Viruses in the families Arteriviridae and Coronaviridae have enveloped virions which contain nonseg-mented, positive-stranded RNA, but the constituent genera differ markedly in genetic complexity and virion structure. Nevertheless, there are striking resemblances among the viruses in the organization and expression of their genomes, and sequence conservation among the polymerase polyproteins strongly suggests that they have a common ancestry. On this basis, the International Committee on Taxonomy of Viruses recently established a new order, Nidovirales, to contain the two families. Here, the common traits and distinguishing features of the Nidovirales are reviewed. r 1997 Academic Press KEY WORDS: arterivirus; coronavirus; torovirus; polyprotein processing; RNA recombination.

Gill-associated virus of Penaeus monodon prawns: an invertebrate virus with ORF1a and ORF1b genes related to arteri- and coronaviruses

Sequence analysis of the C20 kb 5«-terminal portion of the ssRNA genome of gill-associated virus (GAV) of Penaeus monodon prawns has previously established that it contains an ORF1a–1b replicase gene equivalent to those of the coronavirus and arterivirus members of the order Nidovirales. Sequence analysis of the remaining C6–2 kb of the GAV genome downstream of ORF1a–1b to a 3«-poly(A) tail has identified two highly conserved intergenic sequences in which 29/32 nucleotides are conserved. Northern hybridization using probes to the four putative GAV ORFs and either total or poly(A)-selected RNA identified two 3«-coterminal subgenomic (sg) mRNAs of C6 kb and C5–5 kb. Primer extension and 5«-RACE analyses showed that the sgmRNAs initiate at the same 5«-AC positions in the central region of the two conserved intergenic sequences. Neither method provided any evidence that the GAV sgmRNAs are fused to genomic 5«-leader RNA sequences as is the case with vertebrate coronaviruses and arteriviruses. Intracellular double-stranded (ds)RNAs equivalent in size to the 26–2 kb genomic RNA and two sgRNAs were also identified by RNase/DNase digestion of total RNA from GAV-infected prawn tissue. The identification of only two sgmRNAs that initiate at the same position in conserved intergenic sequences and the absence of 5«-genomic leader sequences fused to these sgmRNAs confirms that