Analysis of a genomic segment of white spot syndrome virus of shrimp containing ribonucleotide reductase genes and repeat regions

White spot syndrome is a worldwide disease of penaeid shrimp. The disease agent is a bacilliform, enveloped virus, white spot syndrome virus (WSSV), with a double-stranded DNA genome that probably contains well over 200 kb. Analysis of a 12?3 kb segment of WSSV DNA revealed eight open reading frames (ORFs), including the genes for the large (RR1) and small (RR2) subunits of ribonucleotide reductase. The rr1 and rr2 genes were separated by 5760 bp, containing several putative ORFs and two domains with multiple sequence repeats. The first domain contained six direct repeats of 54 bp and is part of a coding region. The second domain had one partial and two complete direct repeats of 253 bp at an intergenic location. This repeat, located immediately upstream of rr1, has homologues at several other locations on the WSSV genome. Phylogenetic analysis of RR1 and RR2 indicated that WSSV belongs to the eukaryotic branch of an unrooted parsimonious tree and, further, seems to suggest that WSSV and baculoviruses probably do not share an immediate common ancestor. The present analysis of WSSV favours the view that this virus is either a member of a new genus (Whispovirus) within the Baculoviridae or a member of an entirely new virus family.