Novel Escherichia coli mutant, dnaR, thermosensitive in initiation of chromosome replication.

A newly isolated Escherichia coli mutant thermosensitive in DNA synthesis had an allele named dnaR130, which was located at 26.3 minutes on the genetic map. The mutant was defective in initiation of chromosome replication but not in propagation at a high temperature. This mutant was capable of growing in the absence of the rnh function at the high temperature by means of a dnaA-independent replication mechanism. In the mutant exposed to the high temperature, an oriC plasmid was able to replicate, although at a lower rate than at the low temperature. The plasmid replication at the high temperature depended on the dnaA function essential for the initiation of replication from oriC. The mutant lacking the rnh function persistently maintained the oriC plasmid at the high temperature in a dnaA-dependent manner. Thus, the dnaR function was required for initiation of replication of the bacterial chromosome from oriC but not the oriC plasmid. This result reveals that a dnaR-dependent initiation mechanism that is dispensable for oriC plasmid replication operates in the bacterial chromosome replication.     

Initiation of heat-induced replication requires DnaA and the L-13-mer of oriC

An upshift of 10 degrees C or more in the growth temperature of an Escherichia coli culture causes induction of extra rounds of chromosome replication. This stress replication initiates at oriC but has functional requirements different from those of cyclic replication. We named this phenomenon heat-induced replication (HIR). Analysis of HIR in bacterial strains that had complete or partial oriC deletions and were suppressed by F integration showed that no sequence outside oriC is used for HIR. Analysis of a number of oriC mutants showed that deletion of the L-13-mer, which makes oriC inactive for cyclic replication, was the only mutation studied that inactivated HIR. The requirement for this sequence was strictly correlated with Benham's theoretical stress-induced DNA duplex destabilization. oriC mutations at DnaA, FIS, or IHF binding sites showed normal HIR activation, but DnaA was required for HIR. We suggest that strand opening for HIR initiation occurs due to heat-induced destabilization of the L-13-mer, and the stable oligomeric DnaA-single-stranded oriC complex might be required only to load the replicative helicase DnaB.     

IHF redistributes bound initiator protein, DnaA, on supercoiled oriC of Escherichia coli

In Escherichia coli, initiation of chromosome replication requires that DnaA binds to R boxes (9-mer repeats) in oriC, the unique chromosomal replication origin. At the time of initiation, integration host factor (IHF) also binds to a specific site in oriC. IHF stimulates open complex formation by DnaA on supercoiled oriC in cell-free replication systems, but it is unclear whether this stimulation involves specific changes in the oriC nucleoprotein complex. Using dimethylsulphate (DMS) footprinting on supercoiled oriC plasmids, we observed that IHF redistributed prebound DnaA, stimulating binding to sites R2, R3 and R5(M), as well as to three previously unidentified non-R sites with consensus sequence (A/T)G(G/C) (A/T)N(G/C)G(A/T)(A/T)(T/C)A. Redistribution was dependent on IHF binding to its cognate site and also required a functional R4 box. By reducing the DnaA level required to separate DNA strands and trigger initiation of DNA replication at each origin, IHF eliminates competition between strong and weak sites for free DnaA and enhances the precision of initiation synchrony during the cell cycle.     

Subcellular localization of Dna-initiation proteins of Bacillus subtilis: evidence that chromosome replication begins at either edge of the nucleoids

We examined the intracellular distribution of Bacillus subtilis Dna-initiation proteins by immunofluorescence microscopy to visualize the initiation complex of replication in vivo. DnaA was distributed throughout the cytoplasm, but both DnaB and DnaI were always detected as foci during the cell-division cycle. Interaction of DnaI with the DnaC helicase by the yeast two-hybrid assay suggests that DnaI acts as a helicase loader. The number of DnaB and DnaI foci within the cell exceeded that of oriC. Although the foci were not always co-localized with oriC, they seemed to be localized near the outer or inner edges of the nucleoids at initiation of replication. When the replication cycle was synchronized in cells using a temperature-sensitive dnaA mutant, duplication of the oriC region was observed predominantly near an edge of the nucleoid. Before initiation occurred, each one of the DnaB and DnaI foci was frequently observed near there. Furthermore, DnaX-GFP (DnaX is a component of DNA polymerase III) foci were detected near either of the edges of the nucleoids at the onset of replication. These results suggest that the replisome is recruited into oriC near either edge of the nucleoids to initiate chromosome replication in B. subtilis.     

Host cell variations resulting from F plasmid-controlled replication of the Escherichia coli chromosome.

Cell size and DNA concentration were measured in Escherichia coli K-12 ET64. This strain carries a dnaA (Ts) mutation that has been suppressed by the insertion of the F plasmid into the chromosome. ET64 can grow in a balanced steady state of exponential growth at the restrictive temperature for its dnaA allele (39 degrees C), in which chromosome replication is controlled by the F plasmid, and at the permissive temperature (30 degrees C), in which chromosome replication is controlled by dnaA-oriC. When cells grown at the indicated temperatures were compared, it was observed that at 39 degrees C, the cell mass increased and the amount of cellular DNA decreased slightly; therefore, the DNA concentration was strongly reduced. These changes can neither be explained by the reduction of the generation time (which is only 10-15%) nor from observed changes in the replication time and in the time between DNA synthesis termination and cell division. Variations were mainly due to the increase in cell mass per origin of replication, at initiation, in cells grown at 39 degrees C. Control of chromosome replication by the F plasmid appears to be the reason for the increase in the initiation mass. Other possible causes, such as the modification of growth temperature, the generation time, or both, were discarded. These observations suggest that at one growth rate, the F plasmid replicates at a particular cell mass to F particle number ratio, and that this ratio is higher than the cell mass to oriC ratio at the initiation of chromosome replication. This fact might be significant to coordinate the replication of two different replicons in the same cell.     

DnaA Protein of Escherichia coli: oligomerization at the E. coli chromosomal origin is required for initiation and involves specific N-terminal amino acids

Iterated DnaA box sequences within the replication origins of bacteria and prokaryotic plasmids are recognized by the replication initiator, DnaA protein. At the E. coli chromosomal origin, oriC, DnaA is speculated to oligomerize to initiate DNA replication. We developed an assay of oligomer formation at oriC that relies on complementation between two dnaA alleles that are inactive by themselves. One allele is dnaA46; its inactivity at the non-permissive temperature is due to a specific defect in ATP binding. The second allele, T435K, does not support DNA replication because of its inability to bind to DnaA box sequences within oriC. We show that the T435K allele can complement the dnaA46(Ts) allele. The results support a model of oligomer formation in which DnaA box sequences of oriC are bound by DnaA46 to which T435K then binds to form an active complex. Relying on this assay, leucine 5, tryptophan 6 and cysteine 9 in a predicted alpha helix were identified that, when altered, interfere with oligomer formation. Glutamine 8 is additionally needed for oligomer formation on an oriC-containing plasmid, suggesting that the structure of the DnaA-oriC complex at the chromosomal oriC locus is similar but not identical to that assembled on a plasmid. Other evidence suggests that proline 28 of DnaA is involved in the recruitment of DnaB to oriC. These results provide direct evidence that DnaA oligomerization at oriC is required for initiation to occur.     

The structure of the initiation complex at the replication origin, oriC, of Escherichia coli.

Two distinct regions in the replication origin, oriC, of Escherichia coli are separately distorted upon initiation complex formation by the initiator protein DnaA. The AT-rich region in the left part of oriC and the start site region in the right part of oriC. Chemical modification of single-stranded DNA was observed at both regions whereas endonuclease recognition of DNA mini-bulges specifically occurred in the start site region. We show that the helical phasing of binding sites for DnaA protein in oriC is important for origin function. An insertion or deletion of one helical turn between the two rightmost binding sites does not alter the efficiency of replication initiation, whereas all modifications of distance by less or more than one helical turn result in inactivation of oriC. DnaA binding and helical distortions in the AT-rich region as well as in the start site region are not affected in the distance mutants irrespective of their functionality in vivo. We propose a specific compact nucleoprotein structure for the initiation complex.     

Suppression of initiation defects of chromosome replication in Bacillus subtilis dnaA and oriC-deleted mutants by integration of a plasmid replicon into the chromosomes.

We constructed Bacillus subtilis strains in which chromosome replication initiates from the minimal replicon of a plasmid isolated from Bacillus natto, independently of oriC. Integration of the replicon in either orientation at the proA locus (115 degrees on the genetic map) suppressed the temperature-sensitive phenotype caused by a mutation in dnaA, a gene required for initiation of replication from oriC. In addition, in a strain with the plasmid replicon integrated into the chromosome, we were able to delete sequences required for oriC function. These strains were viable but had a slower growth rate than the oriC+ strains. Marker frequency analysis revealed that both pyrD and metD, genes close to proA, showed the highest values among the markers (genes) measured, and those of other markers decreased symmetrically with distance from the site of the integration (proA). These results indicated that the integrated plasmid replicon operated as a new and sole origin of chromosome replication in these strains and that the mode of replication was bidirectional. Interestingly, these mutants produced anucleate cells at a high frequency (about 40% in exponential culture), and the distribution of chromosomes in the cells was irregular. A change in the site and mechanism (from oriC to a plasmid system) of initiation appears to have resulted in a drastic alteration in coordination between chromosome replication and chromosome partition or cell division.     

Initiation of chromosome replication in dnaA and dnaC mutants of Escherichia coli B/r F.

Regulatory aspects of chromosome replication were investigated in dnaA5 and dnaC2 mutants of the Escherichia coli B/r F. When cultures growing at 25 degrees C were shifted to 41 degrees C for extended periods and then returned to 25 degrees C, the subsequent synchronous initiations of chromosome replication were spaced at fixed intervals. When chloramphenicol was added coincident with the temperature downshift, the extend of chromosome replication in the dnaA mutant was greater than that in the dnaC mutant, but the time intervals between initiations were the same in both mutants. Furthermore, the time interval between the first two initiation events was unaffected by alterations in the rate of rifampin-sensitive RNA synthesis or cell mass increase. In the dnaC2 mutant, the capacities for both initiations were achieved in the absence of extensive DNA replication at 25 degrees C as long as protein synthesis was permitted, but the cells did not progress toward the second initiation at 25 degrees C when both protein synthesis and DNA replication were prevented. Cells of the dnaA5 mutant did not achieve the capacity for the second initiation event in the absence of extensive chromosome replication, although delayed initiation may have taken place. A plausible hypothesis to explain the data is that the minimum interval is determined by the time required for formation of a supercoiled, membrane-attached structure in the vicinity of oriC which is required for initiation of DNA synthesis.     

Cooperation of the prs and dnaA gene products for initiation of chromosome replication in Escherichia coli.

A new Escherichia coli mutant allele, named dnaR, that causes thermosensitive initiation of chromosome replication has been identified to be an allele of the prs gene, the gene for phosphoribosylpyrophosphate synthetase (Y. Sakakibara, J. Mol. Biol. 226:979-987, 1992; Y. Sakakibara, J. Mol. Biol. 226:989-996, 1992). The dnaR mutant became temperature resistant by acquisition of a mutation in the dnaA gene that did not affect the intrinsic activity for the initiation of replication. The suppressor mutant was capable of initiating replication from oriC at a high temperature restrictive for the dnaR single mutant. The thermoresistant DNA synthesis was inhibited by the presence of the wild-type dnaA allele at a high but not a low copy number. The synthesis was also inhibited by an elevated dose of a mutant dnaR allele retaining dnaR activity. Therefore, thermoresistant DNA synthesis in the suppressor mutant was dependent on both the dnaA and the dnaR functions. On the basis of these results, I conclude that the initiation of chromosome replication requires cooperation of the prs and dnaA products.     

Localized DNA melting and structural pertubations in the origin of replication, oriC, of Escherichia coli in vitro and in vivo.

The leftmost region of the Escherichia coli origin of DNA replication (oriC) contains three tandemly repeated AT-rich 13mers which have been shown to become single-stranded during the early stages of initiation in vitro. Melting is induced by the ATP form of DnaA, the initiator protein of DNA replication. KMnO4 was used to probe for single-stranded regions and altered DNA conformation during the initiation of DNA replication at oriC in vitro and in vivo. Unpairing in the AT-rich 13mer region is thermodynamically stable even in the absence of DnaA protein, but only when divalent cations are omitted from the reaction. In the presence of Mg2+, oriC melting is strictly DnaA dependent. The sensitive region is distinct from that detected in the absence of DnaA as it is located further to the left within the minimal origin. In addition, the DNA is severely distorted between the three 13mers and the IHF binding site in oriC. A change of conformation can also be observed during the initiation of DNA replication in vivo. This is the first in vivo evidence for a structural change at the 13mers during initiation complex formation.     

Cluster of DnaA boxes involved in regulation of Streptomyces chromosome replication: from in silico to in vivo studies

In Streptomyces coelicolor, replication is initiated by the DnaA protein in the centrally located oriC region and proceeds bidirectionally until the replication forks reach the ends of the linear chromosome. We identified three clusters of DnaA boxes (H69, H24, and D78) which are in a relatively short segment of the chromosome centered on the oriC region. Of the clusters analyzed, D78 exhibited the highest affinity for the DnaA protein; the affinity of DnaA for the D78 cluster was about eightfold higher than the affinity for oriC. The high-affinity DnaA boxes appear to be involved in the control of chromosome replication. Deletion of D78 resulted in more frequent chromosome replication (an elevated ratio of origins to chromosome ends was observed) and activated aerial mycelium formation, leading to earlier colony maturation. In contrast, extra copies of D78 (delivered on a plasmid) caused slow colony growth, presumably because of a reduction in the frequency of initiation of chromosome replication. This suggests that the number of high-affinity DnaA boxes is relatively constant in hyphal compartments and that deletion of D78 therefore permits an increased copy number of either the chromosomal origin region or a plasmid harboring the D78 cluster. This system conceivably influences the timing of decisions to initiate aerial mycelial formation and sporulation.     

Inititation and termination of chromosome replication in Escherichia coli subjected to amino acid starvation.

Initiation and termination of chromosome replication in an Escherichia coli auxotroph subjected to amino acid starvation were examined by following the incorporation of [3H]thymidine into the EcoRI restriction fragments of the chromosome. The pattern of incorporation observed upon restoration of the amino acid showed that starvation blocks the process of initiation prior to deoxyribonucleic acid synthesis within any significant portion of the EcoRI fragment which contains the origin of replication, oriC. In this experiment, no incorporation of [3H]thymidine into EcoRI fragments from the terminus of replication was observed, nor was it found when a dnaC initiation mutant was used to prevent incorporation at the origin which might have obscured labeling of terminus fragments. Thus amino acid starvation does not appear to block replication forks shortly before termination of replication. Attempted synchronization of replication initiation by including a period of thymine starvation subsequent to the amino acid starvation led to simultaneous incorporation of [3H]-thymidine into all EcoRI fragments within the 240-kilobase region that surrounds oriC. It is shown that the thymine starvation step allowed initiation and a variable, but limited, amount of replication to occur.     

The initiation mess?

This review concerns the mechanisms which control initiation of chromosome replication in enterobacteria with respect to cell growth. Initiation control is commonly separated into positive and negative regulatory mechanisms. Four main points are advanced concerning these different aspects of initiation control. (i) The average concentration of the initiator protein DnaA is proportional to the origin concentration, i.e. the origin per cell mass ratio and, thus, inversely proportional to the very often used term of the 'initiation mass'. (ii) The time of initiation of chromosome replication in the cell cycle is set by DnaA protein accumulating to a threshold level, which in concert with a number of other factors allows for a co-operative formation of the initiation complex. (iii) The time of initiation is not determined by the interaction with these other factors or by the transient interaction between newly replicated origins ( oriC  ) and the cell surface. (iv) The aberrant initiation phenotype observed in various mutants, including dnaA (ts) mutants, might be due to a defective pre-initiation DnaA– oriC interaction or it might be due to a defect in the protection of newly initiated origins from reinitiation. Many of these points are discussed and evaluated in view of recent developments concerning the regulation of chromosome replication in Escherichia coli     

In vitro assembly of a prepriming complex at the origin of the Escherichia coli chromosome

During initiation of DNA replication of plasmids containing the origin of the Escherichia coli chromosome (oriC), the proteins dnaA, dnaB, and dnaC interact and assemble a complex at oriC. The complex is larger and more asymmetric than that formed by dnaA protein and embraces an extra 50 base pairs at the left side of the minimal oriC sequence. Both dnaA and dnaB proteins have been identified in the complex by electron microscopy and antibody binding; dnaC protein was not detected. HU protein, which stimulates the activity of the initiation reaction, was often present. Entry of dnaB protein required dnaA and dnaC proteins and a supercoiled template. Thus, a complex structure, involving multiple proteins and a large region of DNA, must be formed at the origin to prepare the template for priming and replication.     

Reinitiation of deoxyribonucleic acid synthesis by deoxyribonucleic acid initiation mutants of Escherichia coli: role of ribonucleic acid synthesis, protein synthesis, and cell division.

The dnaA and dnaC genes are thought to code for two proteins required for the initiation of chromosomal deoxyribonucleic acid replication in Escherichia coli. When a strain carrying a mutation in either of these genes is shifted from a permissive to a restrictive temperature, chromosome replication ceases after a period of residual synthesis. When the strains are reincubated at the permissive temperature, replication again resumes after a short lag. This reinitiation does not require either protein synthesis (as measured by resistance to chloramphenicol) or ribonucleic acid synthesis (as measured by resistance to rifampin). Thus, if there is a requirement for the synthesis of a specific ribonucleic acid to initiate deoxyribonucleic acid replication, this ribonucleic acid can be synthesized prior to the time of initiation and is relatively stable. Furthermore, the synthesis of this hypothetical ribonucleic acid does not require either the dnaA of dnaC gene products. The buildup at the restrictive temperature of the potential to reinitiate deoxyribonucleic acid synthesis at the permissive temperature shows rather complex kinetics the buildup roughly parallels the rate of mass increase of the culture for at least the first mass doubling at the restrictive temperature. At later times there appears to be a gradual loss of initiation potential despite a continued increase in mass. Under optimal conditions the increase in initiation potential can equal, but not exceed, the increase in cell division at the restrictive temperature. These results are most easily interpreted according to models that postulate a relationship between the initiation of deoxyribonucleic acid synthesis and the processes leading to cell division.     

HobA--a novel protein involved in initiation of chromosomal replication in Helicobacter pylori

Replication of the bacterial chromosome is initiated by the binding of the DnaA protein to a unique DNA region, called oriC. Many regulatory factors in numerous species act by controlling the ability of DnaA to bind and unwind DNA, but the Helicobacter pylori genome does not contain homologues to any of these factors. Here, we describe HobA, a novel protein essential for initiation of H. pylori chromosome replication, which is conserved among, and unique to, epsilon proteobacteria. We demonstrate that HobA interacts specifically via DnaA with the oriC-DnaA complex. We postulate that HobA is essential for correct formation and stabilization of the orisome by facilitating the spatial positioning of DnaA at oriC. Consistent with its function, overexpression of hobA had no effect on growth of H. pylori, whereas depletion of HobA led to growth arrest and failure to initiate replication. In conclusion, HobA may be the first identified of a new group of initiation factors common to epsilon proteobacteria.     

Rifampicin-resistant initiation of chromosome replication from oriC in ihf mutants

IHF (integration host factor) mutants exhibit asynchronous initiation of chromosome replication from oriC as determined from flow cytometric analysis of cultures where RNA synthesis was inhibited with rifampicin. However, the run-out kinetics of chromosome replication in ihf mutants shows that they continue to produce oriCs for some time in the absence of RNA synthesis resulting in a twofold increase in the oriC per mass ratio. An ihf dnaA double mutant did not exhibit this continued increase of the oriC per mass ratio. This indicates that ihf mutants can initiate replication from oriC in a rifampicin-resistant initiation mode but requires fully functional DnaA protein. The origin per mass ratio, determined by a quantitative Southern blotting technique, showed that the ihf mutants had an origin per mass ratio that was 60% of the wild type although it had a normal DnaA protein concentration. This shows that the initiation mass was substantially higher in the ihf mutants. The oriC per terminus ratio, which was also determined by Southern blotting, was very low in the ihf mutant, although it grew with the same doubling times as the wild-type strain. This indicates that cells lacking IHF replicate their chromosome(s) very fast.