Biology Reference
In-Depth Information
''coupling'' factors CSA and CSB.
56
The subsequent steps in GG-NER and TC-
NER are similar to each other and to those in NER in prokaryotes. XPB and
XPD, which are subunits of transcription factor TFIIH, have helicase activity
and unwind the DNA at the sites of damage. XPG protein has a structure-specific
endonuclease activity, which makes an incision 3
0
to the damaged DNA. Subse-
quently, the XPF-ERCC1 complex makes the 5
0
incision during the NER. The
dual incision leads to the removal of an ssDNA with a single-strand gap of 25-30
nucleotides. The resulting gap in DNA is filled by DNA ploymerase
d
or
e
by
copying the undamaged strand. Proliferating cell nuclear antigen (PCNA) assists
the DNA polymerase in the reaction, and replication protein A (RPA) protects
the other DNA strand from degradation during NER. Finally, DNA ligase seals
the nicks to finish NER.
Histone acetylation is an important event in NER. Several investigators
have shown that treatment of human cells with sodium butyrate (an inhibitor of
HDACs), stimulates the initial rate of NER
in vivo
and appears to correlate
with an increase in the highest acetylated form of histone H4.
57-59
These
authors have also shown that DNA repair synthesis occurring early after UV
irradiation in mammalian cells is significantly enhanced in hyperacetylated
mononucleosomes and seems not to result from increased UV damage in
hyperacetylated chromatin.
57-59
A study has shown that the TATA-box-binding protein-free TAF-containing
complex (TFTC), a Gcn5-containing HAT complex and a transcriptional
coactivator, is involved in the NER pathway.
60
TFTC has also been previously
reported to acetylate histone H3 both
in vitro
and
in vivo.
61
These authors
demonstrated that TFTC shares strong homology with a subunit of
UV-damaged DNA-binding factor DDB1, which is recruited to UV-induced
DNA lesions
in vivo.
60
TFTC binds preferentially to naked UV-damaged
DNA and to nucleosomes assembled on UV-damaged DNA
in vitro
. Moreover,
TFTC preferentially acetylates nucleosomes assembled on UV-damaged
DNA templates
in vitro.
60
A more recent study also demonstrated that UV
irradiation leads to a dramatic increase in H3 acetylation
in vivo.
62
As
TFTC acts as coactivator in transcription, it might be associated mainly with
TC-NER.
There have also been studies showing that other transcription factors take
part in the acetylation process of histones as well, thereby leading to NER. It
was recently discovered that the E2F1 transcription factor accumulates at sites
of UV-induced DNA damage and directly stimulates NER through a nontran-
scriptional mechanism by associating with the GCN5 acetyltransferase in
response to UV radiation and thus recruiting GCN5 to sites of damage. UV
radiation induces the acetylation of histone H3 lysine 9 (H3K9) and this
requires both GCN5 and E2F1.
63
Moreover, as previously observed for
E2F1, knockdown of GCN5 results in impaired recruitment of NER factors
