PARP-1, illustrating the cleavage point and attachment points for the formation of linear or branched
polymer chains. ( Please refer to color plate section ) (B) Cartoon of PARP-1 binding to single strand break in DNA, leading to enzyme activation,
cleavage of NAD þ with the formation of negatively charged polymer and subsequent recruitment of other elements of single strand break repair.
(Please refer to color plate section).
in the vicinity of the break serves to loosen chromatin,
which facilitates recruitment and access of SSBR/BER
enzymes 21,22 and promote PARP-1 dissociation from
the break. PARP-1 recruits the proteins necessary for
both long-patch and short-patch DNA single-strand
break repair. Short-patch repair occurs after excision of
a damaged base that, following AP endonuclease attack
at the site of base loss, results in an indirect break and
involves the PAR-mediated recruitment of the scaffold
protein XRCC1 followed by DNA pol b and ligase III.
Long-patch repair occurs after direct DNA breaks (for
example, after ionizing radiation damage), involves
recruitment of XRCC1, which then recruits polynucleo-
tide kinase (PNK) to convert the damaged ends to 5'-
phosphate and 3'-hydroxyl moieties. Proliferating cell
nuclear antigen (PCNA) and DNA polymerase d / 3
extend and fill the gap by 2
endonuclease 1 (FEN1) cleaves the resulting flap. The
nick is subsequently ligated by DNA ligase I reviewed
by Hoeijmakers et al . 23 PARP-1 activation is a necessary
prerequisite for the recruitment of XRCC1 to the break as
PAR foci form before XRCC1 repair foci and inhibition of
PARP-1 activity inhibits the formation of XRCCI foci. 24
Automodification of PARP-1 inactivates its catalytic
activity and causes it to dissociate from the DNA. The
(ADP-ribose) polymers are degraded by poly(ADP-
ribose) glycohydrolase (PARG), 25 restoring the catalytic
activity of a now unmodified PARP-1 enzyme and
allowing re-association of the histones with DNA. The
whole process from PARP initially binding to the
DNA, recruitment of repair proteins and polymer degra-
dation is measured in a few minutes. This rapid NAD þ
consumption, synthesis and turnover of the polymer
imposes a high energy cost to the cell, underlying the
15 nucleotides, and flap
e
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