RPB5 is an essential subunit of eukaryotic and archaeal RNA polymerases. It is a proposed target for transcription activator proteins in eukaryotes, but the mechanism of interaction is not known. We have determined the solution structure of the RPB5 subunit from the thermophilic archeon, Methanobacterium thermoautotrophicum. MtRBP5 contains a four-stranded beta-sheet platform supporting two alpha-helices, one on each side of the beta-sheet, resulting in an overall mushroom shape that does not appear to have any structural homologues in the structural database. The position and conservation of charged surface residues suggests possible modes of interaction with other proteins, as well as a rationale for the thermal stability of this protein.