Initiation of RNA-primed DNA synthesis in vitro by DNA polymerase alpha-primase

DNA Damage Tolerance by Eukaryotic DNA Polymerase and Primase PrimPol

PrimPol is a human deoxyribonucleic acid (DNA) polymerase that also possesses primase activity and is involved in DNA damage tolerance, the prevention of genome instability and mitochondrial DNA maintenance. In this review, we focus on recent advances in biochemical and crystallographic studies of PrimPol, as well as in identification of new protein-protein interaction partners. Furthermore, we discuss the possible functions of PrimPol in both the nucleus and the mitochondria.

Thrombin-mediated transcriptional regulation using DNA aptamers in DNA-based cell-free protein synthesis

Realizing the potential of cell-free systems will require development of ligand-sensitive gene promoters that control gene expression in response to a ligand of interest. Here, we describe an approach to designing ligand-sensitive transcriptional control in cell-free systems that is based on the combination of a DNA aptamer that binds thrombin and the T7 bacteriophage promoter. Placement of the aptamer near the T7 promoter, and using a primarily single-stranded template, results in up to a 6-fold change in gene expression in a ligand concentration-dependent manner. We further demonstrate that the sensitivity to thrombin concentration and the fold change in expression can be tuned by altering the position of the aptamer. The results described here pave the way for the use of DNA aptamers to achieve modular regulation of transcription in response to a wide variety of ligands in cell-free systems.

The mitochondrial DNA helicase TWINKLE can assemble on a closed circular template and support initiation of DNA synthesis

Mitochondrial DNA replication is performed by a simple machinery, containing the TWINKLE DNA helicase, a single-stranded DNA-binding protein, and the mitochondrial DNA polymerase γ. In addition, mitochondrial RNA polymerase is required for primer formation at the origins of DNA replication. TWINKLE adopts a hexameric ring-shaped structure that must load on the closed circular mtDNA genome. In other systems, a specialized helicase loader often facilitates helicase loading. We here demonstrate that TWINKLE can function without a specialized loader. We also show that the mitochondrial replication machinery can assemble on a closed circular DNA template and efficiently elongate a DNA primer in a manner that closely resembles initiation of mtDNA synthesis in vivo.

DNA primases

DNA primases are enzymes whose continual activity is required at the DNA replication fork. They catalyze the synthesis of short RNA molecules used as primers for DNA polymerases. Primers are synthesized from ribonucleoside triphosphates and are four to fifteen nucleotides long. Most DNA primases can be divided into two classes. The first class contains bacterial and bacteriophage enzymes found associated with replicative DNA helicases. These prokaryotic primases contain three distinct domains: an amino terminal domain with a zinc ribbon motif involved in binding template DNA, a middle RNA polymerase domain, and a carboxyl-terminal region that either is itself a DNA helicase or interacts with a DNA helicase. The second major primase class comprises heterodimeric eukaryotic primases that form a complex with DNA polymerase alpha and its accessory B subunit. The small eukaryotic primase subunit contains the active site for RNA synthesis, and its activity correlates with DNA replication during the cell cycle.

Ori-somes, nucleoprotein complexes descending from origin regions of animal chromosomal DNA replication. A micromorphological study

Micromorphology of nucleoprotein (NP) complexes designated according to their descent and shape as Ori-somes is presented. These NP complexes of three different types harbor molecules of cytoplasmic "small" polydisperse DNA, which descend from origin regions of chromosomal DNA replication and are equipped, as shown previously, with early DNA-synthesizing activities. By negative staining the Ori-somes are visualized as particles of irregular shape, sometimes of a subunit-like structure. Micromorphological differences in size and structural compactness noted among individual Ori-somes are dependent on their type similarly as earlier shown physico-chemically and biochemically. Such differences were also confirmed by two different spreading techniques. The most unravelled structures with electron diffuse centers belong to Ori-somes of component B associated with most active DNA synthesis. In contrast, the Ori-somes of components A and C, associated with pronounced RNA synthesis, revealed large electron-dense centers. The incidence of replicative structures present in Ori-somes corresponds with the level of their DNA-synthesizing activities.

In the simian virus 40 in vitro replication system, start site selection by the polymerase alpha-primase complex is not significantly altered by changes in the concentration of ribonucleotides

The simian virus 40 (SV40) in vitro replication system was previously used to demonstrate that the human polymerase (Pol) alpha-primase complex preferentially initiates DNA synthesis at pyrimidine-rich trinucleotide sequences. However, it has been reported that under certain conditions, nucleoside triphosphate (NTP) concentrations play a critical role in determining where eukaryotic primase initiates synthesis. Therefore, we have examined whether increased NTP concentrations alter the template locations at which SV40 replication is initiated. Our studies demonstrate that elevated ribonucleotide concentrations do not significantly alter which template sequences serve as initiation sites. Of considerable interest, the sequences that serve as initiation sites in the SV40 system are similar to those that serve as initiation sites for prokaryotic primases. It is also demonstrated that regardless of the concentration of ribonucleotides present in the reactions, DNA synthesis initiated outside of the core origin. These studies provide additional evidence that the Pol alpha-primase complex can initiate DNA synthesis only after a considerable amount of single-stranded DNA is generated.

A cruciform-dumbbell model for inverted dimer formation mediated by inverted repeats

Small inverted repeats (small palindromes) on plasmids have been shown to mediate a recombinational rearrangement event in Escherichia coli leading to the formation of inverted dimers (giant palindromes). This recombinational rearrangement event is efficient and independent of RecA and RecBCD. In this report, we propose a cruciform-dumbbell model to explain the inverted dimer formation mediated by inverted repeats. In this model, the inverted repeats promote the formation of a DNA cruciform which is processed by an endonuclease into a linear DNA with two hairpin loops at its ends. Upon DNA replication, this linear dumbbell-like DNA is then converted to the inverted dimer. In support of this model, linear dumbbell DNA molecules with unidirectional origin of DNA replication (ColE1 ori ) have been constructed and shown to transform E.coli efficiently resulting in the formation of the inverted dimer. The ability of linear dumbbell DNA to transform E.coli suggests that the terminal loops may be important in bypassing the requirement of DNA supercoiling for initiation of replication of the ColE1 ori.

Synthesis of the mammalian telomere lagging strand in vitro

Using a synthetic telomere DNA template and whole cell extracts, we have identified proteins capable of synthesizing the telomere complementary strand. Synthesis of the complementary strand required a DNA template consisting of 10 repeats of the human telomeric sequence d(TTAGGG) and deoxy- and ribonucleosidetriphosphates and was inhibited by neutralizing antibodies to DNA polymerase alpha. No evidence for RNA-independent synthesis of the lagging strand was observed, suggesting that a stable DNA secondary structure capable of priming the lagging strand is unlikely. Purified DNA polymerase alpha/primase was capable of catalyzing synthesis of the lagging strand with the same requirements as those observed in crude cell extracts. A ladder of products was observed with an interval of six bases, suggesting a unique RNA priming site and site-specific pausing or dissociation of polymerase alpha on the d(TTAGGG)10 template. Removal of the RNA primers was observed upon the addition of purified RNase HI. By varying the input rNTP, the RNA priming site was determined to be opposite the 3' thymidine nucleotide generating a five-base RNA primer with the sequence 5'-AACCC. The addition of UTP did not increase the efficiency of priming and extension, suggesting that the five-base RNA primer is sufficient for extension with dNTPs by DNA polymerase alpha. This represents the first experimental evidence for RNA priming and DNA extension as the mechanism of mammalian telomeric lagging strand replication.

DNA replication initiates non-randomly at multiple sites near the c-myc gene in HeLa cells

The origin of replication of the c-myc gene in HeLa cells was previously identified at low resolution within 3.5 kb 5' to the P1 promoter, based on replication fork polarity and the location of DNA nascent strands. To define the initiation events in the c-myc origin at higher resolution the template bias of nascent DNAs in a 12 kb c-myc domain has been analyzed by hybridization to strand specific probes. Strong switches in the asymmetry of nascent strand template preference confirm that replication initiates non-randomly at multiple sites within 2.4 kb 5' to the c-myc P1 promoter, and at other sites over a region of 12 kb or more. The strongest template biases occur in the 2.4 kb region 5' of the P1 promoter, shown earlier to contain sequences which allow the autonomous semiconservative replication of c-myc plasmids. An asymmetric pyrimidine heptanucleotide consensus sequence has been identified which occurs 12 times in the c-myc origin zone, and whose polarity exactly correlates with the polarity of nascent strand synthesis.

The effects of cytosine arabinoside on RNA-primed DNA synthesis by DNA polymerase alpha-primase

Oligonucleotides containing a specific initiation site for polymerase alpha-primase (pol alpha-primase) were used to measure the effects of cytosine arabinoside triphosphate and cytosine arabinoside monophosphate (araCMP) in DNA on RNA-primed DNA synthesis. Primase inserts araCMP at the 3' terminus of a full-length RNA primer with a 400-fold preference over CMP. The araCMP is elongated efficiently by pol alpha in the primase-coupled reaction. Extension from RNA 3'-araCMP is 50-fold less efficient than from CMP, and extension from DNA 3'-araCMP is 1600-fold less efficient than from dCMP. Using araCMP-containing templates, primer synthesis is reduced 2-3-fold, and RNA-primed DNA synthesis is reduced 2-8-fold. The efficiency of polymerization past a template araCMP by pol alpha is reduced 180-fold during insertion of dGMP opposite araCMP and 35-fold during extension from the araCMP:dGMP 3' terminus. These results show that the pol alpha-primase efficiently incorporates araCMP as the border nucleotide between RNA and DNA and suggest that the inhibitory effects of araC most likely result from slowed elongation of pol alpha and less so from inhibition of primer synthesis by primase.