DNA replication is tightly controlled to ensure accurate inheritance of genetic information. In all organisms initiator proteins possessing AAA+ (ATPases associated with various cellular activities) domains bind replication origins to license new rounds of DNA synthesis1. In bacteria the master initiator protein, DnaA, is highly conserved and has two crucial DNA binding activities2. DnaA monomers recognise the replication origin (oriC) by binding double-stranded DNA sequences (DnaA-boxes); subsequently, DnaA filaments assemble and promote duplex unwinding by engaging and stretching a single DNA strand3–5. While the specificity for duplex DnaA-boxes by DnaA has been appreciated for over thirty years, the sequence specificity for single-strand DNA binding remained unknown. Here we identify a new indispensable bacterial replication origin element composed of a repeating 3-mer motif that we term the DnaA-trio. We show that the function of the DnaA-trio is to stabilise DnaA filaments on a single DNA strand, thus providing essential precision to this binding mechanism. Bioinformatic analysis detects DnaA-trios in replication origins throughout the bacterial kingdom, indicating that this element comprises part of the core oriC structure. The discovery and characterisation of the novel DnaA-trio extends our fundamental understanding of bacterial DNA replication initiation, and because of the conserved structure of AAA+ initiator proteins these findings raise the possibility of specific recognition motifs within replication origins of higher organisms.

c Three evolutionarily distinct families of replicative DNA polymerases, designated polymerase B (Pol B), Pol C, and Pol D, have been identified. Members of the Pol B family are present in all three domains of life, whereas Pol C exists only in Bacteria and Pol D exists only in Archaea. Pol B enzymes replicate eukaryotic chromosomal DNA, and as members of the Pol B family are present in all Archaea, it has been assumed that Pol B enzymes also replicate archaeal genomes. Here we report the construction of Thermococcus kodakarensis strains with mutations that delete or inactivate key functions of Pol B. T. kodakarensis strains lacking Pol B had no detectable loss in viability and no growth defects or changes in spontaneous mutation frequency but had increased sensitivity to UV irradiation. In contrast, we were unable to introduce mutations that inactivated either of the genes encoding the two subunits of Pol D. The results reported establish that Pol D is sufficient for viability and genome replication in T. kodakarensis and argue that Pol D rather than Pol B is likely the replicative DNA polymerase in this archaeon. The majority of Archaea contain Pol D, and, as discussed, if Pol D is the predominant replicative polymerase in Archaea, this profoundly impacts hypotheses for the origin(s), evolution, and distribution of the different DNA replication enzymes and systems now employed in the three domains of life. DNA replication, an essential event for all cellular life, is catalyzed by protein complexes designated replisomes, in which individual activities are tightly regulated and coordinated. DNA polymerases are the functional center of the replisome, but structurally distinct DNA polymerases, designated family C (Pol C) and family B (Pol B) polymerases, catalyze genome replication in Bacteria and eukaryotes, respectively (1-3). This difference has led to much debate, most fundamentally regarding whether DNA replication has evolved more than once, possibly independently in different biological lineages (1, 4-10). All known archaeal genomes encode at least one member of the Pol B family, and given that Archaea are evolutionarily closer to eukaryotes than are Bacteria (11, 12), it has been tacitly assumed, but challenged (13,14), that Pol B enzymes must also replicate archaeal genomes. Presumably, this must be the case for the Crenarchaeota, as their genomes appear to encode only Pol B enzymes. This is, however, only an assumption for all members of the Euryarchaeota, Thaumarchaeota, Korarchaeota, Aigarchaeota, and Nanoarchaeota lineages, as their genomes encode not only Pol B enzymes but also members of an archaeon-specific DNA polymerase family designated Pol D (Fig. 1) (11, 13-15).The Thermococcales are hyperthermophilic Euryarchaea, and given the commercial value of thermostable processive DNA polymerases, Pol B polymerases from this genus have received extensive characterization (13,16,17). Within these single polypeptide enzymes, the regions and residues directly responsible for nucleotide polymerization, 3=¡...

Genome duplication is essential for cell proliferation, and DNA synthesis is generally initiated by dedicated replication proteins at specific loci termed origins. In bacteria, the master initiator DnaA binds the chromosome origin ( oriC ) and unwinds the DNA duplex to permit helicase loading. However, despite decades of research it remained unclear how the information encoded within oriC guides DnaA‐dependent strand separation. To address this fundamental question, we took a systematic genetic approach in vivo and identified the core set of essential sequence elements within the Bacillus subtilis chromosome origin unwinding region. Using this information, we then show in vitro that the minimal replication origin sequence elements are necessary and sufficient to promote the mechanical functions of DNA duplex unwinding by DnaA. Because the basal DNA unwinding system characterized here appears to be conserved throughout the bacterial domain, this discovery provides a framework for understanding oriC architecture, activity, regulation and diversity.

Archaeal family B polymerases bind tightly to the deaminated bases uracil and hypoxanthine in single-stranded DNA, stalling replication on encountering these pro-mutagenic deoxynucleosides four steps ahead of the primer–template junction. When uracil is specifically bound, the polymerase–DNA complex exists in the editing rather than the polymerization conformation, despite the duplex region of the primer-template being perfectly base-paired. In this article, the interplay between the 3′–5′ proofreading exonuclease activity and binding of uracil/hypoxanthine is addressed, using the family-B DNA polymerase from Pyrococcus furiosus. When uracil/hypoxanthine is bound four bases ahead of the primer–template junction (+4 position), both the polymerase and the exonuclease are inhibited, profoundly for the polymerase activity. However, if the polymerase approaches closer to the deaminated bases, locating it at +3, +2, +1 or even 0 (paired with the extreme 3′ base in the primer), the exonuclease activity is strongly stimulated. In these situations, the exonuclease activity is actually stronger than that seen with mismatched primer-templates, even though the deaminated base-containing primer-templates are correctly base-paired. The resulting exonucleolytic degradation of the primer serves to move the uracil/hypoxanthine away from the primer–template junction, restoring the stalling position to +4. Thus the 3′–5′ proofreading exonuclease contributes to the inability of the polymerase to replicate beyond deaminated bases.

Archaeal family-D DNA polymerase is inhibited by the presence of uracil in DNA template strands. When the enzyme encounters uracil, following three parameters change: DNA binding increases roughly 2-fold, the rate of polymerization slows by a factor of ∼5 and 3′–5′ proof-reading exonuclease activity is stimulated by a factor of ∼2. Together these changes result in a significant decrease in polymerization activity and a reduction in net DNA synthesis. Pol D appears to interact with template strand uracil irrespective of its distance ahead of the replication fork. Polymerization does not stop at a defined location relative to uracil, rather a general decrease in DNA synthesis is observed. ‘Trans’ inhibition, the slowing of Pol D by uracil on a DNA strand not being replicated is also observed. It is proposed that Pol D is able to interact with uracil by looping out the single-stranded template, allowing simultaneous contact of both the base and the primer-template junction to give a polymerase-DNA complex with diminished extension ability.