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Drosophila X virus (DXV) belongs to the ''Birnaviridae'' family of viruses. ''Birnaviridae'' currently consists of three genera. The first genus is ''Entomobirnavirus'', which contains DXV. The next genus is ''Aquabirnavirus'', containing infectious pancreatic necrosis virus (IPNV).〔 The last genus is ''Avibirnavirus'', which contains infectious bursal disease virus (IBDV).〔 All of these genera contain homology in three specific areas of their transcripts. The homology comes from the amino and carboxyl regions of preVP2, a small 21-residue-long domain near the carboxyl terminal of VP3, and similar small ORFs sequences.〔 DXV was named after ''Drosophila melanogaster'', where it was first isolated. DXV was first isolated and named in 1978.〔 DXV was discovered as a contaminant in adult ''D. melanogaster'' while studying rhabdoviruses.〔 Assay results of DXV showed that DXV induces sensitivity to both carbon dioxide and NH2, which suggests general anoxia. Therefore, the pathogenic pathway for DXV leads to anoxia sensitivity and death of ''D. melanogaster''.〔 By negative contrast electron microscopy the DXV components were first visualized.〔 The origin of DXV is unknown and unclear. It was thought that DXV could have been pre-existent in ''Drosophila'' broods in a non-pathogenic form. Additionally, it was speculated that DXV might have originated as a contaminant from fetal calf serum in infection type studies because it was documented that endogenous bovine viruses were already in fetal calf serum. ==Structure, genome, and replication== DXV is a Baltimore Class III naked (non-enveloped) virus. The capsid of this protein contains an icosahedral geometry (T=13) consisting of 260 trimeric VP2 capsomeres. Specifically, DXV contains a bi-segmented dsRNA genome.〔 Both segments of the DXV genome contain 5’ terminal GGA triplet and a 3’ terminal CCC triplet consensus, which is consistent with birnaviridae (Shwed, 2002). The segment A genome is 3360-bp in length.〔 Segment A encodes a polyprotein sequence as follows: NH2-preVP2-VP4-VP3-COOH. This segment contains a large and small ORF. The segment B genome is 2991-bp in length.〔 Segment B encodes a polypeptide sequence as follows: NH2-VP1-COOH. The 5’ UTR of segment B is homologous to segment A, but unlike segment A, there is only one ORF.〔 Unusually, VP1 can be in two forms; as a free RdRp and as the genome-liked protein (VpG) that attaches to both 5’ end segments of the DXV through a Ser-5’-GMP phosphodiester bond. The replication of DXV follows the characterized dsRNA virus replication cycle. The large ORF of segment A consists of 3069 nucleotides.〔 The UTRs are characterized as 107-bp on 5’ side and 157-bp on 3’ end.〔 The start codons can be at either position 102, or two codons downstream at position 108. However, the initiation codon starts at the 108-bp.〔 The translation of the large ORF transcript produces a 114-kDa polyprotein.〔 The mature VP4 protein, viral protease, assists this process to increase the processing of the polyprotein to generate preVP2 capsid protein, VP3 viral ribonucleoprotein (RNP), and additional VP4 proteins.〔 In addition VP3 proteins can associate with pre-VP2 as a structural protein and with VP1 to function as a transcriptional activator. The small ORF of segment A consists of 711 nucleotides.〔 This ORF is in a location that extends across VP4/VP3 junction, although the precise position is unknown.〔 The mechanism for transcribing the small ORF is unknown. However, the possibility of ribosomal frameshifting has been ruled out since the small ORF site does not contain the characteristic hallmarks, such the 7 nucleotide long “slippery sequence” or downstream pseudoknot that is seen in other members of ''Birnaviridae''. It is hypothesized the small ORF is translated in a mechanism that uses subgenomic transcripts.〔 In any case, the translation of the small ORF transcript produces a 27-kDa polypeptide.〔 This polypeptide consists of 28 of basic, mainly arginine, residues. However this polypeptide has not been detected in infected cells. The segment B transcript encodes it encodes a 112.8-kDa VP1 polypeptide once translated.〔 This polypeptide has been characterized to be the RNA-dependent RNA polymerase (RdRp) and the VpG.〔 This polypeptide is 977 amino acids in length, making it the largest encoded RdRp in the ''Birnaviridae'' family.〔 The RdRp contains a consensus GTP-binding site and is thought to contain self-guanylylation activity, making it consistent with the ''Birnaviridae'' RdRp capacity.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Drosophila X virus」の詳細全文を読む スポンサード リンク
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