Institute of Biochemistry and Biophysics,
Polish Academy of Sciences, 02-106 Warsaw,
Poland,1 and
Laboratory of Molecular
Genetics, National Institute of Environmental Health Sciences,
Research Triangle Park, North Carolina 277092
The mechanisms that control the fidelity of DNA replication are
being investigated by a number of approaches, including detailed kinetic and structural studies. Important tools in these studies are
mutant versions of DNA polymerases that affect the fidelity of DNA
replication. It has been suggested that proper interactions within the
core of DNA polymerase III (Pol III) of Escherichia coli
could be essential for maintaining the optimal fidelity of DNA
replication (H. Maki and A. Kornberg, Proc. Natl. Acad. Sci. USA
84:4389-4392, 1987). We have been particularly interested in elucidating the physiological role of the interactions between the
DnaE (
subunit [possessing DNA polymerase activity]) and DnaQ (
subunit [possessing 3'
5' exonucleolytic proofreading activity])
proteins. In an attempt to achieve this goal, we have used the
Saccharomyces cerevisiae two-hybrid system to analyze specific in vivo protein interactions. In this report, we demonstrate interactions between the DnaE and DnaQ proteins and between the DnaQ
and HolE (
subunit) proteins. We also tested the interactions of the
wild-type DnaE and HolE proteins with three well-known mutant forms of
DnaQ (MutD5, DnaQ926, and DnaQ49), each of which leads to a strong
mutator phenotype. Our results show that the mutD5 and
dnaQ926 mutations do not affect the subunit- subunit and subunit- subunit interactions. However, the
dnaQ49 mutation greatly reduces the strength of interaction
of the subunit with both the and the subunits. Thus, the
mutator phenotype of dnaQ49 may be the result of an altered
conformation of the protein, which leads to altered interactions
within the Pol III core.
 |
INTRODUCTION |
Replication of the Escherichia
coli chromosome is performed by the DNA polymerase III holoenzyme
(Pol III HE) (15, 16, 21). Pol III HE is an asymmetric,
dimeric complex containing a total of 18 subunits (10 distinct), which
are capable of coordinately synthesizing leading and lagging strands.
The complex contains two polymerase cores, one for each strand,
composed of , , and subunits (22).
With regard to the fidelity of the replication process, the and subunits of the Pol III core are of particular importance. The subunit (dnaE gene product) is the DNA polymerase, which selects the correct nucleotides during template-directed DNA synthesis (18). The subunit (dnaQ gene product)
performs the 3'5' exonucleolytic proofreading activity, which
preferentially removes incorrect bases inserted by the polymerase
(27). The function of the third subunit, the subunit,
has yet to be identified. The subunit of the Pol III HE plays a
complex role in DNA replication. Besides its proofreading activity, it
stabilizes the core by tightly binding to both the and subunits
(22, 29). Interestingly, the and subunits are each
less active individually than when bound together in the Pol III core
(19). One may hypothesize that the fidelity of DNA
replication depends not only on the intrinsic accuracy of the
polymerase and the strength of the 3'5' exonuclease activity but
also on the appropriate interactions between the subunits within the
Pol III core. Thus, the decreased fidelity of DNA replication observed
in E. coli strains carrying mutations within the
dnaQ gene could be due either to the defective catalytic properties of the subunit or to aberrant subunit interactions within the Pol III core.
Several mutators which carry mutations in the dnaQ gene have
been isolated, e.g., dnaQ49, mutD5, and
dnaQ926 strains. Two of these mutations, mutD5
and dnaQ49, have been extensively studied. mutD5
is a particularly strong mutator allele, leading to mutation rates of
up to 105-fold above the wild-type level (3, 4).
This is due not only to the proofreading defect but also to the
concomitant impairment (by saturation) of postreplicative mismatch
repair (25, 26). dnaQ49 strains differ from
mutD5 strains in several respects. First, dnaQ49
is a temperature-sensitive mutator, possessing modest mutator activity
below 30°C but strongly enhanced activity at 37°C (7, 10,
11). Second, dnaQ49 strains are unable to grow at
44.5°C in salt-free rich medium because of the inhibition of DNA
synthesis (10). Third, the dnaQ49 allele is
recessive with respect to the wild-type gene, while mutD5 is
dominant (3, 20). The dnaQ49 and mutD5
alleles result from different missense mutations within the
dnaQ gene (9, 30; see also Table 1). On
the basis of genetic data, a model in which the DnaQ49 protein has a
reduced ability to bind to the subunit has been proposed (30). The third allele, dnaQ926, is the strongest
known mutator of E. coli (9). It was constructed
by site-specific mutagenesis by changing the codons for two conserved
amino acid residues in the ExoI motif of the subunit (Table
1) known to be essential for the
catalytic activity of other polymerase-associated proofreading exonucleases (1). When residing on a plasmid,
dnaQ926 confers a strong, dominant mutator phenotype,
suggesting that the protein, although deficient in exonuclease
activity, may still efficiently bind to the subunit. When
dnaQ926 was transferred to the chromosome, replacing the
wild-type gene, the cells were essentially inviable. dnaQ926
strains survived well, however, when carrying a dnaE
antimutator mutation (6, 8) or a multicopy plasmid
containing the E. coli mutL+ gene. Thus, the
poor viability of dnaQ926 strains was proposed to result
from excessively high mutation rates due, as in the case of
mutD5 strains, both to the proofreading defect and to the
collapse of the mismatch repair system (error catastrophe).
A mechanism coordinating DNA polymerization and DNA excision, relying
on structural and functional communication between the different
subunits of Pol III HE, may play an important role in maintaining
optimal fidelity of DNA replication. We have been particularly
interested in elucidating the physiological role of interactions
between the and subunits. In an attempt to achieve this goal,
we used the Saccharomyces cerevisiae two-hybrid system to
investigate the in vivo interactions between mutant and wild-type DnaQ
protein with the wild-type DnaE and HolE proteins.
| |
MATERIALS AND METHODS |
Bacterial and yeast strains and media.
E. coli DH5
[supE44 
lacU169 (
80lacZM15)
hsdR17 recA1 endA1 gyrA96 thi-1 relA1] was the
transformation recipient strain for all plasmid constructions. All
two-hybrid system experiments were done with S. cerevisiae
Y187 (MAT gal4 gal80 his3 trp1-901 ade2-101 ura3-52
leu2-3,112 met URA3::GAL1-lacZ). Strain Y187 was kindly
provided by S. Elledge (Baylor College of Medicine, Houston, Tex.).
Yeast extract-peptone-dextrose medium and synthetic medium (SMM) were
prepared as described previously (23). For drug selection,
Luria broth plates were supplemented with ampicillin (100 µg/ml).
Methods.
Manipulations and sequencing of DNA were carried
out by standard procedures (24). The S. cerevisiae Y187 strain was transformed simultaneously with a
pGBT9-derived plasmid (e.g., pGBT9dnaE) and a
pGAD424-derived plasmid (e.g., pGAD424-2dnaQ) by the method of Chen et al. (2).
-Galactosidase assay.
For quantitative studies, yeast
strains were grown at 25 or 28°C to stationary phase in synthetic
medium (SMM plus 3% glucose) lacking leucine and tryptophan, diluted
10 times in SMM plus 2% ethanol (lacking leucine and tryptophan), and
then incubated at 25 or 28°C for 48 h. The -galactosidase
activity was determined as described previously (23).
Construction of GAL4 protein fusion plasmids.
Plasmids for
the GAL4 two-hybrid fusion assay were prepared by cloning PCR-amplified
fragments into pGBT9 (Clontech) or pGBT9-2 (13) (both
containing amino acids 1 to 147 of the DNA-binding domain of GAL4) and
pGAD424 (Clontech) or pGAD424-2 (13) (both containing amino
acids 768 to 881 from the trans-activation domain of GAL4).
The dnaQ coding region was obtained from plasmid pIP1 (12), the dnaE coding region was obtained from
plasmid pMWE103 (kindly provided by C. McHenry [University of Colorado
Health Sciences Center, Denver]), and the holE coding
region was obtained from the chromosome of E. coli DH5.
In all cases these were obtained via PCR amplification with the
following forward and reverse primers, respectively: dnaQ,
5'-ATGAGCACTGCAATTACACGC-3' and
5'-TTTTTAGCGCCTTCACAGG-3'; dnaE,
5'-ATGTCTGAACCACGTTTCGTA-3' and
5'-AATCAAGGAAATTCAGACTCA-3'; and holE,
5'-ATGCTGAAGAATCTGGCTA-3' and
5'-CAGGCGTTATGTAAGAAAG-3'. The PCR conditions and cloning of
PCR amplification products were as described previously
(13). To confirm the presence of the in-frame junction of
the genes with the respective GAL4 domains, recombinant plasmids
pGBT9-2dnaQ+,
pGBT9-2holE+, and
pGBT9dnaE+ were sequenced with the primer GAL4bd
(5'-GAAGAGAGTAGTAACAAAGG-3'). The entire DNA sequences of
the PCR-derived holE and dnaQ inserts were
verified by dideoxy sequencing. The dnaE sequence was
verified by sequencing pGBT9dnaE+ from the ATG
codon (position 796) to the HpaI site (position 1035) and
from the SmaI site (position 4022) to the 3' end of the gene
(numbering system as in reference 31). Then the
region in pGBT9dnaE+ between the HpaI
site (position 1035) and the SmaI site (position 4022) was
removed and replaced by the HpaI-SmaI fragment
(2,987 bp) from pMWE103 carrying the wild-type dnaE gene.
Plasmids pGBT9-2dnaQ49, pGBT9-2mutD5, and
pGBT9-2dnaQ926 were constructed as follows.
pGBT9-2dnaQ+ has unique BamHI and
MluI sites (at positions 839 and 375 of the dnaQ
gene, respectively) (numbering according to the method of Maki et al.
[17]). The 464-bp BamHI-MluI
fragment was cut out and replaced by the corresponding fragment of
plasmid pIP21, which carries the dnaQ49 gene
(14). The presence of the dnaQ49 mutation in the
resulting plasmid, pGBT9-2dnaQ49, was confirmed by DNA
sequencing. As sources of mutD5 and dnaQ926 DNA
for PCR amplification we used plasmids pIF45 and pIF44, respectively
(9). After the PCR fragments were cloned into pGBT9-2, the
presence of the mutD5 and dnaQ926 mutations in
the resulting plasmids, pGBT9-2mutD5 and
pGBT9-2dnaQ926, was confirmed by sequencing.
| |
RESULTS AND DISCUSSION |
Interactions within the Pol III core.
It has been shown that
the heterotrimer can be formed in vitro (22, 29).
Structural studies showed the subunit to bind to the subunit
and the subunit to bind to the subunit but not to the subunit, indicating a linear arrangement of the , , and subunits in the Pol III core. To evaluate the usefulness of the
two-hybrid system for studying interactions within the Pol III core, we
have examined interactions between the DnaE ( subunit), DnaQ ( subunit), and HolE ( subunit) fusion proteins. We cloned the
complete DNA coding sequences of the dnaE, dnaQ, and holE genes, each from the first ATG codon, into both
pGBT9 and pGAD424. All pairwise combinations of pGBT9,
pGBT9dnaE+, pGBT9-2dnaQ, or
pGBT9-2holE+ and pGAD424,
pGAD424dnaE+,
pGAD424-2dnaQ+, or
pGAD424-2holE+ were introduced into the yeast
reporter strain Y187. After selection, cotransformants were screened
for their ability to produce -galactosidase by filter assay (data
not shown) and by quantitative measurement of -galactosidase
activity (Table 2). The results in Table
2 indicate that the DnaE fusion protein is able to interact
specifically with the DnaQ fusion protein. Also, the DnaQ fusion
protein binds tightly to the HolE fusion protein. The HolE fusion
protein does not interact with DnaE fusion protein. The observed
efficiency of (hetero)dimer formation, measured by the activity of the
lacZ reporter gene, followed the order DnaQ-HolE 
DnaQ-DnaE. This does not necessarily reflect the relative strengths of
the subunit- subunit and subunit- subunit interactions in
E. coli cells, as the two-hybrid system results are
additionally determined by the levels of expression of various fusion
proteins and by any effects of the GAL4 fusion domains on the binding
efficiencies. None of the three tested proteins formed homodimers. We
noticed stronger interactions of the DnaE and HolE fusion proteins with the DnaQ fusion protein when dnaQ was cloned into pGBT9 than
when cloned into pGAD424. We interpret these differences to reflect the
need for appropriate tertiary structures of the respective proteins in
order to permit the specific interactions. The presence of the
additional 113 amino acids of the GAL4 trans-activation domain fused at the N terminus of DnaQ in the case of pGAD424 may
interfere with the optimal folding of the protein and, consequently, affect its interaction with the other proteins. Our results on the
specificities and strengths of the subunit- subunit and subunit- subunit interactions are in good agreement with those obtained from in vitro experiments using purified subunits
(29). The present data as well as previous data
demonstrating specific interactions between the UmuC and UmuD' or UmuD
and UmuD' proteins (13) indicate that the yeast two-hybrid
system can be used successfully for investigating protein-protein
interactions of E. coli proteins.
Interactions of the HolE and DnaE fusion proteins with mutant DnaQ
proteins.
To investigate the importance of proper DnaE-DnaQ
interactions in maintaining the high fidelity of DNA replication, we
examined the ability of the DnaE fusion protein to interact with
several DnaQ mutant proteins. E. coli strains carrying the
dnaQ49, mutD5, and dnaQ926 mutations
were originally isolated as strong mutators (3, 9, 10, 20).
Amino acid substitutions responsible for the mutator phenotype of each
mutant allele (Table 1) have been identified (9, 30). If the
proper interaction of DnaE with DnaQ has biological significance in
maintaining the high fidelity of DNA replication, one may expect that
at least some mutations in the dnaQ gene affect the ability
of DnaQ to interact with DnaE.
To test this hypothesis, we cloned dnaQ49, mutD5,
and dnaQ926 mutant DNA sequences into the pGBT9-2 plasmid
(see Materials and Methods) and assessed the interactions of the mutant
proteins with DnaE and HolE fusion proteins according to their ability to trans-activate the lacZ reporter construct.
The experiment with the dnaQ49 allele was performed at
25°C in addition to the normal temperature of 28°C, because this
strain has a temperature-dependent mutator activity (and hence,
presumably, a temperature-dependent proofreading deficiency).
Unfortunately, the yeast strain Y187 used for the two-hybrid system
experiment grew very poorly above 30°C and experiments could not be
performed at higher temperatures. Our data (Table
3) indicate that the MutD5 and DnaQ926
mutant proteins exhibit the same strengths of interaction with the DnaE and HolE fusion proteins as the wild-type DnaQ fusion protein. In
contrast, the DnaQ49 fusion protein showed a sixfold-weaker interaction
with DnaE than did wild-type DnaQ. Unexpectedly, the DnaQ49 fusion
protein also exhibited a 10-fold-weaker interaction with the HolE
fusion protein at 28°C, although little or no effect was observed at
25°C (Table 3).
View this table:
[in this window]
[in a new window]
|
TABLE 3.
Interaction of HolE and DnaE fusion proteins
with wild-type and mutant DnaQ proteins: quantitative assay
of -galactosidase activity
|
|
One objective of this work was to correlate the phenotypes of the
dnaQ mutators with the altered subunit interactions within the DNA Pol III core. Our results strongly suggest that neither mutD5 nor dnaQ926 affects the subunit-
subunit and subunit- subunit interactions. These data are
consistent with the localization of the mutations within the
dnaQ gene. Both mutations are located in the ExoI region
presumed responsible for the catalytic activity of the subunit
(9). Thus, it is reasonable to assume that the mutator
phenotype of these mutations directly reflects the decreased 3'5'
exonuclease activity (5) without affecting the subunit- subunit interaction. In contrast, our data indicate that
the dnaQ49 mutation results in decreased strength of the subunit- subunit interaction. This result is consistent with the
genetic analysis of dnaQ49 by Takano et al. (30),
who, based on the recessive nature of the dnaQ49 mutation
(in contrast to the dominant mutD5 allele), suggested that
the dnaQ49 subunit is impaired in its binding to the subunit. Such a binding defect could result from a specific affinity
loss if the responsible mutation in the subunit were to reside at
or near the site of interaction with the subunit or, alternatively,
from a global loss of protein structure. Such a global loss would also
likely result in a loss of binding to the subunit, as is indeed
observed at 28°C. However, since binding of DnaQ49 to the subunit
appears to be normal at 25°C, but its interaction with the subunit is severely impaired at this temperature, a specific local
defect in binding to the subunit may be involved, which at higher
temperatures could expand to a global effect. It should be noted that
even at 25°C the dnaQ49 mutant exhibits a significant
mutator phenotype as shown by Fijalkowska et al. (7). At
this temperature, the frequency of Rif mutations in E. coli
cells bearing dnaQ49 is about 50-fold higher than that in
wild-type cells. It is tempting to speculate that the mutator phenotype
observed in dnaQ49 strains at 25°C may reflect poor
communication between the and subunits.
Our results indicating that the DnaQ49 protein also shows decreased
strength of interaction with the HolE ( subunit) protein raise the
question of whether the temperature-dependent loss of subunit-
subunit binding is related to the temperature-dependent dnaQ49 mutator effect. The function of the subunit is
not yet clear. A strain carrying a holE null mutation is
viable and shows no detectable mutant phenotype (28).
Biochemical analysis of the subunit indicated that it has no effect
on the polymerase activity of the subunit or the subunit-
subunit complex (29). However, purified subunit was
shown to stimulate (about threefold) the 3'5' exonuclease activity
of the subunit on a substrate carrying a 3'-terminal G:T mismatch
(29). Since the subunit interacts only with the subunit, this may indicate that the subunit is involved, either
directly or indirectly, in the fidelity of DNA replication by
modulating the activity of the subunit and/or by acting as a
protein that ensures communication between the and subunits;
loss of interaction with the subunit could in principle cause a
mutator effect. However, the lack of a mutator effect with the
holE strain (28) is not consistent with such a
hypothesis. Furthermore, the observed reduction in subunit- subunit binding at 25°C, at which temperature subunit- subunit binding appears to be normal, suggests that the primary defect of
dnaQ49 lies at the level of subunit- subunit
interaction. Obviously, loss of the subunit from the replication
will be highly mutagenic.
We believe that the usefulness of the yeast two-hybrid system for
testing E. coli mutant proteins will facilitate further in
vivo studies of the subunit-subunit interactions within the Pol III HE.
We thank members of our research groups for many helpful
discussions, C. S. McHenry for providing plasmid pMWE102, and S. Elledge for providing yeast strain Y187.
This work was supported by grants from KBN (6P04 A 043 09 to P.J. and
I.J.F. and 6P04 A 015 09 to A.N. and Z.C.) and from the
Polish-U.S.A. M. Sklodowska-Curie Foundation (FMKS/KP-96-739 to
I.J.F. and R.M.S.).
| 1.
|
Blanco, L.,
A. Bernard, and M. Salas.
1992.
Evidence favouring the hypothesis of a conserved 3'-5' exonuclease active site in DNA-dependent DNA polymerases.
Gene
112:139-144[Medline].
|
| 2.
|
Chen, D.,
B. Yang, and T. Kuo.
1992.
One-step transformation of yeast in stationary phase.
Curr. Genet.
21:83-84[Medline].
|
| 3.
|
Cox, E. C., and D. L. Horner.
1982.
Dominant mutators in Escherichia coli.
Genetics
100:7-18[Abstract/Free Full Text].
|
| 4.
|
Degnen, G. E., and E. C. Cox.
1974.
Conditional mutator gene in Escherichia coli: isolation, mapping, and effector studies.
J. Bacteriol.
117:477-487[Abstract/Free Full Text].
|
| 5.
|
Echols, H.,
C. Lu, and P. M. J. Burgers.
1983.
Mutator strains of Escherichia coli, mutD and dnaQ, with defective exonucleolytic editing by DNA polymerase III holoenzyme.
Proc. Natl. Acad. Sci. USA
80:2189-2192[Abstract/Free Full Text].
|
| 6.
|
Fijalkowska, I. J.,
R. L. Dunn, and R. M. Schaaper.
1993.
Mutants of Escherichia coli with increased fidelity of DNA replication.
Genetics
134:1023-1030[Abstract].
|
| 7.
|
Fijalkowska, I. J.,
R. L. Dunn, and R. M. Schaaper.
1997.
Genetic requirements and mutational specificity of the Escherichia coli SOS mutator activity.
J. Bacteriol.
179:7435-7445[Abstract/Free Full Text].
|
| 8.
|
Fijalkowska, I. J., and R. M. Schaaper.
1995.
Effects of Escherichia coli dnaE antimutator alleles in a proofreading-deficient mutD5 strain.
J. Bacteriol.
177:5979-5986[Abstract/Free Full Text].
|
| 9.
|
Fijalkowska, I. J., and R. M. Schaaper.
1996.
Mutants in the Exo I motif of Escherichia coli dnaQ: defective proofreading and inviability due to error catastrophe.
Proc. Natl. Acad. Sci. USA
93:2856-2861[Abstract/Free Full Text].
|
| 10.
|
Horiuchi, T.,
H. Maki, and M. Sekiguchi.
1978.
A new conditional lethal mutator (dnaQ49) in Escherichia coli K12.
Mol. Gen. Genet.
163:277-283[Medline].
|
| 11.
|
Isbell, R. J., and R. G. Fowler.
1989.
Temperature-dependent mutational specificity of an Escherichia coli mutator, dnaQ49, defective in 3'-5' exonuclease (proofreading) activity.
Mutat. Res.
213:149-156[Medline].
|
| 12.
|
Jonczyk, P.,
I. J. Fijalkowska, and Z. Ciesla.
1988.
Overproduction of the subunit of DNA polymerase III counteracts the SOS mutagenic response of Escherichia coli.
Proc. Natl. Acad. Sci. USA
85:9124-9127[Abstract/Free Full Text].
|
| 13.
|
Jonczyk, P., and A. Nowicka.
1996.
Specific in vivo protein-protein interactions between Escherichia coli SOS mutagenesis proteins.
J. Bacteriol.
178:2580-2585[Abstract/Free Full Text].
|
| 14.
|
Kanabus, M.,
A. Nowicka,
E. Sledziewska-Gojska,
P. Jonczyk, and Z. Ciesla.
1995.
The antimutagenic effect of a truncated subunit of DNA polymerase III in Escherichia coli cells irradiated with UV light.
Mol. Gen. Genet.
247:216-221[Medline].
|
| 15.
|
Kelman, Z., and M. O'Donnell.
1995.
DNA polymerase III holoenzyme: structure and function of a chromosomal replicating machine.
Annu. Rev. Biochem.
64:171-200[Medline].
|
| 16.
|
Kornberg, A., and T. Baker.
1992.
.
DNA replication. W. H.
Freeman, New York, N.Y.
|
| 17.
|
Maki, H.,
T. Horiuchi, and M. Sekiguchi.
1983.
Structure and expression of the dnaQ mutator and the RNase H genes of Escherichia coli: overlap of the promoter region.
Proc. Natl. Acad. Sci. USA
80:7137-7141[Abstract/Free Full Text].
|
| 18.
|
Maki, H., and A. Kornberg.
1985.
The polymerase subunit of DNA polymerase III of Escherichia coli.
J. Biol. Chem.
260:12987-12992[Abstract/Free Full Text].
|
| 19.
|
Maki, H., and A. Kornberg.
1987.
Proofreading by DNA polymerase III of Escherichia coli depends on cooperative interaction of the polymerase and exonuclease subunits.
Proc. Natl. Acad. Sci. USA
84:4389-4392[Abstract/Free Full Text].
|
| 20.
|
Maruyama, M.,
H. Horiuchi,
H. Maki, and M. Sekiguchi.
1983.
A dominant (mutD5) and a recessive (dnaQ49) mutator of Escherichia coli.
J. Mol. Biol.
167:757-771[Medline].
|
| 21.
|
McHenry, C. S.
1988.
DNA polymerase III holoenzyme of Escherichia coli.
Annu. Rev. Biochem.
57:519-550[Medline].
|
| 22.
|
McHenry, C. S., and W. Crow.
1979.
DNA polymerase III of E. coli: purification and identification of subunits.
J. Biol. Chem.
254:1748-1753[Abstract/Free Full Text].
|
| 23.
|
Rose, M. D.,
F. Winston, and P. Hieter.
1990.
.
Methods in yeast genetics. A laboratory course manual.
Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.
|
| 24.
|
Sambrook, J.,
E. F. Fritsch, and T. Maniatis.
1989.
.
Molecular cloning: a laboratory manual, 2nd ed.
Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.
|
| 25.
|
Schaaper, R. M.
1988.
Mechanisms of mutagenesis in the Escherichia coli mutator mutD5: role of DNA mismatch repair.
Proc. Natl. Acad. Sci. USA
85:8126-8130[Abstract/Free Full Text].
|
| 26.
|
Schaaper, R. M., and M. Radman.
1989.
The extreme mutator effect of Escherichia coli mutD5 results from saturation of mismatch repair by excessive DNA replication errors.
EMBO J.
8:3511-3516[Medline].
|
| 27.
|
Scheuermann, R. H., and H. Echols.
1984.
A separate editing exonuclease for DNA replication: the subunit of Escherichia coli DNA polymerase III holoenzyme.
Proc. Natl. Acad. Sci. USA
81:7747-7751[Abstract/Free Full Text].
|
| 28.
|
Slater, S. C.,
M. R. Lifsics,
M. O'Donnell, and R. Maurer.
1994.
holE, the gene coding for the subunit of DNA polymerase III of Escherichia coli: characterization of a holE mutant and comparison with a dnaQ (-subunit) mutant.
J. Bacteriol.
176:815-821[Abstract/Free Full Text].
|
| 29.
|
Studwell-Vaughan, P. S., and M. O'Donnell.
1993.
DNA polymerase III accessory proteins. V. encoded by holE.
J. Biol. Chem.
268:11758-11791[Abstract/Free Full Text].
|
| 30.
|
Takano, K.,
Y. Nakabeppu,
H. Maki,
T. Horiuchi, and M. Sekiguchi.
1986.
Structure and function of dnaQ and mutD mutators of Escherichia coli.
Mol. Gen. Genet.
205:9-13[Medline].
|
| 31.
|
Tomasiewicz, H. G., and C. S. McHenry.
1987.
Sequence analysis of the Escherichia coli dnaE gene.
J. Bacteriol.
169:5735-5744[Abstract/Free Full Text].
|
![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
kowska,1
la1
Top
Abstract
Introduction
skiego 5A, Poland. Phone and fax: (48)39 12 16 23. E-mail: piotrekj{at}ibbrain.ibb.waw.pl.
