Biology Reference
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was mostly present as a monomer, and the
UvrA
homodimer was able to bind
two-independent UvrB molecules. Data presented by Pakotiprapha
et al.
,
47
however, favored the heterotrimer. Considering this, one might imagine that
the
UvrA
dimer binds an
Mfd
molecule at a lesion and then attracts a UvrB
molecule, which causes
Mfd
to dissociate; however, it seems more likely that
Mfd
recruits the
UvrA
dimer and transfers it to DNA, then dissociates before
UvrB binds to
UvrA
. This idea is based on the observation that
Mfd
can
displace UvrB from a UvrAB* complex
in vitro
.
12
The D7 is not required for TCR, but its autoregulatory function may
prevent unwanted interactions with
UvrA
that could reduce GGR capacity,
especially if there are more
Mfd
than
UvrA
molecules in a cell.
39
Mfd
may represent a special case of proteins that facilitate recognition of
DNA lesions by UvrAB*. In
in vitro
reactions, photolyase from
E. coli
, but not
from yeast, increases the rate of incision of DNA containing CPDs,
48,49
and it
enhances the survival of UV-irradiated
uvrA
mutants in which the insertion
domain of
UvrA
has been deleted.
50
Similarly, the YbaZ protein, an
alkyltransferase-like protein that binds to O(6)-alkylguanine lesions but does
not repair them, facilitates NER of O(6)-ethylguanine and O(6)-propylguanine
by interacting with
UvrA
.
51
In contrast to photolyase and YbaZ,
Mfd
recognizes
RNA polymerase blocked at a DNA lesion rather than a specific type of
damage, and it can therefore recruit
UvrA
to a variety of lesions. Unlike
Mfd
,
neither photolyase nor YbaZ is known to have significant UvrB homology, and
the basis for their interaction with
UvrA
is not known.
V. The Role of UvrA in TCR
The
E. coli UvrA
is a 103-kD protein in which 11 domains have been
identified
52
(
Fig. 3
). This protein, which is required for both TCR and GGR,
may be the least prevalent of the proteins involved in NER, although estimates
vary from approximately 20 molecules per cell to approximately 200 before
SOS induction.
43,53
The active form of
UvrA
is a homodimer, and the low ratio
of
UvrA
dimers to lesions in irradiated cells suggests that
UvrA
should turn
over relatively rapidly to attain the rates of repair observed. The amount of
UvrA
may be kept at a low level under normal conditions to minimize ''gratu-
itous'' NER in undamaged DNA.
Certain mutations in
uvrA
caused reduced levels of GGR relative to TCR.
This result was interpreted to mean that these mutants are deficient in recog-
nizing DNA damage and initiating GGR but are able to catalyze TCR because
RNA polymerase and
Mfd
contribute to damage recognition.
39
None of the
TCR-proficient
uvrA
mutants tested was completely deficient in GGR, how-
ever, and none of the GGR-proficient
uvrA
mutants lacked TCR.
39
In contrast,