Histones are short proteins involved in chromatin packaging; in eukaryotes, two H2a-H2b and H3-H4 histone dimers form the nucleosomal core, which acts as the fundamental DNA-packaging element. The double histone fold is a rare globular protein fold in which two consecutive regions characterized by the typical structure of histones assemble together, thus originating a histone pseudodimer. This fold is included in a few prokaryotic histones and in the regulatory region of guanine nucleotide exchange factors of the Sos family. For the prokaryotic histones, there is no direct structural counterpart in the nucleosomal core particle, while the pseudodimer from Sos proteins is very similar to the dimer formed by histones H2a and H2b
The absence of a H3-H4-like histone pseudodimer in the available structural databases prompted us to search for proteins that could assume such fold. The application of several secondary structure prediction and fold recognition methods allowed to show that the viral protein gi|22788712 is compatible with the structure of a H3-H4-like histone pseudodimer. Further in silico analyses revealed that this protein module could retain the ability of mediating protein-DNA interactions, and could consequently act as a DNA-binding domain.