The RNA polymerase subunit RPB10 displays a high level of conservation across archaea and eukarya and is required for cell viability in yeast. Structure determination of this RNA polymerase subunit from Methanobacterium thermoautotrophicum reveals a topology, which we term a zinc-bundle, consisting of three α-helices stabilized by a zinc ion. The metal ion is bound within an atypical CX ₂ CX _n CC sequence motif and serves to bridge an N-terminal loop with helix 3. This represents an example of two adjacent zinc-binding Cys residues within an α-helix conformation. Conserved surface features of RPB10 include discrete regions of neutral, acidic, and basic residues, the latter being located around the zinc-binding site. One or more of these regions may contribute to the role of this subunit as a scaffold protein within the polymerase holoenzyme.