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Rolling circle replication (RCR) is a process of unidirectional nucleic acid replication that can rapidly synthesize multiple copies of circular molecules of DNA or RNA, such as plasmids, the genomes of bacteriophages, and the circular RNA genome of viroids. Some eukaryotic viruses also replicate their DNA or RNA via the rolling circle mechanism.
As a simplified version of natural rolling circle replication, an isothermal DNA amplification technique, rolling circle amplification was developed. The RCA mechanism is widely used in molecular biology and biomedical nanotechnology, especially in the field of biosensing (as a method of signal amplification). [1]
Rolling circle DNA replication is initiated by an initiator protein encoded by the plasmid or bacteriophage DNA, which nicks one strand of the double-stranded, circular DNA molecule at a site called the double-strand origin, or DSO. The initiator protein remains bound to the 5' phosphate end of the nicked strand, and the free 3' hydroxyl end is released to serve as a primer for DNA synthesis by DNA polymerase III. Using the unnicked strand as a template, replication proceeds around the circular DNA molecule, displacing the nicked strand as single-stranded DNA. Displacement of the nicked strand is carried out by a host-encoded helicase called PcrA (the abbreviation standing for plasmid copy reduced) in the presence of the plasmid replication initiation protein.
Continued DNA synthesis can produce multiple single-stranded linear copies of the original DNA in a continuous head-to-tail series called a concatemer. These linear copies can be converted to double-stranded circular molecules through the following process:
First, the initiator protein makes another nick in the DNA to terminate synthesis of the first (leading) strand. RNA polymerase and DNA polymerase III then replicate the single-stranded origin (SSO) DNA to make another double-stranded circle. DNA polymerase I removes the primer, replacing it with DNA, and DNA ligase joins the ends to make another molecule of double-stranded circular DNA.
As a summary, a typical DNA rolling circle replication has five steps: [2]
Some DNA viruses replicate their genomic information in host cells via rolling circle replication. For instance, human herpesvirus-6 (HHV-6)(hibv) expresses a set of "early genes" that are believed to be involved in this process. [3] The long concatemers that result are subsequently cleaved between the pac-1 and pac-2 regions of HHV-6's genome by ribozymes when it is packaged into individual virions. [4]
Human Papillomavirus-16 (HPV-16) is another virus that employs rolling replication to produce progeny at a high rate. HPV-16 infects human epithelial cells and has a double stranded circular genome. During replication, at the origin, the E1 hexamer wraps around the single strand DNA and moves in the 3' to 5' direction. In normal bidirectional replication, the two replication proteins will disassociate at time of collision, but in HPV-16 it is believed that the E1 hexamer does not disassociate, hence leading to a continuous rolling replication. It is believed that this replication mechanism of HPV may have physiological implications into the integration of the virus into the host chromosome and eventual progression into cervical cancer. [5]
In addition, geminivirus also utilizes rolling circle replication as its replication mechanism. It is a virus that is responsible for destroying many major crops, such as cassava, cotton, legumes, maize, tomato and okra. The virus has a circular, single stranded, DNA that replicates in host plant cells. The entire process is initiated by the geminiviral replication initiator protein, Rep, which is also responsible for altering the host environment to act as part of the replication machinery. Rep is also strikingly similar to most other rolling replication initiator proteins of eubacteria, with the presence of motifs I, II, and III at is N terminus. During the rolling circle replication, the ssDNA of geminivirus is converted to dsDNA and Rep is then attached to the dsDNA at the origin sequence TAATATTAC. After Rep, along with other replication proteins, binds to the dsDNA it forms a stem loop where the DNA is then cleaved at the nanomer sequence causing a displacement of the strand. This displacement allows the replication fork to progress in the 3’ to 5’ direction which ultimately yields a new ssDNA strand and a concatameric DNA strand. [6]
Bacteriophage T4 DNA replication intermediates include circular and branched circular concatemeric structures. [7] These structures likely reflect a rolling circle mechanism of replication.
Some RNA viruses and viroids also replicate their genome through rolling circle RNA replication. For viroids, there are two alternative RNA replication pathways that respectively followed by members of the family Pospiviroidae (asymmetric replication) and Avsunviroidae (symmetric replication).
In the family Pospiviroidae (PSTVd-like), the circular plus strand RNA is transcribed by a host RNA polymerase into oligomeric minus strands and then oligomeric plus strands. [8] These oligomeric plus strands are cleaved by a host RNase and ligated by a host RNA ligase to reform the monomeric plus strand circular RNA. This is called the asymmetric pathway of rolling circle replication. The viroids in the family Avsunviroidae (ASBVd-like) replicate their genome through the symmetric pathway of rolling circle replication. [9] In this symmetric pathway, oligomeric minus strands are first cleaved and ligated to form monomeric minus strands, and then are transcribed into oligomeric plus strands. These oligomeric plus strands are then cleaved and ligated to reform the monomeric plus strand. The symmetric replication pathway was named because both plus and minus strands are produced the same way.
Cleavage of the oligomeric plus and minus strands is mediated by the self-cleaving hammerhead ribozyme structure present in the Avsunviroidae, but such structure is absent in the Pospiviroidae. [10]
The derivative form of rolling circle replication has been successfully used for amplification of DNA from very small amounts of starting material. [1] This amplification technique is named as rolling circle amplification (RCA). Different from conventional DNA amplification techniques such as polymerase chain reaction (PCR), RCA is an isothermal nucleic acid amplification technique where the polymerase continuously adds single nucleotides to a primer annealed to a circular template which results in a long concatemer ssDNA that contains tens to hundreds of tandem repeats (complementary to the circular template). [11]
There are five important components required for performing a RCA reaction:
The polymerases used in RCA are Phi29, Bst, and Vent exo-DNA polymerase for DNA amplification, and T7 RNA polymerase for RNA amplification. Since Phi29 DNA polymerase has the best processivity and strand displacement ability among all aforementioned polymerases, it has been most frequently used in RCA reactions. Different from polymerase chain reaction (PCR), RCA can be conducted at a constant temperature (room temperature to 65C) in both free solution and on top of immobilized targets (solid phase amplification).
There are typically three steps involved in a DNA RCA reaction:
RCA produces a linear amplification of DNA, as each circular template grows at a given speed for a certain amount of time. To increase yield and achieve exponential amplification as PCR does, several approaches have been investigated. One of them is the hyperbranched rolling circle amplification or HRCA, where primers that anneal to the original RCA products are added, and also extended. [12] In this way the original RCA creates more template that can be amplified. Another is circle to circle amplification or C2CA, where the RCA products are digested with a restriction enzyme and ligated into new circular templates using a restriction oligo, followed by a new round of RCA with a larger amount of circular templates for amplification. [13]
RCA can amplify a single molecular binding event over a thousandfold, making it particularly useful for detecting targets with ultra-low abundance. RCA reactions can be performed in not only free solution environments, but also on a solid surface like glass, micro- or nano-bead, microwell plates, microfluidic devices or even paper strips. This feature makes it a very powerful tool for amplifying signals in solid-phase immunoassays (e.g., ELISA). In this way, RCA is becoming a highly versatile signal amplification tool with wide-ranging applications in genomics, proteomics, diagnosis and biosensing.
Immuno-RCA is an isothermal signal amplification method for high-specificity & high-sensitivity protein detection and quantification. This technique combines two fields: RCA, which allows nucleotide amplification, and immunoassay, which uses antibodies specific to intracellular or free biomarkers. As a result, immuno-RCA gives a specific amplified signal (high signal-to-noise ratio), making it suitable for detecting, quantifying and visualizing low abundance proteic markers in liquid-phase immunoassays [14] [15] [16] and immunohistochemistry.
Immuno-RCA follows a typical immuno-adsorbent reaction in ELISA or immunohistochemistry tissue staining. [17] The detection antibodies used in immuno-RCA reaction are modified by attaching a ssDNA oligonucleotide on the end of the heavy chains. So the Fab (Fragment, antigen binding) section on the detection antibody can still bind to specific antigens and the oligonucleotide can serve as a primer of the RCA reaction.
The typical antibody mediated immuno-RCA procedure is as follows:
1. A detection antibody recognizes a specific proteic target. This antibody is also attached to an oligonucleotide primer.
2. When circular DNA is present, it is annealed, and the primer matches to the circular DNA complementary sequence.
3. The complementary sequence of the circular DNA template is copied hundreds of times and remains attached to the antibody.
4. RCA output (elongated ssDNA) is detected with fluorescent probes using a fluorescent microscope or a microplate reader.
In addition to antibody mediated immuno-RCA, the ssDNA RCA primer can be conjugated to the 3' end of a DNA aptamer as well. The primer tail can be amplified through rolling circle amplification. The product can be visualized through the labeling of fluorescent reporter. [19] The process is illustrated in the figure on the right.
Various derivatives of RCA were widely used in the field of biosensing. For example, RCA has been successfully used for detecting the existence of viral and bacterial DNA from clinical samples, [20] [21] which is very beneficial for rapid diagnostics of infectious diseases. It has also been used as an on-chip signal amplification method for nucleic acid (for both DNA and RNA) microarray assay. [1]
In addition to the amplification function in biosensing applications, RCA technique can be applied to the construction of DNA nanostructures and DNA hydrogels as well. The products of RCA can also be use as templates for periodic assembly of nanospecies or proteins, synthesis of metallic nanowires [22] and formation of nano-islands. [1]
A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and those that have one strand of DNA in their genome, called single-stranded DNA (ssDNA) viruses. dsDNA viruses primarily belong to two realms: Duplodnaviria and Varidnaviria, and ssDNA viruses are almost exclusively assigned to the realm Monodnaviria, which also includes some dsDNA viruses. Additionally, many DNA viruses are unassigned to higher taxa. Reverse transcribing viruses, which have a DNA genome that is replicated through an RNA intermediate by a reverse transcriptase, are classified into the kingdom Pararnavirae in the realm Riboviria.
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part of biological inheritance. This is essential for cell division during growth and repair of damaged tissues, while it also ensures that each of the new cells receives its own copy of the DNA. The cell possesses the distinctive property of division, which makes replication of DNA essential.
The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA sufficiently to enable detailed study. PCR was invented in 1983 by American biochemist Kary Mullis at Cetus Corporation. Mullis and biochemist Michael Smith, who had developed other essential ways of manipulating DNA, were jointly awarded the Nobel Prize in Chemistry in 1993.
A primer is a short, single-stranded nucleic acid used by all living organisms in the initiation of DNA synthesis. A synthetic primer may also be referred to as an oligo, short for oligonucleotide. DNA polymerase enzymes are only capable of adding nucleotides to the 3’-end of an existing nucleic acid, requiring a primer be bound to the template before DNA polymerase can begin a complementary strand. DNA polymerase adds nucleotides after binding to the RNA primer and synthesizes the whole strand. Later, the RNA strands must be removed accurately and replace them with DNA nucleotides forming a gap region known as a nick that is filled in using an enzyme called ligase. The removal process of the RNA primer requires several enzymes, such as Fen1, Lig1, and others that work in coordination with DNA polymerase, to ensure the removal of the RNA nucleotides and the addition of DNA nucleotides. Living organisms use solely RNA primers, while laboratory techniques in biochemistry and molecular biology that require in vitro DNA synthesis usually use DNA primers, since they are more temperature stable. Primers can be designed in laboratory for specific reactions such as polymerase chain reaction (PCR). When designing PCR primers, there are specific measures that must be taken into consideration, like the melting temperature of the primers and the annealing temperature of the reaction itself. Moreover, the DNA binding sequence of the primer in vitro has to be specifically chosen, which is done using a method called basic local alignment search tool (BLAST) that scans the DNA and finds specific and unique regions for the primer to bind.
A reverse transcriptase (RT) is an enzyme used to convert RNA genome to DNA, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genomes, by retrotransposon mobile genetic elements to proliferate within the host genome, and by eukaryotic cells to extend the telomeres at the ends of their linear chromosomes. Contrary to a widely held belief, the process does not violate the flows of genetic information as described by the classical central dogma, as transfers of information from RNA to DNA are explicitly held possible.
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms, and most cause diseases, whose respective economic importance to humans varies widely. A recent metatranscriptomics study suggests that the host diversity of viroids and other viroid-like elements is broader than previously thought and that it would not be limited to plants, encompassing even the prokaryotes.
Virusoids are circular single-stranded RNA(s) dependent on viruses for replication and encapsidation. The genome of virusoids consists of several hundred (200–400) nucleotides and does not code for any proteins.
Helicase-dependent amplification (HDA) is a method for in vitro DNA amplification that takes place at a constant temperature.
Geminiviridae is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: bright yellow mosaic, yellow mosaic, yellow mottle, leaf curling, stunting, streaks, reduced yields. They have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication. According to the Baltimore classification they are considered class II viruses. It is the largest known family of single stranded DNA viruses.
The Pospiviroidae are a incertae sedis family of ssRNA viroids with 5 genera and 39 species, including the first viroid to be discovered, PSTVd, which is part of genus Pospiviroid. Their secondary structure is key to their biological activity. The classification of this family is based on differences in the conserved central region sequence. Pospiviroidae replication occurs in an asymmetric fashion via host cell RNA polymerase, RNase, and RNA ligase. Its hosts are plants, specifically dicotyledons and some monocotyledons.
Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a distinct group. Seven Baltimore groups are described that take into consideration whether the viral genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), whether the genome is single- or double-stranded, and whether the sense of a single-stranded RNA genome is positive or negative.
Non-cellular life, also known as acellular life, is life that exists without a cellular structure for at least part of its life cycle. Historically, most definitions of life postulated that an organism must be composed of one or more cells, but, for some, this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.
RNA-dependent RNA polymerase (RdRp) or RNA replicase is an enzyme that catalyzes the replication of RNA from an RNA template. Specifically, it catalyzes synthesis of the RNA strand complementary to a given RNA template. This is in contrast to typical DNA-dependent RNA polymerases, which all organisms use to catalyze the transcription of RNA from a DNA template.
Nucleic acid sequence-based amplification, commonly referred to as NASBA, is a method in molecular biology which is used to produce multiple copies of single stranded RNA. NASBA is a two-step process that takes RNA and anneals specially designed primers, then utilizes an enzyme cocktail to amplify it.
The Avsunviroidae are a family of viroids. There are four species in three genera. They consist of RNA genomes between 246 and 375 nucleotides in length. They are single-stranded covalent circles and have intramolecular base pairing. All members lack a central conserved region.
Hepatitis B virus DNA polymerase is a hepatitis B viral protein. It is a DNA polymerase that can use either DNA or RNA templates and a ribonuclease H that cuts RNA in the duplex. Both functions are supplied by the reverse transcriptase (RT) domain.
Φ29 DNA polymerase is an enzyme from the bacteriophage Φ29. It is being increasingly used in molecular biology for multiple displacement DNA amplification procedures, and has a number of features that make it particularly suitable for this application. It was discovered and characterized by Spanish scientists Luis Blanco and Margarita Salas.
Reverse transcription loop-mediated isothermal amplification (RT-LAMP) is a one step nucleic acid amplification method to multiply specific sequences of RNA. It is used to diagnose infectious disease caused by RNA viruses.
Ground squirrel hepatitis virus, abbreviated GSHV, is a partially double-stranded DNA virus that is closely related to human Hepatitis B virus (HBV) and Woodchuck hepatitis virus (WHV). It is a member of the family of viruses Hepadnaviridae and the genus Orthohepadnavirus. Like the other members of its family, GSHV has high degree of species and tissue specificity. It was discovered in Beechey ground squirrels, Spermophilus beecheyi, but also infects Arctic ground squirrels, Spermophilus parryi. Commonalities between GSHV and HBV include morphology, DNA polymerase activity in genome repair, cross-reacting viral antigens, and the resulting persistent infection with viral antigen in the blood (antigenemia). As a result, GSHV is used as an experimental model for HBV.
Retrozymes are a family of retrotransposons first discovered in the genomes of plants but now also known in genomes of animals. Retrozymes contain a hammerhead ribozyme (HHR) in their sequences, although they do not possess any coding regions. Retrozymes are nonautonomous retroelements, and so borrow proteins from other elements to move into new regions of a genome. Retrozymes are actively transcribed into covalently closed circular RNAs and are detected in both polarities, which may indicate the use of rolling circle replication in their lifecycle.