Identification and characterization of a novel gene of grouper iridovirus encoding a purine nucleoside phosphorylase

Purine nucleoside phosphorylase (PNP) is a key enzyme in the purine salvage pathway. It catalyses the reversible phosphorolysis of purine (2'-deoxy)ribonucleosides to free bases and (2'-deoxy)ribose 1-phosphates. Here, a novel piscine viral PNP gene that was identified from grouper iridovirus (GIV), a causative agent of an epizootic fish disease, is reported. This putative GIV PNP gene encodes a protein of 285 aa with a predicted molecular mass of 30 332 Da and shows high similarity to the human PNP gene. Northern and Western blot analyses of GIV-infected grouper kidney (GK) cells revealed that PNP expression increased in cells with time from 6 h post-infection. Immunocytochemistry localized GIV PNP in the cytoplasm of GIV-infected host cells. PNP-EGFP fusion protein was also observed in the cytoplasm of PNP-EGFP reporter construct-transfected GK and HeLa cells. From HPLC analysis, the recombinant GIV PNP protein was shown to catalyse the reversible phosphorolysis of purine nucleosides and could accept guanosine, inosine and adenosine as substrates. In conclusion, this is the first report of a viral PNP with enzymic activity.