Varicella-zoster virus (VZV) open reading frame 61 (ORF61) protein transactivates expression of VZV promoters. VZV ORF61 is functionally homologous to herpes simplex virus type 1 (HSV-1)-infected cell protein 0 (ICP0); however, amino acid sequence homology of these two proteins is limited to the RING finger domains, a recently identified sequence motif composed of cysteine and histidine residues, located in their amino-terminal regions. A carboxy-terminal truncation mutant of ICP0 (which retains the RING finger domain) was previously shown to act as a potent transrepressor and as a dominant-negative mutant in the presence of full-length ICP0. We have shown that the corresponding region of ORF61 has similar properties. Amino-terminal truncation mutants of VZV ORF61 and HSV-1 ICP0, which lack the RING finger domain, cannot transactivate VZV promoters; however, fusion of the amino portion of one protein to the carboxy portion of the other restored the transactivating activity of the full-length proteins. To further study the role of the ORF61 RING finger domain, cysteine or histidine residues of this domain were individually replaced with serine or aspargine, respectively. Each of these amino acid substitution mutants in the RING finger abolished the transactivating activity of the full-length ORF61 protein. Each of the substitutions also reduced the transrepressing and dominant-negative properties of a carboxy-terminal truncation mutant of ORF61. These results indicate that the RING finger domain is required for the transregulatory functions of ORF61.