DNA polymerases of low-GC gram-positive eubacteria: identification of the replication-specific enzyme encoded by dnaE

dnaE, the gene encoding one of the two replication-specific DNA polymerases (Pols) of low-GC-content gram-positive bacteria (E. Dervyn et al., Science 294:1716-1719, 2001; R. Inoue et al., Mol. Genet. Genomics 266:564-571, 2001), was cloned from Bacillus subtilis, a model low-GC gram-positive organism. The gene was overexpressed in Escherichia coli. The purified recombinant product displayed inhibitor responses and physical, catalytic, and antigenic properties indistinguishable from those of the low-GC gram-positive-organism-specific enzyme previously named DNA Pol II after the polB-encoded DNA Pol II of E. coli. Whereas a polB-like gene is absent from low-GC gram-positive genomes and whereas the low-GC gram-positive DNA Pol II strongly conserves a dnaE-like, Pol III primary structure, it is proposed that it be renamed DNA polymerase III E (Pol III E) to accurately reflect its replicative function and its origin from dnaE. It is also proposed that DNA Pol III, the other replication-specific Pol of low-GC gram-positive organisms, be renamed DNA polymerase III C (Pol III C) to denote its origin from polC. By this revised nomenclature, the DNA Pols that are expressed constitutively in low-GC gram-positive bacteria would include DNA Pol I, the dispensable repair enzyme encoded by polA, and the two essential, replication-specific enzymes Pol III C and Pol III E, encoded, respectively, by polC and dnaE.     

Bacillus subtilis DNA polymerase III: complete sequence, overexpression, and characterization of the polC gene

Genomic DNA encompassing polC, the structural gene specifying Bacillus subtilis DNA polymerase III (PolIII), was sequenced and found to contain a 4311-bp open reading frame (ORF) encoding a 162.4-kDa polypeptide of 1437 amino acids (aa). The ORF was engineered into an Escherichia coli expression plasmid under the control of the coliphage lambda repressor. Derepression of E. coli transformants carrying the recombinant vector resulted in the high-level synthesis of a recombinant DNA polymerase indistinguishable from native PolIII. N-terminal aa sequence analysis of the recombinant polymerase unequivocally identified the 4311-bp ORF as that of polC. Comparative aa sequence analysis indicated significant homology of the B. subtilis enzyme with the catalytic alpha subunit of the E. coli PolIII and, with the exception of an exonuclease domain, little homology with other DNA polymerases. The respective sequences of the mutant polC alleles, dnaF and ts-6, were identified, and the expression of specifically truncated forms of polC was exploited to assess the dependence of polymerase activity on the structure of the enzyme's C terminus.     

Cloning and characterization of the polC region of Bacillus subtilis.

The polC gene of Bacillus subtilis is defined by five temperature-sensitive mutations and the 6-(p-hydroxyphenylazo)-uracil (HPUra) resistance mutation azp-12. Biochemical evidence suggests that polC codes for the 160-kilodalton DNA polymerase III. A recombinant plasmid, p154t, was isolated and found to contain the azp-12 marker and one end of the polC gene (N. C. Brown and M. H. Barnes, J. Cell. Biochem. 78 [Suppl.]: 116, 1983). The azp-12 marker was localized to a 1-kilobase DNA segment which was used as a probe to isolate recombinant lambda phages containing polC region sequences. A complete polC gene was constructed by in vitro ligation of DNA segments derived from two of the recombinant phages. The resulting plasmid, pRO10, directed the synthesis of four proteins of 160, 76, 39, and 32 kilodaltons in Escherichia coli maxicells. Recombination-deficient (recE) B. subtilis PSL1 containing pRO10 produced an HPUra-resistant polymerase III activity which was lost when the strain was cured of pRO10. In vivo, the HPUra resistance of the plasmid-encoded polymerase III appeared to be recessive to the resident HPUra-sensitive polymerase III enzyme.     

DNA polymerase III of Enterococcus faecalis: expression and characterization of recombinant enzymes encoded by the polC and dnaE genes

Enterococcus faecalis (Ef) dnaE and polC, the respective genes encoding the DNA replication-specific DNA polymerase III E and DNA polymerase III C, were cloned and engineered for expression in Escherichia coli as hexahistidine (his6)-tagged recombinant proteins. Each gene expressed a catalytically active DNA polymerase of the expected molecular weight. The recombinant polymerases were purified and each was characterized with respect to catalytic properties, inhibitor sensitivity, and recognition by specific antibody raised against the corresponding DNA polymerase III of the model Gram-positive (Gr(+)) organism, Bacillus subtilis (Bs). In conclusion, the properties of each Enterococcus polymerase enzymes were similar to those of the respective B. subtilis enzymes.     

Bacillus subtilis DNA polymerase III is required for the replication of DNA of bacteriophages SPP-1 and phi 105.

The replication of the Bacillus subtilis bacteriophages SPP-1 and phi 105 is sensitive to 6-(p-hydroxyphenylazo)-uracil (HPUra), a selective inhibitor of replicative DNA synthesis of B. subtilis which acts specifically at the levels of a replication-specific polymerase, DNA polymerase III (pol III). The origin of the HPUra-sensitive polymerase required for phage replication was examined by comparison of the drug sensitivity of phage development in a normosensitive host with that in a host carrying azp-12, a polC mutation that specifies production of an HPUra-resistant pol III. azp-12 specified HPUra-resistant phage host pol III. The host polIII requirement for SPP-1 replication also was confirmed by the demonstration that phage development was temperature sensitive in a host mutant carrying the polC mutation mut-1 (ts). Examination of the pol III activity of crude and purified cell-free preparations derived from phage-infected cells did not indicate any detectable changes in the specific activity, purification behavior, or drug sensitivity of the enzyme.     

The cloned polC gene of Bacillus subtilis: characterization of the azp12 mutation and controlled in vitro synthesis of active DNA polymerase III

Wild type (wt) Bacillus subtilis polC and polCazp12, a mutant derivative specifying a form of DNA polymerase III resistant to hydroxyphenylazopyrimidines, were cloned as genomic fragments approximating the length required to encode the entire polymerase. The cloned DNA fragments were subjected to restriction and partial sequence analysis to locate the 5' end of the polC-specific coding sequence and the azp12 mutation, which was identified as a T----G transversion specifying replacement of serine with alanine. The cloned wt and azp12-coding sequences were recloned in an Escherichia coli expression vector with their respective 5' ends under the control of the bacteriophage lambda PL promoter and cIts857-encoded repressor. In response to induction, the wt- and azp12-specific recombinant plasmids expressed active DNA polymerases indistinguishable from the native enzymes derived from the respective B. subtilis hosts.     

Genetic analysis of the flaA locus of Bacillus subtilis.

We isolated two clones of recombinant lambda bacteriophage with overlapping inserts of Bacillus subtilis chromosomal DNA corresponding to part of the flaA locus. The flaA4 and flaA15 mutations were localized on the physical map by marker rescue experiments. The flaA locus and the flaB (sigD) gene were mapped in transduction crosses, and the order glnA polC flaB flaA was determined. FlaB was linked to polC in transformation crosses.     

Identification of Escherichia coli dnaE (polC) mutants with altered sensitivity to 2',3'-dideoxyadenosine

Bacteria with reduced DNA polymerase I activity have increased sensitivity to killing by chain-terminating nucleotides (S. A. Rashbaum and N. R. Cozzarelli, Nature 264:679-680, 1976). We have used this observation as the basis of a genetic strategy to identify mutations in the dnaE (polC) gene of Escherichia coli that alter sensitivity to 2',3'-dideoxyadenosine (ddA). Two dnaE (polC) mutant strains with increased sensitivity to ddA and one strain with increased resistance were isolated and characterized. The mutant phenotypes are due to single amino acid substitutions in the alpha subunit, the protein product of the dnaE (polC) gene. Increased sensitivity to ddA is produced by the L329F and H417Y substitutions, and increased resistance is produced by the G365S substitution. The L329F and H417Y substitutions also reduce the accuracy of DNA replication (the mutator phenotype), while the G365S substitution increases accuracy (the antimutator phenotype). All of the amino acid substitutions are in conserved regions near essential aspartate residues. These results prove the effectiveness of the genetic strategy in identifying informative dnaE (polC) mutations that can be used to elucidate the molecular basis of nucleotide interactions in the alpha subunit of the DNA polymerase III holoenzyme.     

The replication of bacteriophage P4 DNA in vitro. Partial purification of the P4 alpha gene product

A soluble enzyme system has been prepared from a phage P4-infected Escherichia coli strain that supports the replication of exogenous, supercoiled P4 DNA. This DNA synthesis in vitro depends upon the four deoxyribonucleotides and ATP, but is enhanced about four- to fivefold by the presence of other ribonucleotides. E. coli DNA polymerase III holoenzyme, the E. coli single-strand DNA binding protein, and the partially purified P4 alpha gene product are required for replication in vitro. Rifamycin does not inhibit P4 replication in vitro. Since the P4 alpha gene codes for a rifamycin-resistant RNA polymerase (Barrett et al., 1983), and since P4 DNA replication is independent of the host primase (Bowden et al., 1975), we believe the alpha gene product is functioning as a P4-specific DNA primase.     

Isolation of temperature-sensitive DNA polymerase III from Saccharomyces cerevisiae cdc2-2.

DNA polymerase III of the yeast Saccharomyces cerevisiae has been reported to be encoded at the CDC2 locus based on two observations. First, the CDC2 gene has homology to known DNA polymerase genes [Boulet et al. (1989) EMBO J. 8, 1849-1854], and second, the mutants cdc2-1 and cdc2-2 yield little or no DNA polymerase III activity in vitro [Boulet et al. (1989); Sitney et al. (1989) Cell 56, 599-605]. We describe here the isolation of temperature-sensitive DNA polymerase III from cdc2-2 strains. Our results provide direct experimental confirmation of the previously inferred gene/enzyme relationship and verify the conclusion that DNA polymerase III is required to replicate the genome. We isolated DNA polymerase III from two cdc2-2 strains, one containing the wild-type allele for DNA polymerase I (CDC17) and the other a mutant DNA polymerase I allele (cdc17-1). Yields from cdc2-2 cells of both DNA polymerase III activity and an associated 3'-5'-exonuclease activity [exonuclease III; Bauer et al. (1988) J. Biol. Chem. 263, 917-924] were decreased relative to yields from CDC2 cells. DNA polymerase III activity from cdc2-2 strains is thermolabile, displaying at least a 4-fold reduction in half-life at 44 degrees C. The activity is also labile at 37 degrees C, a temperature which is restrictive for growth of cdc2-2 but not CDC2 strains. At 23 degrees C, a temperature which is permissive for growth of both cdc2-2 and CDC2 strains, the mutant and wild-type DNA polymerase III activities display equal stability. These observations provide a demonstrable biochemical basis for the thermosensitive phenotype of cdc2-2 cells.     

Role of deoxyribonucleic acid polymerase III in the repair of single-strand breaks produced in Escherichia coli deoxyribonucleic acid by gamma radiation.

Cell survival, deoxyribonucleic acid (DNA) degradation, and the repair of DNA single-strand breaks were measured for Escherichia coli K-12 pol+, polA1, polC1026(ts), and polA1 polC1026(ts) cells after 137Cs gamma irradiation. The results indicate that DNA polymerase III is required for growth medium-dependent (type III) repair in polA+ or polA cells. In pol+ or polC cells, DNA polymerase I performs type II repair efficiently. The relative deficiencies of each of these strains in DNA repair generally correlate with their relative sensitivities to cell killing and with the extent of DNA degradation observed.     

Cloning the polB gene of Escherichia coli and identification of its product

Using an in vivo mini-Mu cloning system, we have cloned the polB gene of Escherichia coli into the multicopy plasmid, pUC18. A chromosomal insert of 4.9 kilobases gave 30-40-fold overproduction of DNA polymerase II, and the cells containing the plasmid showed normal growth. The restriction pattern of the polB gene does not match that of either the polA gene or polC gene. Plasmid-directed protein synthesis demonstrates peptides of 99 and 82 kDa which are not expressed by derivative plasmids without DNA polymerase II activity. It appears from in situ gel assays and high performance liquid chromatography that 82- and 55-kDa proteins are derived from the 99-kDa protein by degradation, but all retain activity. DNA polymerase I or DNA polymerase III antibody does not inhibit the synthesis reaction of partially purified DNA polymerase II, but DNA polymerase II antibody does. By the criteria of restriction pattern of the polB gene, molecular weight of the protein, and antibody inhibition of reaction, DNA polymerase II can be demonstrated to be a distinct DNA polymerase.     

Genetic identification of two distinct DNA polymerases, DnaE and PolC, that are essential for chromosomal DNA replication in Staphylococcus aureus

We isolated and characterized temperature-sensitive mutants for two genes, dnaE and polC, that are essential for DNA replication in Staphylococcus aureus. DNA replication in these mutants had a slow-stop phenotype when the temperature was shifted to a non-permissive level. The dnaE gene encodes a homolog of the alpha-subunit of the DNA polymerase III holoenzyme, the replicase essential for chromosomal DNA replication in Escherichia coli. The polC gene encodes PolC, another catalytic subunit of DNA polymerase, which is specifically found in gram-positive bacteria. The wild-type dnaE or polC gene complemented the temperature-sensitive phenotypes of cell growth and DNA replication in the corresponding mutant. Single mutations resulting in amino-acid exchanges were identified in the dnaE and polC genes of the temperature-sensitive mutants. The results indicate that these genes encode two distinct DNA polymerases which are both essential for chromosomal DNA replication in S. aureus. The number of viable mutant cells decreased at non-permissive temperature, suggesting that inactivation of DnaE and PolC has a bactericidal effect and that these enzymes are potential targets of antibiotics.     

DNA polymerase III requirement for repair of DNA damage caused by methyl methanesulfonate and hydrogen peroxide.

The pcbA1 mutation allows DNA replication dependent on DNA polymerase I at the restrictive temperature in polC(Ts) strains. Cells which carry pcbA1, a functional DNA polymerase I, and a temperature-sensitive DNA polymerase III gene were used to study the role of DNA polymerase III in DNA repair. At the restrictive temperature for DNA polymerase III, these strains were more sensitive to the alkylating agent methyl methanesulfonate (MMS) and hydrogen peroxide than normal cells. The same strains showed no increase in sensitivity to bleomycin, UV light, or psoralen at the restrictive temperature. The sensitivity of these strains to MMS and hydrogen peroxide was not due to the pcbAl allele, and normal sensitivity was restored by the introduction of a chromosomal or cloned DNA polymerase III gene, verifying that the sensitivity was due to loss of DNA polymerase III alpha-subunit activity. A functional DNA polymerase III is required for the reformation of high-molecular-weight DNA after treatment of cells with MMS or hydrogen peroxide, as demonstrated by alkaline sucrose sedimentation results. Thus, it appears that a functional DNA polymerase III is required for the optimal repair of DNA damage by MMS or hydrogen peroxide.     

Effect of deoxyribonucleic acid replication inhibitors on bacterial recombination.

Two inhibitors of replicative deoxyribonucleic acid (DNA) synthesis, nalidixic acid (NAL) and 6-(p-hydroxyphenylazo)-uracil (HPUra), showed different effects on genetic recombination and DNA repair in Bacillus subtilis. Previous work (Pedrini et al., 1972) showed that NAL does not interfere with the transformation process of B. subtilis. The results reported in this work demonstrated that the drug was also without effect on the transfection by SPP1 or SPO-1 phage DNA (a process that requires a recombination event). The drug was also ineffective on the host cell reactivation of ultraviolet-irradiated SPP1 phage, as well as on transfection with ultraviolet-irradiated DNA of the same phage. HPUra instead markedly reduced the transformation process, as well as transfection, by SPO-1 DNA, but it did not affect the host cell reactivation of SPO-1 phage. In conclusion, whereas the NAL target seems to be specific for replicative DNA synthesis, the HPUra target (i.e., the DNA polymerase III of B. subtilis) seems to be involved also in recombination, but not in the excision repair process. The mutations conferring NAL and HPUra resistance used in this work were mapped by PBS-1 transduction.     

On the identity of dnaP and dnaF genes of Bacillus subtilis

Summary The dnaP strains of Bacillus subtilis are altered in the initiation of DNA replication at high temperature (Riva et al., 1975). Fine mapping of the gene shows that it is located very close to the dnaF gene, described by Karamata and Gross (1970) and mapped by Love et al. (1976) in the polC region. The phenotype of both mutants is indistinguishable: the DNA synthesis stops at non permissive temperature after synthesizing an amount of DNA equivalent to the completion of the rounds of replication already initiated; at permissive temperature they are abnormally sensitive to MMS and are reduced in the ability to be transformed. Both mutants are to be considered as belonging to the dnaF locus.The dnaF gene is very close to the polC gene, which specifies the DNA polymerase III of B. subtilis. The DNA polymerase III of the dnaF mutants is not temperature sensitive in vitro, however, the level of this enzyme is lower by a factor of 4 or 5 in the dnaF mutants, at the permissive temperature. Following shift of dnaF cultures to the non permissive temperature, the level of DNA polymerase III activity specifically decreases further by a factor of at least 10 in the mutant, whereas the DNA polymerase I level is unaffected.The possible roles of the dnaF gene in the control of the cellular level of the DNA polymerase III, and the possibility of a regulatory role of DNA polymerase III in the initiation of DNA replication in bacteria are discussed.Abbreviations and symbols HPUra 6-(p-hydroxyphenylazo)-uracil; mic, minimum inhibitory concentration - MMS methyl-methanesufonate - Pol I Pol II and Pol III: DNA polymerase I, II and III respectively - PCMB parachloro-mercuri-benzoate     

Multiple pathways for repair of oxidative DNA damages caused by X rays and hydrogen peroxide in Escherichia coli.

The responses of Escherichia coli to X rays and hydrogen peroxide were examined in mutants which are deficient in one or more DNA repair genes. Mutant cells deficient in either exonuclease III (xthA) or endonuclease IV (nfo) had normal resistance to X rays, but an xthA-nfo double mutant showed a sensitivity increased over that of either parental strain. A DNA polymerase I mutant (polA) was more sensitive than the xthA-nfo mutant. Cells bearing mutations in all of the polA, xthA, and nfo genes were more sensitive to X rays than polA and xthA-nfo mutants. Similar repair responses were obtained by exposing these mutant cells to hydrogen peroxide, with the exception of the xthA mutant, which was hypersensitive to this agent. The DNA polymerase III mutant (polC(Ts)) was slightly more sensitive to the agents than the wild-type strain at the restrictive temperature. The sensitivity of the polC-xthA-nfo mutant to X rays and hydrogen peroxide was greater than that of polC but almost the same as that of the xthA-nfo mutant. From these results it appears that there are at least four repair pathways, the DNA polymerase I-, exonuclease III/endonuclease IV and DNA polymerase I-, exonuclease III/endonuclease IV and DNA polymerase III-, and exonuclease III/endonuclease IV-dependent pathways, for the repair of oxidative DNA damages in E. coli.     

GC content variability of eubacteria is governed by the pol III alpha subunit

Eubacterial genomes have highly variable GC content (0.17-0.75) and the primary mechanism of such variability remains unknown. The place to look for is what actually catalyzes the synthesis of DNA, where DNA polymerase III is at the center stage, particularly one of its 10 subunits--the alpha subunit. According to the dimeric combination of alpha subunits, GC contents of eubacterial genomes were partitioned into three groups with distinct GC content variation spectra: dnaE1 (full-spectrum), dnaE2/dnaE1 (high-GC), and polC/dnaE3 (low-GC). Therefore, genomic GC content variability is believed to be governed primarily by the alpha subunit grouping of DNA polymerase III; it is of essence in genome composition analysis to take full account of such a grouping principle. Since horizontal gene transfer is very frequent among bacterial genomes, exceptions of the grouping scheme, a few percents of the total, are readily identifiable and should be excluded from in-depth analyses on nucleotide compositions.     

Polymerases and UV mutagenesis in Escherichia coli

Evidence for and against the involvement of the known nucleic acid polymerases in UV mutagenesis in Escherichia coli is reviewed. There is no evidence that rules out the participation of any of them when they are present but only one, the alpha subunit of DNA polymerase III holoenzyme (polC gene product) has been shown to be essential. It is argued that the PolC protein that functions in UV mutagenesis may not be immediately recognizable as one of the normal cellular polymerases or polymerase complexes.