Single-stranded DNA (ssDNA)-binding phosphoprotein which stimulates the enzymatic activity of vaccinia ribonucleotide reductase. In the absence of magnesium and at low I3 concentration, I3 forms beaded nucleosome-like structures on ssDNA. In the presence of magnesium and at high I3 concentration, I3 and DNA aggregate into large branched filamentous structures. Might be essential for viral parental DNA replication. Homooctamer (Potential). Homoomultimer (Potential). Interacts with the small subunit of ribonucleotide reductase; this interaction stimulates ribonucleotide reductase activity. Localizes in cytoplasmic virus factories, where it is associated with viral DNA. Expressed in the early phase of the viral replicative cycle. This protein is synthesized in the early as well as at the intermediate time of infection. Belongs to the chordopoxvirinae I3 family.