ICP27 is an essential herpes simplex virus type 1 nuclear regulatory protein that is required for efficient viral gene expression. Although the mechanism by which ICP27 regulates genes is unknown, a variety of evidence suggests that it functions posttranscriptionally, and recent studies indicate that it is an RNA-binding protein. Previously, we noted that a short arginine- and glycine-rich sequence in ICP27 (residues 138 to 152) is similar to an RGG box motif, a putative RNA-binding determinant found in a number of cellular proteins (W. Mears, V. Lain, and S. Rice, J. Virol. 69:935-947, 1995). In the present study, we have further investigated ICP27's association with RNA and examined the role of the RGG box in RNA binding. We find that ICP27 binds efficiently to RNA homopolymers composed of poly(G) and weakly to poly(U) RNA homopolymers. Poly(G) binding activity maps to the N-terminal 189 residues of ICP27 and requires the RGG box sequence. Using a northwestern blotting assay, we demonstrate that the RGG box alone (residues 140 to 152) can mediate RNA binding when attached to a heterologous protein. As many cellular RGG box proteins are methylated on arginine residues, we also investigated the in vivo methylation status of ICP27. Our results demonstrate that ICP27 is methylated in herpes simplex virus-infected cells. Methylation is dependent on the presence of the RGG box, suggesting that one or more arginine residues in the RGG box sequence are modified. These data demonstrate that ICP27 displays the characteristics of an RGG box-type RNA- binding protein.