We report the structural features of a C-terminal deletion construct of the Epstein-Barr virus single-stranded DNA-binding protein, Balf2 (Balf2 Delta C), which like the herpes simplex virus I encoded protein, infected cell protein 8 (ICP8), binds non-sequence specifically to single-stranded DNA (ssDNA). ICP8, in the absence of ssDNA, assembles into long filamentous structures. Removal of the 60 C-terminal amino acids of ICP8 (ICP8 Delta C) prevents the formation of such filaments, whereas addition of circular ssDNA to ICP8 Delta C induces formation of "super helical" filaments. Balf2 Delta C, which we show is a zinc-binding protein, does not form these filaments under the same conditions but does bind ssDNA in a weakly cooperative manner. Further structural comparison of both proteins in solution by small-angle X-ray scattering shows proteins with similar molecular envelopes. One major difference is the tendency of Balf2 Delta C to dimerize on different surfaces to that used for oligomerization when binding to ssDNA, and this may have implications for the mechanism of replication initiation. (C) 2007 Elsevier Inc. All rights reserved.