RNA-dependent RNA polymerase or RNA replicase is an enzyme that catalyzes the replication of RNA from an RNA template. Specifically, it catalyses 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. RdRP is an essential protein encoded in the genomes of all RNA-containing viruses with no DNA stage, i.e. of the RNA viruses. Some eukaryotes also contains RdRP.
History
Viral RdRPs were discovered in the early 1960s from studies on mengovirus and polio virus when it was observed that these viruses were not sensitive to actinomycin D, a drug that inhibits cellular DNA-directed RNA synthesis. This lack of sensitivity suggested that there is a virus-specific enzyme that could copy RNA from an RNA template and not from a DNA template.
Distribution
RdRPs are highly conserved throughout viruses and is even related to telomerase, though the reason for this is an ongoing question as of 2009. The similarity has led to speculation that viral RdRps are ancestral to human telomerase. The most famous example of RdRP is that of the polio virus. The viral genome is composed of RNA, which enters the cell through receptor-mediated endocytosis. From there, the RNA is able to act as a template for complementary RNA synthesis, immediately. The complementary strand is then, itself, able to act as a template for the production of new viral genomes that are further packaged and released from the cell ready to infect more host cells. The advantage of this method of replication is that there is no DNA stage; replication is quick and easy. The disadvantage is that there is no 'back-up' DNA copy. Many RdRPs are associated tightly with membranes and are, therefore, difficult to study. The best-known RdRPs are polioviral 3Dpol, vesicular stomatitis virus L, and hepatitis C virusNS5B protein. Many eukaryotes also have RdRPs involved in RNA interference; these amplify microRNAs and small temporal RNAs and produce double-stranded RNA using small interfering RNAs as primers. In fact these same RdRPs that are used in the defense mechanisms can be usurped by RNA viruses for their benefit. Their evolutionary history has been reviewed.
Replication process
RdRP catalyses synthesis of the RNA strand complementary to a given RNA template. The RNA replication process is a two-step mechanism. First, the initiation step of RNA synthesis begins at or near the 3' end of the RNA template by means of a primer-independent, or a primer-dependent mechanism that utilizes a viral protein genome-linkedprimer. The de novo initiation consists in the addition of a nucleoside triphosphate to the 3'-OH of the first initiating NTP. During the following so-called elongation phase, this nucleotidyl transfer reaction is repeated with subsequent NTPs to generate the complementary RNA product.
Structure
Viral/prokaryotic RNA-directed RNA polymerases, along with many single-subunit DNA-directed polymerases, employ a fold whose organization has been likened to the shape of a right hand with three subdomains termed fingers, palm, and thumb. Only the palm subdomain, composed of a four-stranded antiparallelbeta sheet with two alpha helices, is well conserved among all of these enzymes. In RdRP, the palm subdomain comprises three well-conserved motifs. Motif A and motif C are spatially juxtaposed; the aspartic acid residues of these motifs are implied in the binding of Mg2+ and/or Mn2+. The asparagine residue of motif B is involved in selection of ribonucleoside triphosphates over dNTPs and, thus, determines whether RNA rather than DNA is synthesized. The domain organization and the 3D structure of the catalytic centre of a wide range of RdRPs, even those with a low overall sequence homology, are conserved. The catalytic centre is formed by several motifs containing a number of conserved amino acid residues. Eukaryotic RNA interference requires a cellular RNA-dependent RNA polymerase. Unlike the "hand" polymerases, they resemble simplified multi-subunit DNA-dependent RNA polymerases, specifically in the catalytic β/β' subunits, in that they use two sets of double-psi β-barrels in the active site. QDE1 in Neurospora crassa, which forms a homodimer, is an example of such an enzyme. Bacteriophage homologs, including yonO, appear to be closer to cRdRPs than DdRPs are. yonO is a DNA-dependent RNA polymerase.
In viruses
There are 4 superfamilies of viruses that cover all RNA-containing viruses with no DNA stage:
Viruses containing positive-strand RNA or double-strand RNA, except retroviruses and Birnaviridae
* All positive-strand RNA eukaryotic viruses with no DNA stage
* All RNA-containing bacteriophages; there are two families of RNA-containing bacteriophages: Leviviridae and Cystoviridae
RNA transcription is similar to but not the same as DNA replication. Flaviviruses produce a polyprotein from the ssRNA genome. The polyprotein is cleaved to a number of products, one of which is NS5, an RNA-dependent RNA polymerase. This RNA-directed RNA polymerase possesses a number of short regions and motifs homologous to other RNA-directed RNA polymerases. RNA replicase found in positive-strand ssRNA viruses are related to each other, forming three large superfamilies. Birnaviral RNA replicase is unique in that it lacks motif C in the palm. Mononegaviral RdRP has been automatically classified as similar to -ssRNA RdRPs, specifically one from Pestivirus and one from Leviviridae. Bunyaviral RdRP monomer resembles the heterotrimeric complex of Orthomyxoviral RdRP. Since it is a protein universal to RNA-containing viruses, RdRP is a useful marker for understanding their evolution. The overall structural evolution of viral RdRPs has been reviewed.
Recombination
When replicating its ssRNA genome, the poliovirus RdRP is able to carry out recombination. Recombination appears to occur by a copy choice mechanism in which the RdRP switches ssRNA templates during negative strand synthesis. Recombination frequency is determined in part by the fidelity of RdRP replication. RdRP variants with high replication fidelity show reduced recombination, and low fidelity RdRps exhibit increased recombination. Recombination by RdRP strand switching also occurs frequently during replication in the ssRNA plant carmoviruses and tombusviruses.
Intragenic complementation
has a linear, single stranded, negative-sense, nonsegmented RNA genome. The viral RdRP consists of two virus-encoded subunits, a smaller one P and a larger one L. When different inactive RdRP mutants with defects throughout the length of the L subunit where tested in pairwise combinations, restoration of viral RNA synthesis was observed in some combinations. This positive L-L interaction is referred to as intragenic complementation and indicates that the L protein is an oligomer in the viral RNA polymerase complex.
