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mutants and to ascertain whether the mutations have differential effects on the
repair of the transcribed and nontranscribed strands of an active gene. A clearer
understanding of the interactions of Mfd and NusA, both of which can associate
with the
subunit of RNA polymerase, would also be helpful.
Several other proteins that associate with RNA polymerase (e.g., NusG)
have yet to be assessed for their relationship to DNA repair.
b
B. Base Excision Repair
While NER primarily removes ''bulky'' or distorting DNA lesions, base
excision repair (BER) usually removes oxidative base damage and abnormal
bases. Although the evidence for TCR of ''bulky'' lesions that arrest RNA
polymerase, including CPDs, cisplatin, cholesterol, and menthol, is convincing,
the results of studies of oxidative lesions have been more controversial, and the
reports of TCR of thymine glycols in human cells based on an antibody assay
have been retracted. 62-65 In contrast, recent analyses of the repair of 5-hy-
droxy-uracil in human cells and cell extracts have shown that the repair of this
lesion is coupled to transcription. 66
Studies of transcriptional mutagenesis (TM) in nondividing E. coli have
implicated NER and TCR in the repair of uracil and 8-oxoguanine at a specific
site in the transcribed strand of a luciferase gene. 67,68 The evidence is based on
the effects of uvrA and mfd mutations on the frequency of repair compared to
TM, the measured end point being luciferase activity resulting from TM.
Although neither uracil nor 8-oxoguanine appears to be a significant block to
the E. coli RNA polymerase in vitro , 69 Mfd appears to contribute to the repair
of these lesions. The experimental system does not allow measurements of
repair of the nontranscribed strand of the gene, however, so the relative repair
rates of the transcribed and the nontranscribed strands under these conditions
is not known. Because the mfd mutation was not completely epistatic to the
uvrA mutation, it was proposed that BER components are involved in some
of the Mfd -dependent repair. Further studies are needed to identify these
components and to validate the model proposed. 68
VIII. Conclusions
The study of E. coli continues to provide important mechanistic insights
into DNA damage-processing pathways such as TCR, but with new questions
being generated as previous ones are answered. For example, direct measure-
ments of lesion removal have shown that Mfd is absolutely required for TCR as
it was originally described, but inactivating Mfd has little effect on the survival
of E. coli exposed to DNA-damaging agents such as UV. In contrast, defects in
the CSB protein, thought to be the functional homolog of Mfd in mammals
including humans, can result in serious developmental and neurological
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