Biology Reference
In-Depth Information
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,
Search WWH ::




Custom Search