Genome organization and translation products of Providence virus: insight into a unique tetravirus

Providence virus (PrV) is a member of the family Tetraviridae, a family of small, positive-sense, ssRNA viruses that exclusively infect lepidopteran insects. PrV is the only known tetravirus that replicates in tissue culture. We have analysed the genome and characterized the viral translation products, showing that PrV has a monopartite genome encoding three ORFs: (i) p130, unique to PrV and of unknown function; (ii) p104, which contains a read-through stop signal, producing an N-terminal product of 40 kDa (p40) and (iii) the capsid protein precursor (p81). There are three 2A-like processing sequences: one at the N terminus of p130 (PrV-2A₁) and two more (PrV-2A₂ and PrV-2A₃) at the N terminus of p81. Metabolic radiolabelling identified viral translation products corresponding to all three ORFs in persistently infected cells and showed that the read-through stop in p104 and PrV-2A₃ in p81 are functional in vivo and these results were confirmed by in vitro translation experiments. The RNA-dependent RNA polymerase domain of the PrV replicase is phylogenetically most closely related to members of the families Tombusviridae and Umbraviridae rather than to members of the family Tetraviridae. The unique genome organization, translational control systems and phylogenetic relationship with the replicases of (+ve) plant viruses lead us to propose that PrV represents a novel family of small insect RNA viruses, distinct from current members of the family Tetraviridae.