M. Kanabus et al., THE ANTIMUTAGENIC EFFECT OF A TRUNCATED EPSILON-SUBUNIT OF DNA-POLYMERASE-III IN ESCHERICHIA-COLI-CELLS IRRADIATED WITH UV-LIGHT, MGG. Molecular & general genetics, 247(2), 1995, pp. 216-221
It has previously been suggested that inhibition of the proofreading 3
'-5' exonuclease activity of DNA polymerase may play an important role
in generation of UV-induced mutations in Escherichia coli. Our previo
us work showing that overproduction of epsilon, the proofreading subun
it of DNA polymerase III, counteracts the SOS mutagenic response of E.
coli seemed to be consistent with this hypothesis. To explore further
the nature of the antimutagenic effect of epsilon we constructed plas
mid pMK17, which encodes only two of the three highly conserved segmen
ts of epsilon - ExoI and ExoII; the third segment, ExoIII, which is es
sential for 3'-5' exonuclease activity, is deleted. We show that at 40
degrees C, overproduction of the truncated epsilon subunit significan
tly delays production of M13 phage, suggesting that the protein retain
s its capacity to bind to DNA. On the other hand, the presence of pMK1
7 in a trpE65 strain growing at 40 degrees C causes a 10-fold decrease
in the frequency of UV-induced Trp(+) mutations. This antimutagenic e
ffect of the truncated epsilon is effectively relieved by excess UmuD,
C proteins. We also show that the presence of plasmid pIP21, which con
tains the dnaQ49 allele encoding an epsilon subunit that is defective
in proofreading activity, almost completely prevents generation of UV-
induced mutations in the trpE65 strain. We propose that the DNA bindin
g ability of free epsilon, rather than its 3'-5' exonuclease activity,
affects processing of premutagenic UV-induced lesions, possibly by in
terfering with the interaction between the UmuC-UmuD'-RecA complex and
pol III holoenzyme. This interaction is probably a necessary conditio
n for translesion synthesis.