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Chapter 24 : Molecular Biology and Pathogenesis of Roniviruses
Category: Viruses and Viral Pathogenesis
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Currently all available information on the Roniviridae has come from studies of yellow head virus (YHV), gill-associated virus (GAV), and genotypic variants of these viruses detected in disparate populations of the black tiger shrimp species, Penaeus monodon. These viruses are very closely related and are currently classified as the type species GAV of the genus Okavirus, the only recognized genus in the family. This chapter covers what is known of the biology of okaviruses as well as molecular characteristics that place these viruses within the order Nidovirales and distinguish them from the coronaviruses, toroviruses, and arteriviruses. It also covers virus-host interactions, virion morphology, assembly, and structure, genome organization, mechanism of subgenomic RNA (sgRNA) synthesis structural proteins, and nidovirus evolutionary considerations learned from okaviruses.
Key Concept Ranking
- White spot syndrome virus
Transmission electron micrographs of negative-stained YHV particles. (Top left) Enveloped virions showing diffuse surface projections; (top right) virion in which the envelope has been damaged, exposing the internal helical nucleocapsid; (bottom left) virions with envelope extensions allowing them to assume circular structures similar to those of toroviruses; (bottom right) ultra-thin section showing the cytoplasm of a gill cell of an infected shrimp with nonenveloped nucleocapsid filaments displaying striations due to their helical symmetry. Scale bars =100 nm. The first three images are reproduced with permission from reference 74 .
Phylogenetic tree constructed from an alignment of a 671-nt sequence in an ORF1b gene region encompassing part a of the helicase domain obtained from 26 viruses detected in P. monodon from the Indo-Pacific region. Viruses clustered into the six described yellow head complex genotypes, including YHV (genotype 1), GAV (genotype 2) and four other genotypes (genotypes 3, 4, 5, and 6) detected only in healthy shrimp. The viruses were detected in shrimp collected in Australia (AUS), India (IND), Indonesia (IDN), Malaysia (MYS), Mozambique (MOZ), the Philippines (PHL), Taiwan (TWN), Thailand (THA), and Vietnam (VNM) between 1997 and 2004 (Wijegoonawardane et al., unpublished).
Schematic representation of the rod-shaped virions of okaviruses highlighting the positioning of the p20 protein in the helical nucleocapsid and the gp116 and gp64 surface glycoproteins forming the envelope protrusions. Reproduced with permission from reference 100a.
Genome organization of GAV indicating positions of identified domains in the ORF1a (multi-TM regions [TM1 to 4] and 3CLpro) and ORF1b (SDD RdRp, zinc-binding domain [ZBD], helicase [HEL], 3´-to-5´ exonuclease [ExoN], uridylatespecific endoribonuclease [NendoU], and C-terminal ribose-2´-0-methyltransferase [MT]) regions of the pp1a and pp1ab replicase polyproteins. Also indicated is the position of the AAAUUUU slippery sequence (•) of the -1 ribosomal frameshift site (RFS), the positions of the two identified 3CLpro cleavage sites (∇) in the pp1ab replicase polyprotein, and the positions of the two signal peptidase type 1-like cleavage sites (▼) used in the posttranslational release of the virion TM glycoproteins gp116 and gp64 from the ORF3 precursor protein. The start positions of the capped, non-leader-containing and 3´-polyadenylated genomic RNA1 and two sg mRNAs (sg mRNA2 and -3) that initiate directly at TREs in the ORF1ab-ORF2 and ORF2-ORF3 IGRs are also shown. The YHV genome organization is similar to that of GAV except that ORF4 is severely truncated.
Schematic representation of the GAV ORF3 polyprotein membrane orientation indicating the relative positions of the six predicted TM domains, the 15 potential N-linked glycosylation sites (•), and the signal peptidase type 1 cleavage sites immediately preceding TM domains 3 and 5 used to generate the mature envelope glycoproteins gp116 and gp64.