Protein Domain : RNA-directed RNA polymerase, luteovirus IPR001795

Type	Family
Description	RNA-directed RNA polymerase (RdRp) ( ) is an essential protein encoded in the genomes of all RNA containing viruses with no DNA stage [ , ]. It catalyses synthesis of the RNA strand complementary to a given RNA template, but the precise molecular mechanism remains unclear.The postulated 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 (de novo) mechanism. The de novo initiation consists in the addition of a nucleotide tri-phosphate (NTP) 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 [ ]. All the RNA-directed RNA polymerases, and many DNA-directed polymerases, employ a fold whose organisation has been likened to the shape of a right hand with three subdomains termed fingers, palm and thumb [ ]. Only the catalytic palm subdomain, composed of a four-stranded antiparallel β-sheet with two α-helices, is well conserved among all of these enzymes. In RdRp, the palm subdomain comprises three well conserved motifs (A, B and C). Motif A (D-x(4,5)-D) and motif C (GDD) are spatially juxtaposed; the Asp residues of these motifs are implied in the binding of Mg2+ and/or Mn2+. The Asn residue of motif B is involved in selection of ribonucleoside triphosphates over dNTPs and thus determines whether RNA is synthesised rather than DNA [].The domain organisation [ ] and the 3D structure of the catalytic centre of a wide range of RdPp's, 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.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: viral RNA-directed RNA polymerases including all positive-strand RNA viruses with no DNA stage, double-strand RNA viruses, and the Cystoviridae, Reoviridae, Hypoviridae, Partitiviridae, Totiviridae families.Mononegavirales (negative-strand RNA viruses with non-segmented genomes).Negative-strand RNA viruses with segmented genomes, i.e. Orthomyxoviruses (including influenza A, B, and C viruses, Thogotoviruses, and the infectious salmon anemia virus), Arenaviruses, Bunyaviruses, Hantaviruses, Nairoviruses, Phleboviruses, Tenuiviruses and Tospoviruses.Birnaviridae family of dsRNA viruses.The RNA-directed RNA polymerases in the first of the above superfamilies can be divided into the following three subgroups:All positive-strand RNA eukaryotic viruses with no DNA stage.All RNA-containing bacteriophages -there are two families of RNA-containing bacteriophages: Leviviridae (positive ssRNA phages) and Cystoviridae (dsRNA phages).Reoviridae family of dsRNA viruses.The nucleotide sequence for the RNA of Potato leafroll virus (PLrV) has been determined [ , ]. The sequence contains six large open reading frames (ORFs). The 5' coding region encodes two polypeptides of 28K and 70K, which overlap in different reading frames; it is suggested that the third ORF in the 5' block is translated by frameshift read through near the end of the 70K protein, yielding a 118K polypeptide []. The C-terminal part of the 118K protein contains a consensus sequence for RNA-dependent RNA-polymerases [].The genomic RNA sequence of Southern bean mosaic virus (SBMV) has been determined [ ]. The genome contains four ORFs. The largest ORF encodes the two largest proteins translated in cell-free extracts from full-length virion RNA []. Segments of the predicted amino acid sequence of this ORF resemble those of known viral RNA-polymerases, ATP-binding proteins and viral genome-linked proteins [].The genome sequence of Pea enation mosaic virus (PEMV) RNA 1 shows strong organisational relationships and sequence similarities to the Beet western yellows virus (BWYV) and PLrV [ ]. Sequence analysis reveals five predominant ORFs. The third ORF is characterised by a number of RNA-polymerase motifs and a helicase-like motif typical of RNA-dependent RNA-polymerases []. It overlaps (out of frame) the ORF 2 product and is proposed to be expressed by a frameshift fusion of ORF 2 and ORF 3 [].The PLrV sequence shows some similarities to the putative polymerase of SBMV [ ], and more extensive similarities to the corresponding BWYV polypeptide [].
Short Name	RNA-dir_pol_luteovirus