A model for transition of 5'-nuclease domain of DNA polymerase I from inert to active modes

DNA Polymerase I Large Fragment from Deinococcus radiodurans, a Candidate for a Cutting-Edge Room-Temperature LAMP

In recent years, the loop-mediated isothermal amplification (LAMP) technique, designed for microbial pathogen detection, has acquired fundamental importance in the biomedical field, providing rapid and precise responses. However, it still has some drawbacks, mainly due to the need for a thermostatic block, necessary to reach 63 °C, which is the BstI DNA polymerase working temperature. Here, we report the identification and characterization of the DNA polymerase I Large Fragment from Deinococcus radiodurans (DraLF-PolI) that functions at room temperature and is resistant to various environmental stress conditions. We demonstrated that DraLF-PolI displays efficient catalytic activity over a wide range of temperatures and pH, maintains its activity even after storage under various stress conditions, including desiccation, and retains its strand-displacement activity required for isothermal amplification technology. All of these characteristics make DraLF-PolI an excellent candidate for a cutting-edge room-temperature LAMP that promises to be very useful for the rapid and simple detection of pathogens at the point of care.

Bacillus subtilis encodes a discrete flap endonuclease that cleaves RNA-DNA hybrids

The current model for Okazaki fragment maturation in bacteria invokes RNA cleavage by RNase H, followed by strand displacement synthesis and 5' RNA flap removal by DNA polymerase I (Pol I). RNA removal by Pol I is thought to occur through the 5'-3' flap endo/exonuclease (FEN) domain, located in the N-terminus of the protein. In addition to Pol I, many bacteria encode a second, Pol I-independent FEN. The contribution of Pol I and Pol I-independent FENs to DNA replication and genome stability remains unclear. In this work we purified Bacillus subtilis Pol I and FEN, then assayed these proteins on a variety of RNA-DNA hybrid and DNA-only substrates. We found that FEN is far more active than Pol I on nicked double-flap, 5' single flap, and nicked RNA-DNA hybrid substrates. We show that the 5' nuclease activity of B. subtilis Pol I is feeble, even during DNA synthesis when a 5' flapped substrate is formed modeling an Okazaki fragment intermediate. Examination of Pol I and FEN on DNA-only substrates shows that FEN is more active than Pol I on most substrates tested. Further experiments show that ΔpolA phenotypes are completely rescued by expressing the C-terminal polymerase domain while expression of the N-terminal 5' nuclease domain fails to complement ΔpolA. Cells lacking FEN (ΔfenA) show a phenotype in conjunction with an RNase HIII defect, providing genetic evidence for the involvement of FEN in Okazaki fragment processing. With these results, we propose a model where cells remove RNA primers using FEN while upstream Okazaki fragments are extended through synthesis by Pol I. Our model resembles Okazaki fragment processing in eukaryotes, where Pol δ catalyzes strand displacement synthesis followed by 5' flap cleavage using FEN-1. Together our work highlights the conservation of ordered steps for Okazaki fragment processing in cells ranging from bacteria to human.

Crystal structure of DNA polymerase I from Thermus phage G20c

This study describes the structure of DNA polymerase I from Thermus phage G20c, termed PolI_G20c. This is the first structure of a DNA polymerase originating from a group of related thermophilic bacteriophages infecting Thermus thermophilus, including phages G20c, TSP4, P74-26, P23-45 and phiFA and the novel phage Tth15-6. Sequence and structural analysis of PolI_G20c revealed a 3'-5' exonuclease domain and a DNA polymerase domain, and activity screening confirmed that both domains were functional. No functional 5'-3' exonuclease domain was present. Structural analysis also revealed a novel specific structure motif, here termed SβαR, that was not previously identified in any polymerase belonging to the DNA polymerases I (or the DNA polymerase A family). The SβαR motif did not show any homology to the sequences or structures of known DNA polymerases. The exception was the sequence conservation of the residues in this motif in putative DNA polymerases encoded in the genomes of a group of thermophilic phages related to Thermus phage G20c. The structure of PolI_G20c was determined with the aid of another structure that was determined in parallel and was used as a model for molecular replacement. This other structure was of a 3'-5' exonuclease termed ExnV1. The cloned and expressed gene encoding ExnV1 was isolated from a thermophilic virus metagenome that was collected from several hot springs in Iceland. The structure of ExnV1, which contains the novel SβαR motif, was first determined to 2.19 Å resolution. With these data at hand, the structure of PolI_G20c was determined to 2.97 Å resolution. The structures of PolI_G20c and ExnV1 are most similar to those of the Klenow fragment of DNA polymerase I (PDB entry 2kzz) from Escherichia coli, DNA polymerase I from Geobacillus stearothermophilus (PDB entry 1knc) and Taq polymerase (PDB entry 1bgx) from Thermus aquaticus.

Single-molecule view of coordination in a multi-functional DNA polymerase

Replication and repair of genomic DNA requires the actions of multiple enzymatic functions that must be coordinated in order to ensure efficient and accurate product formation. Here, we have used single-molecule FRET microscopy to investigate the physical basis of functional coordination in DNA polymerase I (Pol I) from Escherichia coli, a key enzyme involved in lagging-strand replication and base excision repair. Pol I contains active sites for template-directed DNA polymerization and 5' flap processing in separate domains. We show that a DNA substrate can spontaneously transfer between polymerase and 5' nuclease domains during a single encounter with Pol I. Additionally, we show that the flexibly tethered 5' nuclease domain adopts different positions within Pol I-DNA complexes, depending on the nature of the DNA substrate. Our results reveal the structural dynamics that underlie functional coordination in Pol I and are likely relevant to other multi-functional DNA polymerases.

Understanding the Effect of Multiple Domain Deletion in DNA Polymerase I from Geobacillus Sp. Strain SK72

The molecular structure of DNA polymerase I or family A polymerases is made up of three major domains that consist of a single polymerase domain with two extra exonuclease domains. When the N-terminal was deleted, the enzyme was still able to perform basic polymerase activity with additional traits that used isothermal amplification. However, the 3′-5′ exonuclease domain that carries a proofreading activity was disabled. Yet, the structure remained attached to the 5′-3′ polymerization domain without affecting its ability. The purpose of this non-functional domain still remains scarce. It either gives negative effects or provides structural support to the DNA polymerase. Here, we compared the effect of deleting each domain against the polymerase activity. The recombinant wild type and its variants were successfully purified and characterized. Interestingly, SK72-Exo (a large fragment excluding the 5′-3′ exonuclease domain) exhibited better catalytic activity than the native SK72 (with all three domains) at similar optimum temperature and pH profile, and it showed longer stability at 70 °C. Meanwhile, SK72-Exo2 (polymerization domain without both the 5′-3′ and 3′-5′ exonuclease domain) displayed the lowest activity with an optimum at 40 °C and favored a more neutral environment. It was also the least stable among the variants, with almost no activity at 50 °C for the first 10 min. In conclusion, cutting both exonuclease domains in DNA polymerase I has a detrimental effect on the polymerization activity and structural stability.

Optimal numbers of residues in linkers of DNA polymerase I, T7 primase and DNA polymerase IV

DNA polymerase I (PolI), T7 primase and DNA polymerase IV (Dpo4) have a common feature in their structures that the two main domains are connected by an unstructured polypeptide linker. To perform their specific enzymatic activities, the enzymes are required to rearrange the position and orientation of one domain relative to the other into an active mode. Here, we show that the three enzymes share the same mechanism of the transition from the inert to active modes and use the minimum numbers of residues in their linkers to achieve the most efficient transitions. The transition time to the finally active mode is sensitively dependent on the stretched length of the linker in the finally active mode while is insensitive to the position and orientation in the initially inert state. Moreover, we find that for any enzyme whose two domains are connected by an unstructured flexible linker, the stretched length (L) of the linker in the finally active mode and the optimal number (Nopt) of the residues in the linker satisfy relation L ≈ αNopt, with α = 0.24-0.27 nm being a constant insensitive to the system.

A model for dynamics of primer extension by eukaryotic DNA primase

A mathematical model is proposed for processive primer extension by eukaryotic DNA primase. The model uses available experimental data to predict rate constants for the dynamic behavior of primase activity as a function of NTP concentration. The model also predicts some data such as the binding affinities of the primase for the DNA template and for the RNA primer.

Modeling translocation dynamics of strand displacement DNA synthesis by DNA polymerase I

A model is presented for the translocation dynamics of the strand displacement DNA synthesis by DNA polymerases such as polymerase I family. (i) The model gives an explanation to the experimental results which showed that the rate of strand displacement DNA synthesis is nearly consistent with that of single stranded primer extension synthesis, although the two are expected to have substantial differences in their energetics. (ii) During strand displacement DNA synthesis, the pausing at the specific sequence is considered to be due to an affinity of the fingers subdomain for the specific sequence of dsDNA downstream of the single strand. The theoretical results on the sequence-dependent pausing dynamics such as the mean pausing lifetimes and the distribution of the pausing lifetime are consistent with the experimental data. Moreover, predicted results are presented for the binding affinity of the fingers subdomain for the specific sequence of dsDNA and the dependence of the mean sequence-dependent pausing lifetime on the external force acting on the polymerase.